Merge branch 'x86-mds-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip

Pull x86 MDS mitigations from Thomas Gleixner:
 "Microarchitectural Data Sampling (MDS) is a hardware vulnerability
  which allows unprivileged speculative access to data which is
  available in various CPU internal buffers. This new set of misfeatures
  has the following CVEs assigned:

     CVE-2018-12126  MSBDS  Microarchitectural Store Buffer Data Sampling
     CVE-2018-12130  MFBDS  Microarchitectural Fill Buffer Data Sampling
     CVE-2018-12127  MLPDS  Microarchitectural Load Port Data Sampling
     CVE-2019-11091  MDSUM  Microarchitectural Data Sampling Uncacheable Memory

  MDS attacks target microarchitectural buffers which speculatively
  forward data under certain conditions. Disclosure gadgets can expose
  this data via cache side channels.

  Contrary to other speculation based vulnerabilities the MDS
  vulnerability does not allow the attacker to control the memory target
  address. As a consequence the attacks are purely sampling based, but
  as demonstrated with the TLBleed attack samples can be postprocessed
  successfully.

  The mitigation is to flush the microarchitectural buffers on return to
  user space and before entering a VM. It's bolted on the VERW
  instruction and requires a microcode update. As some of the attacks
  exploit data structures shared between hyperthreads, full protection
  requires to disable hyperthreading. The kernel does not do that by
  default to avoid breaking unattended updates.

  The mitigation set comes with documentation for administrators and a
  deeper technical view"

* 'x86-mds-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip: (23 commits)
  x86/speculation/mds: Fix documentation typo
  Documentation: Correct the possible MDS sysfs values
  x86/mds: Add MDSUM variant to the MDS documentation
  x86/speculation/mds: Add 'mitigations=' support for MDS
  x86/speculation/mds: Print SMT vulnerable on MSBDS with mitigations off
  x86/speculation/mds: Fix comment
  x86/speculation/mds: Add SMT warning message
  x86/speculation: Move arch_smt_update() call to after mitigation decisions
  x86/speculation/mds: Add mds=full,nosmt cmdline option
  Documentation: Add MDS vulnerability documentation
  Documentation: Move L1TF to separate directory
  x86/speculation/mds: Add mitigation mode VMWERV
  x86/speculation/mds: Add sysfs reporting for MDS
  x86/speculation/mds: Add mitigation control for MDS
  x86/speculation/mds: Conditionally clear CPU buffers on idle entry
  x86/kvm/vmx: Add MDS protection when L1D Flush is not active
  x86/speculation/mds: Clear CPU buffers on exit to user
  x86/speculation/mds: Add mds_clear_cpu_buffers()
  x86/kvm: Expose X86_FEATURE_MD_CLEAR to guests
  x86/speculation/mds: Add BUG_MSBDS_ONLY
  ...

27 files changed, 921 insertions, 82 deletions

diff --git a/Documentation/ABI/testing/sysfs-devices-system-cpu b/Documentation/ABI/testing/sysfs-devices-system-cpu
index 4fb76c0e8d30..1528239f69b2 100644
--- a/Documentation/ABI/testing/sysfs-devices-system-cpu
+++ b/Documentation/ABI/testing/sysfs-devices-system-cpu
@@ -484,6 +484,7 @@ What:		/sys/devices/system/cpu/vulnerabilities
                 /sys/devices/system/cpu/vulnerabilities/spectre_v2
                 /sys/devices/system/cpu/vulnerabilities/spec_store_bypass
                 /sys/devices/system/cpu/vulnerabilities/l1tf
+                /sys/devices/system/cpu/vulnerabilities/mds
 Date:           January 2018
 Contact:        Linux kernel mailing list <linux-kernel@vger.kernel.org>
 Description:    Information about CPU vulnerabilities
@@ -496,8 +497,7 @@ Description:	Information about CPU vulnerabilities
                 "Vulnerable"      CPU is affected and no mitigation in effect
                 "Mitigation: $M"  CPU is affected and mitigation $M is in effect
 
-                Details about the l1tf file can be found in
-                Documentation/admin-guide/l1tf.rst
+                See also: Documentation/admin-guide/hw-vuln/index.rst
 
 What:           /sys/devices/system/cpu/smt
                 /sys/devices/system/cpu/smt/active
diff --git a/Documentation/admin-guide/hw-vuln/index.rst b/Documentation/admin-guide/hw-vuln/index.rst
new file mode 100644
index 000000000000..ffc064c1ec68
--- /dev/null
+++ b/Documentation/admin-guide/hw-vuln/index.rst
@@ -0,0 +1,13 @@
+========================
+Hardware vulnerabilities
+========================
+
+This section describes CPU vulnerabilities and provides an overview of the
+possible mitigations along with guidance for selecting mitigations if they
+are configurable at compile, boot or run time.
+
+.. toctree::
+   :maxdepth: 1
+
+   l1tf
+   mds
diff --git a/Documentation/admin-guide/l1tf.rst b/Documentation/admin-guide/hw-vuln/l1tf.rst
index 9af977384168..31653a9f0e1b 100644
--- a/Documentation/admin-guide/l1tf.rst
+++ b/Documentation/admin-guide/hw-vuln/l1tf.rst
@@ -445,6 +445,7 @@ The default is 'cond'. If 'l1tf=full,force' is given on the kernel command
 line, then 'always' is enforced and the kvm-intel.vmentry_l1d_flush
 module parameter is ignored and writes to the sysfs file are rejected.
 
+.. _mitigation_selection:
 
 Mitigation selection guide
 --------------------------
diff --git a/Documentation/admin-guide/hw-vuln/mds.rst b/Documentation/admin-guide/hw-vuln/mds.rst
new file mode 100644
index 000000000000..e3a796c0d3a2
--- /dev/null
+++ b/Documentation/admin-guide/hw-vuln/mds.rst
@@ -0,0 +1,308 @@
+MDS - Microarchitectural Data Sampling
+======================================
+
+Microarchitectural Data Sampling is a hardware vulnerability which allows
+unprivileged speculative access to data which is available in various CPU
+internal buffers.
+
+Affected processors
+-------------------
+
+This vulnerability affects a wide range of Intel processors. The
+vulnerability is not present on:
+
+   - Processors from AMD, Centaur and other non Intel vendors
+
+   - Older processor models, where the CPU family is < 6
+
+   - Some Atoms (Bonnell, Saltwell, Goldmont, GoldmontPlus)
+
+   - Intel processors which have the ARCH_CAP_MDS_NO bit set in the
+     IA32_ARCH_CAPABILITIES MSR.
+
+Whether a processor is affected or not can be read out from the MDS
+vulnerability file in sysfs. See :ref:`mds_sys_info`.
+
+Not all processors are affected by all variants of MDS, but the mitigation
+is identical for all of them so the kernel treats them as a single
+vulnerability.
+
+Related CVEs
+------------
+
+The following CVE entries are related to the MDS vulnerability:
+
+   ==============  =====  ===================================================
+   CVE-2018-12126  MSBDS  Microarchitectural Store Buffer Data Sampling
+   CVE-2018-12130  MFBDS  Microarchitectural Fill Buffer Data Sampling
+   CVE-2018-12127  MLPDS  Microarchitectural Load Port Data Sampling
+   CVE-2019-11091  MDSUM  Microarchitectural Data Sampling Uncacheable Memory
+   ==============  =====  ===================================================
+
+Problem
+-------
+
+When performing store, load, L1 refill operations, processors write data
+into temporary microarchitectural structures (buffers). The data in the
+buffer can be forwarded to load operations as an optimization.
+
+Under certain conditions, usually a fault/assist caused by a load
+operation, data unrelated to the load memory address can be speculatively
+forwarded from the buffers. Because the load operation causes a fault or
+assist and its result will be discarded, the forwarded data will not cause
+incorrect program execution or state changes. But a malicious operation
+may be able to forward this speculative data to a disclosure gadget which
+allows in turn to infer the value via a cache side channel attack.
+
+Because the buffers are potentially shared between Hyper-Threads cross
+Hyper-Thread attacks are possible.
+
+Deeper technical information is available in the MDS specific x86
+architecture section: :ref:`Documentation/x86/mds.rst <mds>`.
+
+
+Attack scenarios
+----------------
+
+Attacks against the MDS vulnerabilities can be mounted from malicious non
+priviledged user space applications running on hosts or guest. Malicious
+guest OSes can obviously mount attacks as well.
+
+Contrary to other speculation based vulnerabilities the MDS vulnerability
+does not allow the attacker to control the memory target address. As a
+consequence the attacks are purely sampling based, but as demonstrated with
+the TLBleed attack samples can be postprocessed successfully.
+
+Web-Browsers
+^^^^^^^^^^^^
+
+  It's unclear whether attacks through Web-Browsers are possible at
+  all. The exploitation through Java-Script is considered very unlikely,
+  but other widely used web technologies like Webassembly could possibly be
+  abused.
+
+
+.. _mds_sys_info:
+
+MDS system information
+-----------------------
+
+The Linux kernel provides a sysfs interface to enumerate the current MDS
+status of the system: whether the system is vulnerable, and which
+mitigations are active. The relevant sysfs file is:
+
+/sys/devices/system/cpu/vulnerabilities/mds
+
+The possible values in this file are:
+
+  .. list-table::
+
+     * - 'Not affected'
+       - The processor is not vulnerable
+     * - 'Vulnerable'
+       - The processor is vulnerable, but no mitigation enabled
+     * - 'Vulnerable: Clear CPU buffers attempted, no microcode'
+       - The processor is vulnerable but microcode is not updated.
+
+         The mitigation is enabled on a best effort basis. See :ref:`vmwerv`
+     * - 'Mitigation: Clear CPU buffers'
+       - The processor is vulnerable and the CPU buffer clearing mitigation is
+         enabled.
+
+If the processor is vulnerable then the following information is appended
+to the above information:
+
+    ========================  ============================================
+    'SMT vulnerable'          SMT is enabled
+    'SMT mitigated'           SMT is enabled and mitigated
+    'SMT disabled'            SMT is disabled
+    'SMT Host state unknown'  Kernel runs in a VM, Host SMT state unknown
+    ========================  ============================================
+
+.. _vmwerv:
+
+Best effort mitigation mode
+^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+  If the processor is vulnerable, but the availability of the microcode based
+  mitigation mechanism is not advertised via CPUID the kernel selects a best
+  effort mitigation mode.  This mode invokes the mitigation instructions
+  without a guarantee that they clear the CPU buffers.
+
+  This is done to address virtualization scenarios where the host has the
+  microcode update applied, but the hypervisor is not yet updated to expose
+  the CPUID to the guest. If the host has updated microcode the protection
+  takes effect otherwise a few cpu cycles are wasted pointlessly.
+
+  The state in the mds sysfs file reflects this situation accordingly.
+
+
+Mitigation mechanism
+-------------------------
+
+The kernel detects the affected CPUs and the presence of the microcode
+which is required.
+
+If a CPU is affected and the microcode is available, then the kernel
+enables the mitigation by default. The mitigation can be controlled at boot
+time via a kernel command line option. See
+:ref:`mds_mitigation_control_command_line`.
+
+.. _cpu_buffer_clear:
+
+CPU buffer clearing
+^^^^^^^^^^^^^^^^^^^
+
+  The mitigation for MDS clears the affected CPU buffers on return to user
+  space and when entering a guest.
+
+  If SMT is enabled it also clears the buffers on idle entry when the CPU
+  is only affected by MSBDS and not any other MDS variant, because the
+  other variants cannot be protected against cross Hyper-Thread attacks.
+
+  For CPUs which are only affected by MSBDS the user space, guest and idle
+  transition mitigations are sufficient and SMT is not affected.
+
+.. _virt_mechanism:
+
+Virtualization mitigation
+^^^^^^^^^^^^^^^^^^^^^^^^^
+
+  The protection for host to guest transition depends on the L1TF
+  vulnerability of the CPU:
+
+  - CPU is affected by L1TF:
+
+    If the L1D flush mitigation is enabled and up to date microcode is
+    available, the L1D flush mitigation is automatically protecting the
+    guest transition.
+
+    If the L1D flush mitigation is disabled then the MDS mitigation is
+    invoked explicit when the host MDS mitigation is enabled.
+
+    For details on L1TF and virtualization see:
+    :ref:`Documentation/admin-guide/hw-vuln//l1tf.rst <mitigation_control_kvm>`.
+
+  - CPU is not affected by L1TF:
+
+    CPU buffers are flushed before entering the guest when the host MDS
+    mitigation is enabled.
+
+  The resulting MDS protection matrix for the host to guest transition:
+
+  ============ ===== ============= ============ =================
+   L1TF         MDS   VMX-L1FLUSH   Host MDS     MDS-State
+
+   Don't care   No    Don't care    N/A          Not affected
+
+   Yes          Yes   Disabled      Off          Vulnerable
+
+   Yes          Yes   Disabled      Full         Mitigated
+
+   Yes          Yes   Enabled       Don't care   Mitigated
+
+   No           Yes   N/A           Off          Vulnerable
+
+   No           Yes   N/A           Full         Mitigated
+  ============ ===== ============= ============ =================
+
+  This only covers the host to guest transition, i.e. prevents leakage from
+  host to guest, but does not protect the guest internally. Guests need to
+  have their own protections.
+
+.. _xeon_phi:
+
+XEON PHI specific considerations
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+  The XEON PHI processor family is affected by MSBDS which can be exploited
+  cross Hyper-Threads when entering idle states. Some XEON PHI variants allow
+  to use MWAIT in user space (Ring 3) which opens an potential attack vector
+  for malicious user space. The exposure can be disabled on the kernel
+  command line with the 'ring3mwait=disable' command line option.
+
+  XEON PHI is not affected by the other MDS variants and MSBDS is mitigated
+  before the CPU enters a idle state. As XEON PHI is not affected by L1TF
+  either disabling SMT is not required for full protection.
+
+.. _mds_smt_control:
+
+SMT control
+^^^^^^^^^^^
+
+  All MDS variants except MSBDS can be attacked cross Hyper-Threads. That
+  means on CPUs which are affected by MFBDS or MLPDS it is necessary to
+  disable SMT for full protection. These are most of the affected CPUs; the
+  exception is XEON PHI, see :ref:`xeon_phi`.
+
+  Disabling SMT can have a significant performance impact, but the impact
+  depends on the type of workloads.
+
+  See the relevant chapter in the L1TF mitigation documentation for details:
+  :ref:`Documentation/admin-guide/hw-vuln/l1tf.rst <smt_control>`.
+
+
+.. _mds_mitigation_control_command_line:
+
+Mitigation control on the kernel command line
+---------------------------------------------
+
+The kernel command line allows to control the MDS mitigations at boot
+time with the option "mds=". The valid arguments for this option are:
+
+  ============  =============================================================
+  full          If the CPU is vulnerable, enable all available mitigations
+                for the MDS vulnerability, CPU buffer clearing on exit to
+                userspace and when entering a VM. Idle transitions are
+                protected as well if SMT is enabled.
+
+                It does not automatically disable SMT.
+
+  full,nosmt    The same as mds=full, with SMT disabled on vulnerable
+                CPUs.  This is the complete mitigation.
+
+  off           Disables MDS mitigations completely.
+
+  ============  =============================================================
+
+Not specifying this option is equivalent to "mds=full".
+
+
+Mitigation selection guide
+--------------------------
+
+1. Trusted userspace
+^^^^^^^^^^^^^^^^^^^^
+
+   If all userspace applications are from a trusted source and do not
+   execute untrusted code which is supplied externally, then the mitigation
+   can be disabled.
+
+
+2. Virtualization with trusted guests
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+   The same considerations as above versus trusted user space apply.
+
+3. Virtualization with untrusted guests
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+   The protection depends on the state of the L1TF mitigations.
+   See :ref:`virt_mechanism`.
+
+   If the MDS mitigation is enabled and SMT is disabled, guest to host and
+   guest to guest attacks are prevented.
+
+.. _mds_default_mitigations:
+
+Default mitigations
+-------------------
+
+  The kernel default mitigations for vulnerable processors are:
+
+  - Enable CPU buffer clearing
+
+  The kernel does not by default enforce the disabling of SMT, which leaves
+  SMT systems vulnerable when running untrusted code. The same rationale as
+  for L1TF applies.
+  See :ref:`Documentation/admin-guide/hw-vuln//l1tf.rst <default_mitigations>`.
diff --git a/Documentation/admin-guide/index.rst b/Documentation/admin-guide/index.rst
index 5b8286fdd91b..8001917ee012 100644
--- a/Documentation/admin-guide/index.rst
+++ b/Documentation/admin-guide/index.rst
@@ -17,14 +17,12 @@ etc.
    kernel-parameters
    devices
 
-This section describes CPU vulnerabilities and provides an overview of the
-possible mitigations along with guidance for selecting mitigations if they
-are configurable at compile, boot or run time.
+This section describes CPU vulnerabilities and their mitigations.
 
 .. toctree::
    :maxdepth: 1
 
-   l1tf
+   hw-vuln/index
 
 Here is a set of documents aimed at users who are trying to track down
 problems and bugs in particular.
diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 08df58805703..43176340c73d 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -2143,7 +2143,7 @@
 
                         Default is 'flush'.
 
-                        For details see: Documentation/admin-guide/l1tf.rst
+                        For details see: Documentation/admin-guide/hw-vuln/l1tf.rst
 
         l2cr=           [PPC]
 
@@ -2389,6 +2389,32 @@
                         Format: <first>,<last>
                         Specifies range of consoles to be captured by the MDA.
 
+        mds=            [X86,INTEL]
+                        Control mitigation for the Micro-architectural Data
+                        Sampling (MDS) vulnerability.
+
+                        Certain CPUs are vulnerable to an exploit against CPU
+                        internal buffers which can forward information to a
+                        disclosure gadget under certain conditions.
+
+                        In vulnerable processors, the speculatively
+                        forwarded data can be used in a cache side channel
+                        attack, to access data to which the attacker does
+                        not have direct access.
+
+                        This parameter controls the MDS mitigation. The
+                        options are:
+
+                        full       - Enable MDS mitigation on vulnerable CPUs
+                        full,nosmt - Enable MDS mitigation and disable
+                                     SMT on vulnerable CPUs
+                        off        - Unconditionally disable MDS mitigation
+
+                        Not specifying this option is equivalent to
+                        mds=full.
+
+                        For details see: Documentation/admin-guide/hw-vuln/mds.rst
+
         mem=nn[KMG]     [KNL,BOOT] Force usage of a specific amount of memory
                         Amount of memory to be used when the kernel is not able
                         to see the whole system memory or for test.
@@ -2565,6 +2591,7 @@
                                                spec_store_bypass_disable=off [X86,PPC]
                                                ssbd=force-off [ARM64]
                                                l1tf=off [X86]
+                                               mds=off [X86]
 
                         auto (default)
                                 Mitigate all CPU vulnerabilities, but leave SMT
@@ -2579,6 +2606,7 @@
                                 if needed.  This is for users who always want to
                                 be fully mitigated, even if it means losing SMT.
                                 Equivalent to: l1tf=flush,nosmt [X86]
+                                               mds=full,nosmt [X86]
 
         mminit_loglevel=
                         [KNL] When CONFIG_DEBUG_MEMORY_INIT is set, this
diff --git a/Documentation/index.rst b/Documentation/index.rst
index 9e01aace4f48..a7566ef62411 100644
--- a/Documentation/index.rst
+++ b/Documentation/index.rst
@@ -114,6 +114,7 @@ implementation.
 
    x86/index
    sh/index
+   x86/index
 
 Filesystem Documentation
 ------------------------
diff --git a/Documentation/x86/conf.py b/Documentation/x86/conf.py
new file mode 100644
index 000000000000..33c5c3142e20
--- /dev/null
+++ b/Documentation/x86/conf.py
@@ -0,0 +1,10 @@
+# -*- coding: utf-8; mode: python -*-
+
+project = "X86 architecture specific documentation"
+
+tags.add("subproject")
+
+latex_documents = [
+    ('index', 'x86.tex', project,
+     'The kernel development community', 'manual'),
+]
diff --git a/Documentation/x86/index.rst b/Documentation/x86/index.rst
index 73a487957fd4..ae36fc5fc649 100644
--- a/Documentation/x86/index.rst
+++ b/Documentation/x86/index.rst
@@ -23,6 +23,7 @@ x86-specific Documentation
    intel_mpx
    amd-memory-encryption
    pti
+   mds
    microcode
    resctrl_ui
    usb-legacy-support
diff --git a/Documentation/x86/mds.rst b/Documentation/x86/mds.rst
new file mode 100644
index 000000000000..534e9baa4e1d
--- /dev/null
+++ b/Documentation/x86/mds.rst
@@ -0,0 +1,225 @@
+Microarchitectural Data Sampling (MDS) mitigation
+=================================================
+
+.. _mds:
+
+Overview
+--------
+
+Microarchitectural Data Sampling (MDS) is a family of side channel attacks
+on internal buffers in Intel CPUs. The variants are:
+
+ - Microarchitectural Store Buffer Data Sampling (MSBDS) (CVE-2018-12126)
+ - Microarchitectural Fill Buffer Data Sampling (MFBDS) (CVE-2018-12130)
+ - Microarchitectural Load Port Data Sampling (MLPDS) (CVE-2018-12127)
+ - Microarchitectural Data Sampling Uncacheable Memory (MDSUM) (CVE-2019-11091)
+
+MSBDS leaks Store Buffer Entries which can be speculatively forwarded to a
+dependent load (store-to-load forwarding) as an optimization. The forward
+can also happen to a faulting or assisting load operation for a different
+memory address, which can be exploited under certain conditions. Store
+buffers are partitioned between Hyper-Threads so cross thread forwarding is
+not possible. But if a thread enters or exits a sleep state the store
+buffer is repartitioned which can expose data from one thread to the other.
+
+MFBDS leaks Fill Buffer Entries. Fill buffers are used internally to manage
+L1 miss situations and to hold data which is returned or sent in response
+to a memory or I/O operation. Fill buffers can forward data to a load
+operation and also write data to the cache. When the fill buffer is
+deallocated it can retain the stale data of the preceding operations which
+can then be forwarded to a faulting or assisting load operation, which can
+be exploited under certain conditions. Fill buffers are shared between
+Hyper-Threads so cross thread leakage is possible.
+
+MLPDS leaks Load Port Data. Load ports are used to perform load operations
+from memory or I/O. The received data is then forwarded to the register
+file or a subsequent operation. In some implementations the Load Port can
+contain stale data from a previous operation which can be forwarded to
+faulting or assisting loads under certain conditions, which again can be
+exploited eventually. Load ports are shared between Hyper-Threads so cross
+thread leakage is possible.
+
+MDSUM is a special case of MSBDS, MFBDS and MLPDS. An uncacheable load from
+memory that takes a fault or assist can leave data in a microarchitectural
+structure that may later be observed using one of the same methods used by
+MSBDS, MFBDS or MLPDS.
+
+Exposure assumptions
+--------------------
+
+It is assumed that attack code resides in user space or in a guest with one
+exception. The rationale behind this assumption is that the code construct
+needed for exploiting MDS requires:
+
+ - to control the load to trigger a fault or assist
+
+ - to have a disclosure gadget which exposes the speculatively accessed
+   data for consumption through a side channel.
+
+ - to control the pointer through which the disclosure gadget exposes the
+   data
+
+The existence of such a construct in the kernel cannot be excluded with
+100% certainty, but the complexity involved makes it extremly unlikely.
+
+There is one exception, which is untrusted BPF. The functionality of
+untrusted BPF is limited, but it needs to be thoroughly investigated
+whether it can be used to create such a construct.
+
+
+Mitigation strategy
+-------------------
+
+All variants have the same mitigation strategy at least for the single CPU
+thread case (SMT off): Force the CPU to clear the affected buffers.
+
+This is achieved by using the otherwise unused and obsolete VERW
+instruction in combination with a microcode update. The microcode clears
+the affected CPU buffers when the VERW instruction is executed.
+
+For virtualization there are two ways to achieve CPU buffer
+clearing. Either the modified VERW instruction or via the L1D Flush
+command. The latter is issued when L1TF mitigation is enabled so the extra
+VERW can be avoided. If the CPU is not affected by L1TF then VERW needs to
+be issued.
+
+If the VERW instruction with the supplied segment selector argument is
+executed on a CPU without the microcode update there is no side effect
+other than a small number of pointlessly wasted CPU cycles.
+
+This does not protect against cross Hyper-Thread attacks except for MSBDS
+which is only exploitable cross Hyper-thread when one of the Hyper-Threads
+enters a C-state.
+
+The kernel provides a function to invoke the buffer clearing:
+
+    mds_clear_cpu_buffers()
+
+The mitigation is invoked on kernel/userspace, hypervisor/guest and C-state
+(idle) transitions.
+
+As a special quirk to address virtualization scenarios where the host has
+the microcode updated, but the hypervisor does not (yet) expose the
+MD_CLEAR CPUID bit to guests, the kernel issues the VERW instruction in the
+hope that it might actually clear the buffers. The state is reflected
+accordingly.
+
+According to current knowledge additional mitigations inside the kernel
+itself are not required because the necessary gadgets to expose the leaked
+data cannot be controlled in a way which allows exploitation from malicious
+user space or VM guests.
+
+Kernel internal mitigation modes
+--------------------------------
+
+ ======= ============================================================
+ off      Mitigation is disabled. Either the CPU is not affected or
+          mds=off is supplied on the kernel command line
+
+ full     Mitigation is enabled. CPU is affected and MD_CLEAR is
+          advertised in CPUID.
+
+ vmwerv   Mitigation is enabled. CPU is affected and MD_CLEAR is not
+          advertised in CPUID. That is mainly for virtualization
+          scenarios where the host has the updated microcode but the
+          hypervisor does not expose MD_CLEAR in CPUID. It's a best
+          effort approach without guarantee.
+ ======= ============================================================
+
+If the CPU is affected and mds=off is not supplied on the kernel command
+line then the kernel selects the appropriate mitigation mode depending on
+the availability of the MD_CLEAR CPUID bit.
+
+Mitigation points
+-----------------
+
+1. Return to user space
+^^^^^^^^^^^^^^^^^^^^^^^
+
+   When transitioning from kernel to user space the CPU buffers are flushed
+   on affected CPUs when the mitigation is not disabled on the kernel
+   command line. The migitation is enabled through the static key
+   mds_user_clear.
+
+   The mitigation is invoked in prepare_exit_to_usermode() which covers
+   most of the kernel to user space transitions. There are a few exceptions
+   which are not invoking prepare_exit_to_usermode() on return to user
+   space. These exceptions use the paranoid exit code.
+
+   - Non Maskable Interrupt (NMI):
+
+     Access to sensible data like keys, credentials in the NMI context is
+     mostly theoretical: The CPU can do prefetching or execute a
+     misspeculated code path and thereby fetching data which might end up
+     leaking through a buffer.
+
+     But for mounting other attacks the kernel stack address of the task is
+     already valuable information. So in full mitigation mode, the NMI is
+     mitigated on the return from do_nmi() to provide almost complete
+     coverage.
+
+   - Double fault (#DF):
+
+     A double fault is usually fatal, but the ESPFIX workaround, which can
+     be triggered from user space through modify_ldt(2) is a recoverable
+     double fault. #DF uses the paranoid exit path, so explicit mitigation
+     in the double fault handler is required.
+
+   - Machine Check Exception (#MC):
+
+     Another corner case is a #MC which hits between the CPU buffer clear
+     invocation and the actual return to user. As this still is in kernel
+     space it takes the paranoid exit path which does not clear the CPU
+     buffers. So the #MC handler repopulates the buffers to some
+     extent. Machine checks are not reliably controllable and the window is
+     extremly small so mitigation would just tick a checkbox that this
+     theoretical corner case is covered. To keep the amount of special
+     cases small, ignore #MC.
+
+   - Debug Exception (#DB):
+
+     This takes the paranoid exit path only when the INT1 breakpoint is in
+     kernel space. #DB on a user space address takes the regular exit path,
+     so no extra mitigation required.
+
+
+2. C-State transition
+^^^^^^^^^^^^^^^^^^^^^
+
+   When a CPU goes idle and enters a C-State the CPU buffers need to be
+   cleared on affected CPUs when SMT is active. This addresses the
+   repartitioning of the store buffer when one of the Hyper-Threads enters
+   a C-State.
+
+   When SMT is inactive, i.e. either the CPU does not support it or all
+   sibling threads are offline CPU buffer clearing is not required.
+
+   The idle clearing is enabled on CPUs which are only affected by MSBDS
+   and not by any other MDS variant. The other MDS variants cannot be
+   protected against cross Hyper-Thread attacks because the Fill Buffer and
+   the Load Ports are shared. So on CPUs affected by other variants, the
+   idle clearing would be a window dressing exercise and is therefore not
+   activated.
+
+   The invocation is controlled by the static key mds_idle_clear which is
+   switched depending on the chosen mitigation mode and the SMT state of
+   the system.
+
+   The buffer clear is only invoked before entering the C-State to prevent
+   that stale data from the idling CPU from spilling to the Hyper-Thread
+   sibling after the store buffer got repartitioned and all entries are
+   available to the non idle sibling.
+
+   When coming out of idle the store buffer is partitioned again so each
+   sibling has half of it available. The back from idle CPU could be then
+   speculatively exposed to contents of the sibling. The buffers are
+   flushed either on exit to user space or on VMENTER so malicious code
+   in user space or the guest cannot speculatively access them.
+
+   The mitigation is hooked into all variants of halt()/mwait(), but does
+   not cover the legacy ACPI IO-Port mechanism because the ACPI idle driver
+   has been superseded by the intel_idle driver around 2010 and is
+   preferred on all affected CPUs which are expected to gain the MD_CLEAR
+   functionality in microcode. Aside of that the IO-Port mechanism is a
+   legacy interface which is only used on older systems which are either
+   not affected or do not receive microcode updates anymore.
diff --git a/arch/x86/entry/common.c b/arch/x86/entry/common.c
index 51beb8d29123..a986b3c8294c 100644
--- a/arch/x86/entry/common.c
+++ b/arch/x86/entry/common.c
@@ -32,6 +32,7 @@
 #include <asm/vdso.h>
 #include <asm/cpufeature.h>
 #include <asm/fpu/api.h>
+#include <asm/nospec-branch.h>
 
 #define CREATE_TRACE_POINTS
 #include <trace/events/syscalls.h>
@@ -220,6 +221,8 @@ __visible inline void prepare_exit_to_usermode(struct pt_regs *regs)
 #endif
 
         user_enter_irqoff();
+
+        mds_user_clear_cpu_buffers();
 }
 
 #define SYSCALL_EXIT_WORK_FLAGS                         \
diff --git a/arch/x86/include/asm/cpufeatures.h b/arch/x86/include/asm/cpufeatures.h
index 981ff9479648..75f27ee2c263 100644
--- a/arch/x86/include/asm/cpufeatures.h
+++ b/arch/x86/include/asm/cpufeatures.h
@@ -344,6 +344,7 @@
 /* Intel-defined CPU features, CPUID level 0x00000007:0 (EDX), word 18 */
 #define X86_FEATURE_AVX512_4VNNIW       (18*32+ 2) /* AVX-512 Neural Network Instructions */
 #define X86_FEATURE_AVX512_4FMAPS       (18*32+ 3) /* AVX-512 Multiply Accumulation Single precision */
+#define X86_FEATURE_MD_CLEAR            (18*32+10) /* VERW clears CPU buffers */
 #define X86_FEATURE_TSX_FORCE_ABORT     (18*32+13) /* "" TSX_FORCE_ABORT */
 #define X86_FEATURE_PCONFIG             (18*32+18) /* Intel PCONFIG */
 #define X86_FEATURE_SPEC_CTRL           (18*32+26) /* "" Speculation Control (IBRS + IBPB) */
@@ -382,5 +383,7 @@
 #define X86_BUG_SPECTRE_V2              X86_BUG(16) /* CPU is affected by Spectre variant 2 attack with indirect branches */
 #define X86_BUG_SPEC_STORE_BYPASS       X86_BUG(17) /* CPU is affected by speculative store bypass attack */
 #define X86_BUG_L1TF                    X86_BUG(18) /* CPU is affected by L1 Terminal Fault */
+#define X86_BUG_MDS                     X86_BUG(19) /* CPU is affected by Microarchitectural data sampling */
+#define X86_BUG_MSBDS_ONLY              X86_BUG(20) /* CPU is only affected by the  MSDBS variant of BUG_MDS */
 
 #endif /* _ASM_X86_CPUFEATURES_H */
diff --git a/arch/x86/include/asm/irqflags.h b/arch/x86/include/asm/irqflags.h
index 058e40fed167..8a0e56e1dcc9 100644
--- a/arch/x86/include/asm/irqflags.h
+++ b/arch/x86/include/asm/irqflags.h
@@ -6,6 +6,8 @@
 
 #ifndef __ASSEMBLY__
 
+#include <asm/nospec-branch.h>
+
 /* Provide __cpuidle; we can't safely include <linux/cpu.h> */
 #define __cpuidle __attribute__((__section__(".cpuidle.text")))
 
@@ -54,11 +56,13 @@ static inline void native_irq_enable(void)
 
 static inline __cpuidle void native_safe_halt(void)
 {
+        mds_idle_clear_cpu_buffers();
         asm volatile("sti; hlt": : :"memory");
 }
 
 static inline __cpuidle void native_halt(void)
 {
+        mds_idle_clear_cpu_buffers();
         asm volatile("hlt": : :"memory");
 }
 
diff --git a/arch/x86/include/asm/msr-index.h b/arch/x86/include/asm/msr-index.h
index 1378518cf63f..88dd202c8b00 100644
--- a/arch/x86/include/asm/msr-index.h
+++ b/arch/x86/include/asm/msr-index.h
@@ -2,6 +2,8 @@
 #ifndef _ASM_X86_MSR_INDEX_H
 #define _ASM_X86_MSR_INDEX_H
 
+#include <linux/bits.h>
+
 /*
  * CPU model specific register (MSR) numbers.
  *
@@ -40,14 +42,14 @@
 /* Intel MSRs. Some also available on other CPUs */
 
 #define MSR_IA32_SPEC_CTRL              0x00000048 /* Speculation Control */
-#define SPEC_CTRL_IBRS                  (1 << 0)   /* Indirect Branch Restricted Speculation */
+#define SPEC_CTRL_IBRS                  BIT(0)     /* Indirect Branch Restricted Speculation */
 #define SPEC_CTRL_STIBP_SHIFT           1          /* Single Thread Indirect Branch Predictor (STIBP) bit */
-#define SPEC_CTRL_STIBP                 (1 << SPEC_CTRL_STIBP_SHIFT)    /* STIBP mask */
+#define SPEC_CTRL_STIBP                 BIT(SPEC_CTRL_STIBP_SHIFT)      /* STIBP mask */
 #define SPEC_CTRL_SSBD_SHIFT            2          /* Speculative Store Bypass Disable bit */
-#define SPEC_CTRL_SSBD                  (1 << SPEC_CTRL_SSBD_SHIFT)     /* Speculative Store Bypass Disable */
+#define SPEC_CTRL_SSBD                  BIT(SPEC_CTRL_SSBD_SHIFT)       /* Speculative Store Bypass Disable */
 
 #define MSR_IA32_PRED_CMD               0x00000049 /* Prediction Command */
-#define PRED_CMD_IBPB                   (1 << 0)   /* Indirect Branch Prediction Barrier */
+#define PRED_CMD_IBPB                   BIT(0)     /* Indirect Branch Prediction Barrier */
 
 #define MSR_PPIN_CTL                    0x0000004e
 #define MSR_PPIN                        0x0000004f
@@ -69,20 +71,25 @@
 #define MSR_MTRRcap                     0x000000fe
 
 #define MSR_IA32_ARCH_CAPABILITIES      0x0000010a
-#define ARCH_CAP_RDCL_NO                (1 << 0)   /* Not susceptible to Meltdown */
-#define ARCH_CAP_IBRS_ALL               (1 << 1)   /* Enhanced IBRS support */
-#define ARCH_CAP_SKIP_VMENTRY_L1DFLUSH  (1 << 3)   /* Skip L1D flush on vmentry */
-#define ARCH_CAP_SSB_NO                 (1 << 4)   /*
-                                                    * Not susceptible to Speculative Store Bypass
-                                                    * attack, so no Speculative Store Bypass
-                                                    * control required.
-                                                    */
+#define ARCH_CAP_RDCL_NO                BIT(0)  /* Not susceptible to Meltdown */
+#define ARCH_CAP_IBRS_ALL               BIT(1)  /* Enhanced IBRS support */
+#define ARCH_CAP_SKIP_VMENTRY_L1DFLUSH  BIT(3)  /* Skip L1D flush on vmentry */
+#define ARCH_CAP_SSB_NO                 BIT(4)  /*
+                                                 * Not susceptible to Speculative Store Bypass
+                                                 * attack, so no Speculative Store Bypass
+                                                 * control required.
+                                                 */
+#define ARCH_CAP_MDS_NO                 BIT(5)   /*
+                                                  * Not susceptible to
+                                                  * Microarchitectural Data
+                                                  * Sampling (MDS) vulnerabilities.
+                                                  */
 
 #define MSR_IA32_FLUSH_CMD              0x0000010b
-#define L1D_FLUSH                       (1 << 0)   /*
-                                                    * Writeback and invalidate the
-                                                    * L1 data cache.
-                                                    */
+#define L1D_FLUSH                       BIT(0)  /*
+                                                 * Writeback and invalidate the
+                                                 * L1 data cache.
+                                                 */
 
 #define MSR_IA32_BBL_CR_CTL             0x00000119
 #define MSR_IA32_BBL_CR_CTL3            0x0000011e
diff --git a/arch/x86/include/asm/mwait.h b/arch/x86/include/asm/mwait.h
index 39a2fb29378a..eb0f80ce8524 100644
--- a/arch/x86/include/asm/mwait.h
+++ b/arch/x86/include/asm/mwait.h
@@ -6,6 +6,7 @@
 #include <linux/sched/idle.h>
 
 #include <asm/cpufeature.h>
+#include <asm/nospec-branch.h>
 
 #define MWAIT_SUBSTATE_MASK             0xf
 #define MWAIT_CSTATE_MASK               0xf
@@ -40,6 +41,8 @@ static inline void __monitorx(const void *eax, unsigned long ecx,
 
 static inline void __mwait(unsigned long eax, unsigned long ecx)
 {
+        mds_idle_clear_cpu_buffers();
+
         /* "mwait %eax, %ecx;" */
         asm volatile(".byte 0x0f, 0x01, 0xc9;"
                      :: "a" (eax), "c" (ecx));
@@ -74,6 +77,8 @@ static inline void __mwait(unsigned long eax, unsigned long ecx)
 static inline void __mwaitx(unsigned long eax, unsigned long ebx,
                             unsigned long ecx)
 {
+        /* No MDS buffer clear as this is AMD/HYGON only */
+
         /* "mwaitx %eax, %ebx, %ecx;" */
         asm volatile(".byte 0x0f, 0x01, 0xfb;"
                      :: "a" (eax), "b" (ebx), "c" (ecx));
@@ -81,6 +86,8 @@ static inline void __mwaitx(unsigned long eax, unsigned long ebx,
 
 static inline void __sti_mwait(unsigned long eax, unsigned long ecx)
 {
+        mds_idle_clear_cpu_buffers();
+
         trace_hardirqs_on();
         /* "mwait %eax, %ecx;" */
         asm volatile("sti; .byte 0x0f, 0x01, 0xc9;"
diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/nospec-branch.h
index daf25b60c9e3..109f974f9835 100644
--- a/arch/x86/include/asm/nospec-branch.h
+++ b/arch/x86/include/asm/nospec-branch.h
@@ -308,6 +308,56 @@ DECLARE_STATIC_KEY_FALSE(switch_to_cond_stibp);
 DECLARE_STATIC_KEY_FALSE(switch_mm_cond_ibpb);
 DECLARE_STATIC_KEY_FALSE(switch_mm_always_ibpb);
 
+DECLARE_STATIC_KEY_FALSE(mds_user_clear);
+DECLARE_STATIC_KEY_FALSE(mds_idle_clear);
+
+#include <asm/segment.h>
+
+/**
+ * mds_clear_cpu_buffers - Mitigation for MDS vulnerability
+ *
+ * This uses the otherwise unused and obsolete VERW instruction in
+ * combination with microcode which triggers a CPU buffer flush when the
+ * instruction is executed.
+ */
+static inline void mds_clear_cpu_buffers(void)
+{
+        static const u16 ds = __KERNEL_DS;
+
+        /*
+         * Has to be the memory-operand variant because only that
+         * guarantees the CPU buffer flush functionality according to
+         * documentation. The register-operand variant does not.
+         * Works with any segment selector, but a valid writable
+         * data segment is the fastest variant.
+         *
+         * "cc" clobber is required because VERW modifies ZF.
+         */
+        asm volatile("verw %[ds]" : : [ds] "m" (ds) : "cc");
+}
+
+/**
+ * mds_user_clear_cpu_buffers - Mitigation for MDS vulnerability
+ *
+ * Clear CPU buffers if the corresponding static key is enabled
+ */
+static inline void mds_user_clear_cpu_buffers(void)
+{
+        if (static_branch_likely(&mds_user_clear))
+                mds_clear_cpu_buffers();
+}
+
+/**
+ * mds_idle_clear_cpu_buffers - Mitigation for MDS vulnerability
+ *
+ * Clear CPU buffers if the corresponding static key is enabled
+ */
+static inline void mds_idle_clear_cpu_buffers(void)
+{
+        if (static_branch_likely(&mds_idle_clear))
+                mds_clear_cpu_buffers();
+}
+
 #endif /* __ASSEMBLY__ */
 
 /*
diff --git a/arch/x86/include/asm/processor.h b/arch/x86/include/asm/processor.h
index 7e99ef67bff0..c34a35c78618 100644
--- a/arch/x86/include/asm/processor.h
+++ b/arch/x86/include/asm/processor.h
@@ -978,4 +978,10 @@ enum l1tf_mitigations {
 
 extern enum l1tf_mitigations l1tf_mitigation;
 
+enum mds_mitigations {
+        MDS_MITIGATION_OFF,
+        MDS_MITIGATION_FULL,
+        MDS_MITIGATION_VMWERV,
+};
+
 #endif /* _ASM_X86_PROCESSOR_H */
diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index 29630393f300..03b4cc0ec3a7 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -37,6 +37,7 @@
 static void __init spectre_v2_select_mitigation(void);
 static void __init ssb_select_mitigation(void);
 static void __init l1tf_select_mitigation(void);
+static void __init mds_select_mitigation(void);
 
 /* The base value of the SPEC_CTRL MSR that always has to be preserved. */
 u64 x86_spec_ctrl_base;
@@ -63,6 +64,13 @@ DEFINE_STATIC_KEY_FALSE(switch_mm_cond_ibpb);
 /* Control unconditional IBPB in switch_mm() */
 DEFINE_STATIC_KEY_FALSE(switch_mm_always_ibpb);
 
+/* Control MDS CPU buffer clear before returning to user space */
+DEFINE_STATIC_KEY_FALSE(mds_user_clear);
+EXPORT_SYMBOL_GPL(mds_user_clear);
+/* Control MDS CPU buffer clear before idling (halt, mwait) */
+DEFINE_STATIC_KEY_FALSE(mds_idle_clear);
+EXPORT_SYMBOL_GPL(mds_idle_clear);
+
 void __init check_bugs(void)
 {
         identify_boot_cpu();
@@ -101,6 +109,10 @@ void __init check_bugs(void)
 
         l1tf_select_mitigation();
 
+        mds_select_mitigation();
+
+        arch_smt_update();
+
 #ifdef CONFIG_X86_32
         /*
          * Check whether we are able to run this kernel safely on SMP.
@@ -207,6 +219,61 @@ static void x86_amd_ssb_disable(void)
 }
 
 #undef pr_fmt
+#define pr_fmt(fmt)     "MDS: " fmt
+
+/* Default mitigation for MDS-affected CPUs */
+static enum mds_mitigations mds_mitigation __ro_after_init = MDS_MITIGATION_FULL;
+static bool mds_nosmt __ro_after_init = false;
+
+static const char * const mds_strings[] = {
+        [MDS_MITIGATION_OFF]    = "Vulnerable",
+        [MDS_MITIGATION_FULL]   = "Mitigation: Clear CPU buffers",
+        [MDS_MITIGATION_VMWERV] = "Vulnerable: Clear CPU buffers attempted, no microcode",
+};
+
+static void __init mds_select_mitigation(void)
+{
+        if (!boot_cpu_has_bug(X86_BUG_MDS) || cpu_mitigations_off()) {
+                mds_mitigation = MDS_MITIGATION_OFF;
+                return;
+        }
+
+        if (mds_mitigation == MDS_MITIGATION_FULL) {
+                if (!boot_cpu_has(X86_FEATURE_MD_CLEAR))
+                        mds_mitigation = MDS_MITIGATION_VMWERV;
+
+                static_branch_enable(&mds_user_clear);
+
+                if (!boot_cpu_has(X86_BUG_MSBDS_ONLY) &&
+                    (mds_nosmt || cpu_mitigations_auto_nosmt()))
+                        cpu_smt_disable(false);
+        }
+
+        pr_info("%s\n", mds_strings[mds_mitigation]);
+}
+
+static int __init mds_cmdline(char *str)
+{
+        if (!boot_cpu_has_bug(X86_BUG_MDS))
+                return 0;
+
+        if (!str)
+                return -EINVAL;
+
+        if (!strcmp(str, "off"))
+                mds_mitigation = MDS_MITIGATION_OFF;
+        else if (!strcmp(str, "full"))
+                mds_mitigation = MDS_MITIGATION_FULL;
+        else if (!strcmp(str, "full,nosmt")) {
+                mds_mitigation = MDS_MITIGATION_FULL;
+                mds_nosmt = true;
+        }
+
+        return 0;
+}
+early_param("mds", mds_cmdline);
+
+#undef pr_fmt
 #define pr_fmt(fmt)     "Spectre V2 : " fmt
 
 static enum spectre_v2_mitigation spectre_v2_enabled __ro_after_init =
@@ -575,9 +642,6 @@ specv2_set_mode:
 
         /* Set up IBPB and STIBP depending on the general spectre V2 command */
         spectre_v2_user_select_mitigation(cmd);
-
-        /* Enable STIBP if appropriate */
-        arch_smt_update();
 }
 
 static void update_stibp_msr(void * __unused)
@@ -611,6 +675,31 @@ static void update_indir_branch_cond(void)
                 static_branch_disable(&switch_to_cond_stibp);
 }
 
+#undef pr_fmt
+#define pr_fmt(fmt) fmt
+
+/* Update the static key controlling the MDS CPU buffer clear in idle */
+static void update_mds_branch_idle(void)
+{
+        /*
+         * Enable the idle clearing if SMT is active on CPUs which are
+         * affected only by MSBDS and not any other MDS variant.
+         *
+         * The other variants cannot be mitigated when SMT is enabled, so
+         * clearing the buffers on idle just to prevent the Store Buffer
+         * repartitioning leak would be a window dressing exercise.
+         */
+        if (!boot_cpu_has_bug(X86_BUG_MSBDS_ONLY))
+                return;
+
+        if (sched_smt_active())
+                static_branch_enable(&mds_idle_clear);
+        else
+                static_branch_disable(&mds_idle_clear);
+}
+
+#define MDS_MSG_SMT "MDS CPU bug present and SMT on, data leak possible. See https://www.kernel.org/doc/html/latest/admin-guide/hw-vuln/mds.html for more details.\n"
+
 void arch_smt_update(void)
 {
         /* Enhanced IBRS implies STIBP. No update required. */
@@ -632,6 +721,17 @@ void arch_smt_update(void)
                 break;
         }
 
+        switch (mds_mitigation) {
+        case MDS_MITIGATION_FULL:
+        case MDS_MITIGATION_VMWERV:
+                if (sched_smt_active() && !boot_cpu_has(X86_BUG_MSBDS_ONLY))
+                        pr_warn_once(MDS_MSG_SMT);
+                update_mds_branch_idle();
+                break;
+        case MDS_MITIGATION_OFF:
+                break;
+        }
+
         mutex_unlock(&spec_ctrl_mutex);
 }
 
@@ -1043,7 +1143,7 @@ static void __init l1tf_select_mitigation(void)
                 pr_info("You may make it effective by booting the kernel with mem=%llu parameter.\n",
                                 half_pa);
                 pr_info("However, doing so will make a part of your RAM unusable.\n");
-                pr_info("Reading https://www.kernel.org/doc/html/latest/admin-guide/l1tf.html might help you decide.\n");
+                pr_info("Reading https://www.kernel.org/doc/html/latest/admin-guide/hw-vuln/l1tf.html might help you decide.\n");
                 return;
         }
 
@@ -1076,6 +1176,7 @@ static int __init l1tf_cmdline(char *str)
 early_param("l1tf", l1tf_cmdline);
 
 #undef pr_fmt
+#define pr_fmt(fmt) fmt
 
 #ifdef CONFIG_SYSFS
 
@@ -1114,6 +1215,23 @@ static ssize_t l1tf_show_state(char *buf)
 }
 #endif
 
+static ssize_t mds_show_state(char *buf)
+{
+        if (!hypervisor_is_type(X86_HYPER_NATIVE)) {
+                return sprintf(buf, "%s; SMT Host state unknown\n",
+                               mds_strings[mds_mitigation]);
+        }
+
+        if (boot_cpu_has(X86_BUG_MSBDS_ONLY)) {
+                return sprintf(buf, "%s; SMT %s\n", mds_strings[mds_mitigation],
+                               (mds_mitigation == MDS_MITIGATION_OFF ? "vulnerable" :
+                                sched_smt_active() ? "mitigated" : "disabled"));
+        }
+
+        return sprintf(buf, "%s; SMT %s\n", mds_strings[mds_mitigation],
+                       sched_smt_active() ? "vulnerable" : "disabled");
+}
+
 static char *stibp_state(void)
 {
         if (spectre_v2_enabled == SPECTRE_V2_IBRS_ENHANCED)
@@ -1180,6 +1298,10 @@ static ssize_t cpu_show_common(struct device *dev, struct device_attribute *attr
                 if (boot_cpu_has(X86_FEATURE_L1TF_PTEINV))
                         return l1tf_show_state(buf);
                 break;
+
+        case X86_BUG_MDS:
+                return mds_show_state(buf);
+
         default:
                 break;
         }
@@ -1211,4 +1333,9 @@ ssize_t cpu_show_l1tf(struct device *dev, struct device_attribute *attr, char *b
 {
         return cpu_show_common(dev, attr, buf, X86_BUG_L1TF);
 }
+
+ssize_t cpu_show_mds(struct device *dev, struct device_attribute *attr, char *buf)
+{
+        return cpu_show_common(dev, attr, buf, X86_BUG_MDS);
+}
 #endif
diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
index 8739bdfe9bdf..d7f55ad2dfb1 100644
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -940,61 +940,77 @@ static void identify_cpu_without_cpuid(struct cpuinfo_x86 *c)
 #endif
 }
 
-static const __initconst struct x86_cpu_id cpu_no_speculation[] = {
-        { X86_VENDOR_INTEL,     6, INTEL_FAM6_ATOM_SALTWELL,    X86_FEATURE_ANY },
-        { X86_VENDOR_INTEL,     6, INTEL_FAM6_ATOM_SALTWELL_TABLET,     X86_FEATURE_ANY },
-        { X86_VENDOR_INTEL,     6, INTEL_FAM6_ATOM_BONNELL_MID, X86_FEATURE_ANY },
-        { X86_VENDOR_INTEL,     6, INTEL_FAM6_ATOM_SALTWELL_MID,        X86_FEATURE_ANY },
-        { X86_VENDOR_INTEL,     6, INTEL_FAM6_ATOM_BONNELL,     X86_FEATURE_ANY },
-        { X86_VENDOR_CENTAUR,   5 },
-        { X86_VENDOR_INTEL,     5 },
-        { X86_VENDOR_NSC,       5 },
-        { X86_VENDOR_ANY,       4 },
+#define NO_SPECULATION  BIT(0)
+#define NO_MELTDOWN     BIT(1)
+#define NO_SSB          BIT(2)
+#define NO_L1TF         BIT(3)
+#define NO_MDS          BIT(4)
+#define MSBDS_ONLY      BIT(5)
+
+#define VULNWL(_vendor, _family, _model, _whitelist)    \
+        { X86_VENDOR_##_vendor, _family, _model, X86_FEATURE_ANY, _whitelist }
+
+#define VULNWL_INTEL(model, whitelist)          \
+        VULNWL(INTEL, 6, INTEL_FAM6_##model, whitelist)
+
+#define VULNWL_AMD(family, whitelist)           \
+        VULNWL(AMD, family, X86_MODEL_ANY, whitelist)
+
+#define VULNWL_HYGON(family, whitelist)         \
+        VULNWL(HYGON, family, X86_MODEL_ANY, whitelist)
+
+static const __initconst struct x86_cpu_id cpu_vuln_whitelist[] = {
+        VULNWL(ANY,     4, X86_MODEL_ANY,       NO_SPECULATION),
+        VULNWL(CENTAUR, 5, X86_MODEL_ANY,       NO_SPECULATION),
+        VULNWL(INTEL,   5, X86_MODEL_ANY,       NO_SPECULATION),
+        VULNWL(NSC,     5, X86_MODEL_ANY,       NO_SPECULATION),
+
+        /* Intel Family 6 */
+        VULNWL_INTEL(ATOM_SALTWELL,             NO_SPECULATION),
+        VULNWL_INTEL(ATOM_SALTWELL_TABLET,      NO_SPECULATION),
+        VULNWL_INTEL(ATOM_SALTWELL_MID,         NO_SPECULATION),
+        VULNWL_INTEL(ATOM_BONNELL,              NO_SPECULATION),
+        VULNWL_INTEL(ATOM_BONNELL_MID,          NO_SPECULATION),
+
+        VULNWL_INTEL(ATOM_SILVERMONT,           NO_SSB | NO_L1TF | MSBDS_ONLY),
+        VULNWL_INTEL(ATOM_SILVERMONT_X,         NO_SSB | NO_L1TF | MSBDS_ONLY),
+        VULNWL_INTEL(ATOM_SILVERMONT_MID,       NO_SSB | NO_L1TF | MSBDS_ONLY),
+        VULNWL_INTEL(ATOM_AIRMONT,              NO_SSB | NO_L1TF | MSBDS_ONLY),
+        VULNWL_INTEL(XEON_PHI_KNL,              NO_SSB | NO_L1TF | MSBDS_ONLY),
+        VULNWL_INTEL(XEON_PHI_KNM,              NO_SSB | NO_L1TF | MSBDS_ONLY),
+
+        VULNWL_INTEL(CORE_YONAH,                NO_SSB),
+
+        VULNWL_INTEL(ATOM_AIRMONT_MID,          NO_L1TF | MSBDS_ONLY),
+
+        VULNWL_INTEL(ATOM_GOLDMONT,             NO_MDS | NO_L1TF),
+        VULNWL_INTEL(ATOM_GOLDMONT_X,           NO_MDS | NO_L1TF),
+        VULNWL_INTEL(ATOM_GOLDMONT_PLUS,        NO_MDS | NO_L1TF),
+
+        /* AMD Family 0xf - 0x12 */
+        VULNWL_AMD(0x0f,        NO_MELTDOWN | NO_SSB | NO_L1TF | NO_MDS),
+        VULNWL_AMD(0x10,        NO_MELTDOWN | NO_SSB | NO_L1TF | NO_MDS),
+        VULNWL_AMD(0x11,        NO_MELTDOWN | NO_SSB | NO_L1TF | NO_MDS),
+        VULNWL_AMD(0x12,        NO_MELTDOWN | NO_SSB | NO_L1TF | NO_MDS),
+
+        /* FAMILY_ANY must be last, otherwise 0x0f - 0x12 matches won't work */
+        VULNWL_AMD(X86_FAMILY_ANY,      NO_MELTDOWN | NO_L1TF | NO_MDS),
+        VULNWL_HYGON(X86_FAMILY_ANY,    NO_MELTDOWN | NO_L1TF | NO_MDS),
         {}
 };
 
-static const __initconst struct x86_cpu_id cpu_no_meltdown[] = {
-        { X86_VENDOR_AMD },
-        { X86_VENDOR_HYGON },
-        {}
-};
-
-/* Only list CPUs which speculate but are non susceptible to SSB */
-static const __initconst struct x86_cpu_id cpu_no_spec_store_bypass[] = {
-        { X86_VENDOR_INTEL,     6,      INTEL_FAM6_ATOM_SILVERMONT      },
-        { X86_VENDOR_INTEL,     6,      INTEL_FAM6_ATOM_AIRMONT         },
-        { X86_VENDOR_INTEL,     6,      INTEL_FAM6_ATOM_SILVERMONT_X    },
-        { X86_VENDOR_INTEL,     6,      INTEL_FAM6_ATOM_SILVERMONT_MID  },
-        { X86_VENDOR_INTEL,     6,      INTEL_FAM6_CORE_YONAH           },
-        { X86_VENDOR_INTEL,     6,      INTEL_FAM6_XEON_PHI_KNL         },
-        { X86_VENDOR_INTEL,     6,      INTEL_FAM6_XEON_PHI_KNM         },
-        { X86_VENDOR_AMD,       0x12,                                   },
-        { X86_VENDOR_AMD,       0x11,                                   },
-        { X86_VENDOR_AMD,       0x10,                                   },
-        { X86_VENDOR_AMD,       0xf,                                    },
-        {}
-};
+static bool __init cpu_matches(unsigned long which)
+{
+        const struct x86_cpu_id *m = x86_match_cpu(cpu_vuln_whitelist);
 
-static const __initconst struct x86_cpu_id cpu_no_l1tf[] = {
-        /* in addition to cpu_no_speculation */
-        { X86_VENDOR_INTEL,     6,      INTEL_FAM6_ATOM_SILVERMONT      },
-        { X86_VENDOR_INTEL,     6,      INTEL_FAM6_ATOM_SILVERMONT_X    },
-        { X86_VENDOR_INTEL,     6,      INTEL_FAM6_ATOM_AIRMONT         },
-        { X86_VENDOR_INTEL,     6,      INTEL_FAM6_ATOM_SILVERMONT_MID  },
-        { X86_VENDOR_INTEL,     6,      INTEL_FAM6_ATOM_AIRMONT_MID     },
-        { X86_VENDOR_INTEL,     6,      INTEL_FAM6_ATOM_GOLDMONT        },
-        { X86_VENDOR_INTEL,     6,      INTEL_FAM6_ATOM_GOLDMONT_X      },
-        { X86_VENDOR_INTEL,     6,      INTEL_FAM6_ATOM_GOLDMONT_PLUS   },
-        { X86_VENDOR_INTEL,     6,      INTEL_FAM6_XEON_PHI_KNL         },
-        { X86_VENDOR_INTEL,     6,      INTEL_FAM6_XEON_PHI_KNM         },
-        {}
-};
+        return m && !!(m->driver_data & which);
+}
 
 static void __init cpu_set_bug_bits(struct cpuinfo_x86 *c)
 {
         u64 ia32_cap = 0;
 
-        if (x86_match_cpu(cpu_no_speculation))
+        if (cpu_matches(NO_SPECULATION))
                 return;
 
         setup_force_cpu_bug(X86_BUG_SPECTRE_V1);
@@ -1003,15 +1019,20 @@ static void __init cpu_set_bug_bits(struct cpuinfo_x86 *c)
         if (cpu_has(c, X86_FEATURE_ARCH_CAPABILITIES))
                 rdmsrl(MSR_IA32_ARCH_CAPABILITIES, ia32_cap);
 
-        if (!x86_match_cpu(cpu_no_spec_store_bypass) &&
-           !(ia32_cap & ARCH_CAP_SSB_NO) &&
+        if (!cpu_matches(NO_SSB) && !(ia32_cap & ARCH_CAP_SSB_NO) &&
            !cpu_has(c, X86_FEATURE_AMD_SSB_NO))
                 setup_force_cpu_bug(X86_BUG_SPEC_STORE_BYPASS);
 
         if (ia32_cap & ARCH_CAP_IBRS_ALL)
                 setup_force_cpu_cap(X86_FEATURE_IBRS_ENHANCED);
 
-        if (x86_match_cpu(cpu_no_meltdown))
+        if (!cpu_matches(NO_MDS) && !(ia32_cap & ARCH_CAP_MDS_NO)) {
+                setup_force_cpu_bug(X86_BUG_MDS);
+                if (cpu_matches(MSBDS_ONLY))
+                        setup_force_cpu_bug(X86_BUG_MSBDS_ONLY);
+        }
+
+        if (cpu_matches(NO_MELTDOWN))
                 return;
 
         /* Rogue Data Cache Load? No! */
@@ -1020,7 +1041,7 @@ static void __init cpu_set_bug_bits(struct cpuinfo_x86 *c)
 
         setup_force_cpu_bug(X86_BUG_CPU_MELTDOWN);
 
-        if (x86_match_cpu(cpu_no_l1tf))
+        if (cpu_matches(NO_L1TF))
                 return;
 
         setup_force_cpu_bug(X86_BUG_L1TF);
diff --git a/arch/x86/kernel/nmi.c b/arch/x86/kernel/nmi.c
index 3755d0310026..05b09896cfaf 100644
--- a/arch/x86/kernel/nmi.c
+++ b/arch/x86/kernel/nmi.c
@@ -35,6 +35,7 @@
 #include <asm/x86_init.h>
 #include <asm/reboot.h>
 #include <asm/cache.h>
+#include <asm/nospec-branch.h>
 
 #define CREATE_TRACE_POINTS
 #include <trace/events/nmi.h>
@@ -551,6 +552,9 @@ nmi_restart:
                 write_cr2(this_cpu_read(nmi_cr2));
         if (this_cpu_dec_return(nmi_state))
                 goto nmi_restart;
+
+        if (user_mode(regs))
+                mds_user_clear_cpu_buffers();
 }
 NOKPROBE_SYMBOL(do_nmi);
 
diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
index 8b6d03e55d2f..7de466eb960b 100644
--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -58,6 +58,7 @@
 #include <asm/alternative.h>
 #include <asm/fpu/xstate.h>
 #include <asm/trace/mpx.h>
+#include <asm/nospec-branch.h>
 #include <asm/mpx.h>
 #include <asm/vm86.h>
 #include <asm/umip.h>
@@ -367,6 +368,13 @@ dotraplinkage void do_double_fault(struct pt_regs *regs, long error_code)
                 regs->ip = (unsigned long)general_protection;
                 regs->sp = (unsigned long)&gpregs->orig_ax;
 
+                /*
+                 * This situation can be triggered by userspace via
+                 * modify_ldt(2) and the return does not take the regular
+                 * user space exit, so a CPU buffer clear is required when
+                 * MDS mitigation is enabled.
+                 */
+                mds_user_clear_cpu_buffers();
                 return;
         }
 #endif
diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
index fd3951638ae4..bbbe611f0c49 100644
--- a/arch/x86/kvm/cpuid.c
+++ b/arch/x86/kvm/cpuid.c
@@ -410,7 +410,8 @@ static inline int __do_cpuid_ent(struct kvm_cpuid_entry2 *entry, u32 function,
         /* cpuid 7.0.edx*/
         const u32 kvm_cpuid_7_0_edx_x86_features =
                 F(AVX512_4VNNIW) | F(AVX512_4FMAPS) | F(SPEC_CTRL) |
-                F(SPEC_CTRL_SSBD) | F(ARCH_CAPABILITIES) | F(INTEL_STIBP);
+                F(SPEC_CTRL_SSBD) | F(ARCH_CAPABILITIES) | F(INTEL_STIBP) |
+                F(MD_CLEAR);
 
         /* all calls to cpuid_count() should be made on the same cpu */
         get_cpu();
diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index 9663d41cc2bc..e1fa935a545f 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -6431,8 +6431,11 @@ static void vmx_vcpu_run(struct kvm_vcpu *vcpu)
          */
         x86_spec_ctrl_set_guest(vmx->spec_ctrl, 0);
 
+        /* L1D Flush includes CPU buffer clear to mitigate MDS */
         if (static_branch_unlikely(&vmx_l1d_should_flush))
                 vmx_l1d_flush(vcpu);
+        else if (static_branch_unlikely(&mds_user_clear))
+                mds_clear_cpu_buffers();
 
         if (vcpu->arch.cr2 != read_cr2())
                 write_cr2(vcpu->arch.cr2);
@@ -6668,8 +6671,8 @@ free_partial_vcpu:
         return ERR_PTR(err);
 }
 
-#define L1TF_MSG_SMT "L1TF CPU bug present and SMT on, data leak possible. See CVE-2018-3646 and https://www.kernel.org/doc/html/latest/admin-guide/l1tf.html for details.\n"
-#define L1TF_MSG_L1D "L1TF CPU bug present and virtualization mitigation disabled, data leak possible. See CVE-2018-3646 and https://www.kernel.org/doc/html/latest/admin-guide/l1tf.html for details.\n"
+#define L1TF_MSG_SMT "L1TF CPU bug present and SMT on, data leak possible. See CVE-2018-3646 and https://www.kernel.org/doc/html/latest/admin-guide/hw-vuln/l1tf.html for details.\n"
+#define L1TF_MSG_L1D "L1TF CPU bug present and virtualization mitigation disabled, data leak possible. See CVE-2018-3646 and https://www.kernel.org/doc/html/latest/admin-guide/hw-vuln/l1tf.html for details.\n"
 
 static int vmx_vm_init(struct kvm *kvm)
 {
diff --git a/drivers/base/cpu.c b/drivers/base/cpu.c
index 668139cfa664..cc37511de866 100644
--- a/drivers/base/cpu.c
+++ b/drivers/base/cpu.c
@@ -548,11 +548,18 @@ ssize_t __weak cpu_show_l1tf(struct device *dev,
         return sprintf(buf, "Not affected\n");
 }
 
+ssize_t __weak cpu_show_mds(struct device *dev,
+                            struct device_attribute *attr, char *buf)
+{
+        return sprintf(buf, "Not affected\n");
+}
+
 static DEVICE_ATTR(meltdown, 0444, cpu_show_meltdown, NULL);
 static DEVICE_ATTR(spectre_v1, 0444, cpu_show_spectre_v1, NULL);
 static DEVICE_ATTR(spectre_v2, 0444, cpu_show_spectre_v2, NULL);
 static DEVICE_ATTR(spec_store_bypass, 0444, cpu_show_spec_store_bypass, NULL);
 static DEVICE_ATTR(l1tf, 0444, cpu_show_l1tf, NULL);
+static DEVICE_ATTR(mds, 0444, cpu_show_mds, NULL);
 
 static struct attribute *cpu_root_vulnerabilities_attrs[] = {
         &dev_attr_meltdown.attr,
@@ -560,6 +567,7 @@ static struct attribute *cpu_root_vulnerabilities_attrs[] = {
         &dev_attr_spectre_v2.attr,
         &dev_attr_spec_store_bypass.attr,
         &dev_attr_l1tf.attr,
+        &dev_attr_mds.attr,
         NULL
 };
 
diff --git a/include/linux/cpu.h b/include/linux/cpu.h
index 732745f865b7..3813fe45effd 100644
--- a/include/linux/cpu.h
+++ b/include/linux/cpu.h
@@ -57,6 +57,8 @@ extern ssize_t cpu_show_spec_store_bypass(struct device *dev,
                                           struct device_attribute *attr, char *buf);
 extern ssize_t cpu_show_l1tf(struct device *dev,
                              struct device_attribute *attr, char *buf);
+extern ssize_t cpu_show_mds(struct device *dev,
+                            struct device_attribute *attr, char *buf);
 
 extern __printf(4, 5)
 struct device *cpu_device_create(struct device *parent, void *drvdata,
diff --git a/tools/power/x86/turbostat/Makefile b/tools/power/x86/turbostat/Makefile
index 1598b4fa0b11..045f5f7d68ab 100644
--- a/tools/power/x86/turbostat/Makefile
+++ b/tools/power/x86/turbostat/Makefile
@@ -9,7 +9,7 @@ ifeq ("$(origin O)", "command line")
 endif
 
 turbostat : turbostat.c
-override CFLAGS +=      -Wall
+override CFLAGS +=      -Wall -I../../../include
 override CFLAGS +=      -DMSRHEADER='"../../../../arch/x86/include/asm/msr-index.h"'
 override CFLAGS +=      -DINTEL_FAMILY_HEADER='"../../../../arch/x86/include/asm/intel-family.h"'
 
diff --git a/tools/power/x86/x86_energy_perf_policy/Makefile b/tools/power/x86/x86_energy_perf_policy/Makefile
index ae7a0e09b722..1fdeef864e7c 100644
--- a/tools/power/x86/x86_energy_perf_policy/Makefile
+++ b/tools/power/x86/x86_energy_perf_policy/Makefile
@@ -9,7 +9,7 @@ ifeq ("$(origin O)", "command line")
 endif
 
 x86_energy_perf_policy : x86_energy_perf_policy.c
-override CFLAGS +=      -Wall
+override CFLAGS +=      -Wall -I../../../include
 override CFLAGS +=      -DMSRHEADER='"../../../../arch/x86/include/asm/msr-index.h"'
 
 %: %.c