Documentation: prctl/seccomp_filter

Documents how system call filtering using Berkeley Packet
Filter programs works and how it may be used.
Includes an example for x86 and a semi-generic
example using a macro-based code generator.

Acked-by: Eric Paris <eparis@redhat.com>
Signed-off-by: Will Drewry <wad@chromium.org>
Acked-by: Kees Cook <keescook@chromium.org>

v18: - added acked by
     - update no new privs numbers
v17: - remove @compat note and add Pitfalls section for arch checking
       (keescook@chromium.org)
v16: -
v15: -
v14: - rebase/nochanges
v13: - rebase on to 88ebdda6159ffc15699f204c33feb3e431bf9bdc
v12: - comment on the ptrace_event use
     - update arch support comment
     - note the behavior of SECCOMP_RET_DATA when there are multiple filters
       (keescook@chromium.org)
     - lots of samples/ clean up incl 64-bit bpf-direct support
       (markus@chromium.org)
     - rebase to linux-next
v11: - overhaul return value language, updates (keescook@chromium.org)
     - comment on do_exit(SIGSYS)
v10: - update for SIGSYS
     - update for new seccomp_data layout
     - update for ptrace option use
v9: - updated bpf-direct.c for SIGILL
v8: - add PR_SET_NO_NEW_PRIVS to the samples.
v7: - updated for all the new stuff in v7: TRAP, TRACE
    - only talk about PR_SET_SECCOMP now
    - fixed bad JLE32 check (coreyb@linux.vnet.ibm.com)
    - adds dropper.c: a simple system call disabler
v6: - tweak the language to note the requirement of
      PR_SET_NO_NEW_PRIVS being called prior to use. (luto@mit.edu)
v5: - update sample to use system call arguments
    - adds a "fancy" example using a macro-based generator
    - cleaned up bpf in the sample
    - update docs to mention arguments
    - fix prctl value (eparis@redhat.com)
    - language cleanup (rdunlap@xenotime.net)
v4: - update for no_new_privs use
    - minor tweaks
v3: - call out BPF <-> Berkeley Packet Filter (rdunlap@xenotime.net)
    - document use of tentative always-unprivileged
    - guard sample compilation for i386 and x86_64
v2: - move code to samples (corbet@lwn.net)
Signed-off-by: James Morris <james.l.morris@oracle.com>

1 files changed, 163 insertions, 0 deletions

diff --git a/Documentation/prctl/seccomp_filter.txt b/Documentation/prctl/seccomp_filter.txt
new file mode 100644
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+++ b/Documentation/prctl/seccomp_filter.txt
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+                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.  The kernel will
+        rollback the register state to just before the system call
+        entry such that a signal handler in the task will be able to
+        inspect the ucontext_t->uc_mcontext registers and emulate
+        system call success or failure upon return from the signal
+        handler.
+
+        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_TRACESECCOMP
+        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.
+
+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.