doc: ReSTify seccomp_filter.txt

This updates seccomp_filter.txt for ReST markup, and moves it under the
user-space API index, since it describes how application author can use
seccomp.

Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

2 files changed, 230 insertions, 0 deletions

diff --git a/Documentation/userspace-api/index.rst b/Documentation/userspace-api/index.rst
index a9d01b44a659..15ff12342db8 100644
--- a/Documentation/userspace-api/index.rst
+++ b/Documentation/userspace-api/index.rst
@@ -16,6 +16,7 @@ place where this information is gathered.
 .. toctree::
    :maxdepth: 2
 
+   seccomp_filter
    unshare
 
 .. only::  subproject and html
diff --git a/Documentation/userspace-api/seccomp_filter.rst b/Documentation/userspace-api/seccomp_filter.rst
new file mode 100644
index 000000000000..f71eb5ef1f2d
--- /dev/null
+++ b/Documentation/userspace-api/seccomp_filter.rst
@@ -0,0 +1,229 @@
+===========================================
+Seccomp BPF (SECure COMPuting with filters)
+===========================================
+
+Introduction
+============
+
+A large number of system calls are exposed to every userland process
+with many of them going unused for the entire lifetime of the process.
+As system calls change and mature, bugs are found and eradicated.  A
+certain subset of userland applications benefit by having a reduced set
+of available system calls.  The resulting set reduces the total kernel
+surface exposed to the application.  System call filtering is meant for
+use with those applications.
+
+Seccomp filtering provides a means for a process to specify a filter for
+incoming system calls.  The filter is expressed as a Berkeley Packet
+Filter (BPF) program, as with socket filters, except that the data
+operated on is related to the system call being made: system call
+number and the system call arguments.  This allows for expressive
+filtering of system calls using a filter program language with a long
+history of being exposed to userland and a straightforward data set.
+
+Additionally, BPF makes it impossible for users of seccomp to fall prey
+to time-of-check-time-of-use (TOCTOU) attacks that are common in system
+call interposition frameworks.  BPF programs may not dereference
+pointers which constrains all filters to solely evaluating the system
+call arguments directly.
+
+What it isn't
+=============
+
+System call filtering isn't a sandbox.  It provides a clearly defined
+mechanism for minimizing the exposed kernel surface.  It is meant to be
+a tool for sandbox developers to use.  Beyond that, policy for logical
+behavior and information flow should be managed with a combination of
+other system hardening techniques and, potentially, an LSM of your
+choosing.  Expressive, dynamic filters provide further options down this
+path (avoiding pathological sizes or selecting which of the multiplexed
+system calls in socketcall() is allowed, for instance) which could be
+construed, incorrectly, as a more complete sandboxing solution.
+
+Usage
+=====
+
+An additional seccomp mode is added and is enabled using the same
+prctl(2) call as the strict seccomp.  If the architecture has
+``CONFIG_HAVE_ARCH_SECCOMP_FILTER``, then filters may be added as below:
+
+``PR_SET_SECCOMP``:
+        Now takes an additional argument which specifies a new filter
+        using a BPF program.
+        The BPF program will be executed over struct seccomp_data
+        reflecting the system call number, arguments, and other
+        metadata.  The BPF program must then return one of the
+        acceptable values to inform the kernel which action should be
+        taken.
+
+        Usage::
+
+                prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, prog);
+
+        The 'prog' argument is a pointer to a struct sock_fprog which
+        will contain the filter program.  If the program is invalid, the
+        call will return -1 and set errno to ``EINVAL``.
+
+        If ``fork``/``clone`` and ``execve`` are allowed by @prog, any child
+        processes will be constrained to the same filters and system
+        call ABI as the parent.
+
+        Prior to use, the task must call ``prctl(PR_SET_NO_NEW_PRIVS, 1)`` or
+        run with ``CAP_SYS_ADMIN`` privileges in its namespace.  If these are not
+        true, ``-EACCES`` will be returned.  This requirement ensures that filter
+        programs cannot be applied to child processes with greater privileges
+        than the task that installed them.
+
+        Additionally, if ``prctl(2)`` is allowed by the attached filter,
+        additional filters may be layered on which will increase evaluation
+        time, but allow for further decreasing the attack surface during
+        execution of a process.
+
+The above call returns 0 on success and non-zero on error.
+
+Return values
+=============
+
+A seccomp filter may return any of the following values. If multiple
+filters exist, the return value for the evaluation of a given system
+call will always use the highest precedent value. (For example,
+``SECCOMP_RET_KILL`` will always take precedence.)
+
+In precedence order, they are:
+
+``SECCOMP_RET_KILL``:
+        Results in the task exiting immediately without executing the
+        system call.  The exit status of the task (``status & 0x7f``) will
+        be ``SIGSYS``, not ``SIGKILL``.
+
+``SECCOMP_RET_TRAP``:
+        Results in the kernel sending a ``SIGSYS`` signal to the triggering
+        task without executing the system call. ``siginfo->si_call_addr``
+        will show the address of the system call instruction, and
+        ``siginfo->si_syscall`` and ``siginfo->si_arch`` will indicate which
+        syscall was attempted.  The program counter will be as though
+        the syscall happened (i.e. it will not point to the syscall
+        instruction).  The return value register will contain an arch-
+        dependent value -- if resuming execution, set it to something
+        sensible.  (The architecture dependency is because replacing
+        it with ``-ENOSYS`` could overwrite some useful information.)
+
+        The ``SECCOMP_RET_DATA`` portion of the return value will be passed
+        as ``si_errno``.
+
+        ``SIGSYS`` triggered by seccomp will have a si_code of ``SYS_SECCOMP``.
+
+``SECCOMP_RET_ERRNO``:
+        Results in the lower 16-bits of the return value being passed
+        to userland as the errno without executing the system call.
+
+``SECCOMP_RET_TRACE``:
+        When returned, this value will cause the kernel to attempt to
+        notify a ``ptrace()``-based tracer prior to executing the system
+        call.  If there is no tracer present, ``-ENOSYS`` is returned to
+        userland and the system call is not executed.
+
+        A tracer will be notified if it requests ``PTRACE_O_TRACESECCOM``P
+        using ``ptrace(PTRACE_SETOPTIONS)``.  The tracer will be notified
+        of a ``PTRACE_EVENT_SECCOMP`` and the ``SECCOMP_RET_DATA`` portion of
+        the BPF program return value will be available to the tracer
+        via ``PTRACE_GETEVENTMSG``.
+
+        The tracer can skip the system call by changing the syscall number
+        to -1.  Alternatively, the tracer can change the system call
+        requested by changing the system call to a valid syscall number.  If
+        the tracer asks to skip the system call, then the system call will
+        appear to return the value that the tracer puts in the return value
+        register.
+
+        The seccomp check will not be run again after the tracer is
+        notified.  (This means that seccomp-based sandboxes MUST NOT
+        allow use of ptrace, even of other sandboxed processes, without
+        extreme care; ptracers can use this mechanism to escape.)
+
+``SECCOMP_RET_ALLOW``:
+        Results in the system call being executed.
+
+If multiple filters exist, the return value for the evaluation of a
+given system call will always use the highest precedent value.
+
+Precedence is only determined using the ``SECCOMP_RET_ACTION`` mask.  When
+multiple filters return values of the same precedence, only the
+``SECCOMP_RET_DATA`` from the most recently installed filter will be
+returned.
+
+Pitfalls
+========
+
+The biggest pitfall to avoid during use is filtering on system call
+number without checking the architecture value.  Why?  On any
+architecture that supports multiple system call invocation conventions,
+the system call numbers may vary based on the specific invocation.  If
+the numbers in the different calling conventions overlap, then checks in
+the filters may be abused.  Always check the arch value!
+
+Example
+=======
+
+The ``samples/seccomp/`` directory contains both an x86-specific example
+and a more generic example of a higher level macro interface for BPF
+program generation.
+
+
+
+Adding architecture support
+===========================
+
+See ``arch/Kconfig`` for the authoritative requirements.  In general, if an
+architecture supports both ptrace_event and seccomp, it will be able to
+support seccomp filter with minor fixup: ``SIGSYS`` support and seccomp return
+value checking.  Then it must just add ``CONFIG_HAVE_ARCH_SECCOMP_FILTER``
+to its arch-specific Kconfig.
+
+
+
+Caveats
+=======
+
+The vDSO can cause some system calls to run entirely in userspace,
+leading to surprises when you run programs on different machines that
+fall back to real syscalls.  To minimize these surprises on x86, make
+sure you test with
+``/sys/devices/system/clocksource/clocksource0/current_clocksource`` set to
+something like ``acpi_pm``.
+
+On x86-64, vsyscall emulation is enabled by default.  (vsyscalls are
+legacy variants on vDSO calls.)  Currently, emulated vsyscalls will
+honor seccomp, with a few oddities:
+
+- A return value of ``SECCOMP_RET_TRAP`` will set a ``si_call_addr`` pointing to
+  the vsyscall entry for the given call and not the address after the
+  'syscall' instruction.  Any code which wants to restart the call
+  should be aware that (a) a ret instruction has been emulated and (b)
+  trying to resume the syscall will again trigger the standard vsyscall
+  emulation security checks, making resuming the syscall mostly
+  pointless.
+
+- A return value of ``SECCOMP_RET_TRACE`` will signal the tracer as usual,
+  but the syscall may not be changed to another system call using the
+  orig_rax register. It may only be changed to -1 order to skip the
+  currently emulated call. Any other change MAY terminate the process.
+  The rip value seen by the tracer will be the syscall entry address;
+  this is different from normal behavior.  The tracer MUST NOT modify
+  rip or rsp.  (Do not rely on other changes terminating the process.
+  They might work.  For example, on some kernels, choosing a syscall
+  that only exists in future kernels will be correctly emulated (by
+  returning ``-ENOSYS``).
+
+To detect this quirky behavior, check for ``addr & ~0x0C00 ==
+0xFFFFFFFFFF600000``.  (For ``SECCOMP_RET_TRACE``, use rip.  For
+``SECCOMP_RET_TRAP``, use ``siginfo->si_call_addr``.)  Do not check any other
+condition: future kernels may improve vsyscall emulation and current
+kernels in vsyscall=native mode will behave differently, but the
+instructions at ``0xF...F600{0,4,8,C}00`` will not be system calls in these
+cases.
+
+Note that modern systems are unlikely to use vsyscalls at all -- they
+are a legacy feature and they are considerably slower than standard
+syscalls.  New code will use the vDSO, and vDSO-issued system calls
+are indistinguishable from normal system calls.