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>
This commit is contained in:
Kees Cook
Jonathan Corbet
parent
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@ -16,6 +16,7 @@ place where this information is gathered.
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.. toctree::
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:maxdepth: 2
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seccomp_filter
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unshare
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.. only:: subproject and html
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@ -1,8 +1,9 @@
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SECure COMPuting with filters
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=============================
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===========================================
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Seccomp BPF (SECure COMPuting with filters)
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===========================================
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Introduction
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------------
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============
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A large number of system calls are exposed to every userland process
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with many of them going unused for the entire lifetime of the process.
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@ -27,7 +28,7 @@ pointers which constrains all filters to solely evaluating the system
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call arguments directly.
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What it isn't
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-------------
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=============
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System call filtering isn't a sandbox. It provides a clearly defined
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mechanism for minimizing the exposed kernel surface. It is meant to be
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@ -40,13 +41,13 @@ system calls in socketcall() is allowed, for instance) which could be
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construed, incorrectly, as a more complete sandboxing solution.
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Usage
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-----
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=====
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An additional seccomp mode is added and is enabled using the same
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prctl(2) call as the strict seccomp. If the architecture has
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CONFIG_HAVE_ARCH_SECCOMP_FILTER, then filters may be added as below:
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``CONFIG_HAVE_ARCH_SECCOMP_FILTER``, then filters may be added as below:
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PR_SET_SECCOMP:
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``PR_SET_SECCOMP``:
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Now takes an additional argument which specifies a new filter
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using a BPF program.
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The BPF program will be executed over struct seccomp_data
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@ -55,24 +56,25 @@ PR_SET_SECCOMP:
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acceptable values to inform the kernel which action should be
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taken.
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Usage:
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Usage::
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prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, prog);
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The 'prog' argument is a pointer to a struct sock_fprog which
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will contain the filter program. If the program is invalid, the
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call will return -1 and set errno to EINVAL.
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call will return -1 and set errno to ``EINVAL``.
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If fork/clone and execve are allowed by @prog, any child
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If ``fork``/``clone`` and ``execve`` are allowed by @prog, any child
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processes will be constrained to the same filters and system
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call ABI as the parent.
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Prior to use, the task must call prctl(PR_SET_NO_NEW_PRIVS, 1) or
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run with CAP_SYS_ADMIN privileges in its namespace. If these are not
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true, -EACCES will be returned. This requirement ensures that filter
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Prior to use, the task must call ``prctl(PR_SET_NO_NEW_PRIVS, 1)`` or
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run with ``CAP_SYS_ADMIN`` privileges in its namespace. If these are not
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true, ``-EACCES`` will be returned. This requirement ensures that filter
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programs cannot be applied to child processes with greater privileges
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than the task that installed them.
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Additionally, if prctl(2) is allowed by the attached filter,
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Additionally, if ``prctl(2)`` is allowed by the attached filter,
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additional filters may be layered on which will increase evaluation
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time, but allow for further decreasing the attack surface during
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execution of a process.
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@ -80,51 +82,52 @@ PR_SET_SECCOMP:
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The above call returns 0 on success and non-zero on error.
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Return values
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-------------
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=============
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A seccomp filter may return any of the following values. If multiple
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filters exist, the return value for the evaluation of a given system
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call will always use the highest precedent value. (For example,
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SECCOMP_RET_KILL will always take precedence.)
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``SECCOMP_RET_KILL`` will always take precedence.)
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In precedence order, they are:
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SECCOMP_RET_KILL:
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``SECCOMP_RET_KILL``:
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Results in the task exiting immediately without executing the
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system call. The exit status of the task (status & 0x7f) will
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be SIGSYS, not SIGKILL.
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system call. The exit status of the task (``status & 0x7f``) will
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be ``SIGSYS``, not ``SIGKILL``.
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SECCOMP_RET_TRAP:
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Results in the kernel sending a SIGSYS signal to the triggering
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task without executing the system call. siginfo->si_call_addr
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``SECCOMP_RET_TRAP``:
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Results in the kernel sending a ``SIGSYS`` signal to the triggering
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task without executing the system call. ``siginfo->si_call_addr``
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will show the address of the system call instruction, and
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siginfo->si_syscall and siginfo->si_arch will indicate which
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``siginfo->si_syscall`` and ``siginfo->si_arch`` will indicate which
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syscall was attempted. The program counter will be as though
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the syscall happened (i.e. it will not point to the syscall
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instruction). The return value register will contain an arch-
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dependent value -- if resuming execution, set it to something
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sensible. (The architecture dependency is because replacing
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it with -ENOSYS could overwrite some useful information.)
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it with ``-ENOSYS`` could overwrite some useful information.)
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The SECCOMP_RET_DATA portion of the return value will be passed
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as si_errno.
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The ``SECCOMP_RET_DATA`` portion of the return value will be passed
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as ``si_errno``.
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SIGSYS triggered by seccomp will have a si_code of SYS_SECCOMP.
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``SIGSYS`` triggered by seccomp will have a si_code of ``SYS_SECCOMP``.
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SECCOMP_RET_ERRNO:
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``SECCOMP_RET_ERRNO``:
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Results in the lower 16-bits of the return value being passed
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to userland as the errno without executing the system call.
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SECCOMP_RET_TRACE:
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``SECCOMP_RET_TRACE``:
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When returned, this value will cause the kernel to attempt to
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notify a ptrace()-based tracer prior to executing the system
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call. If there is no tracer present, -ENOSYS is returned to
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notify a ``ptrace()``-based tracer prior to executing the system
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call. If there is no tracer present, ``-ENOSYS`` is returned to
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userland and the system call is not executed.
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A tracer will be notified if it requests PTRACE_O_TRACESECCOMP
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using ptrace(PTRACE_SETOPTIONS). The tracer will be notified
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of a PTRACE_EVENT_SECCOMP and the SECCOMP_RET_DATA portion of
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A tracer will be notified if it requests ``PTRACE_O_TRACESECCOM``P
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using ``ptrace(PTRACE_SETOPTIONS)``. The tracer will be notified
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of a ``PTRACE_EVENT_SECCOMP`` and the ``SECCOMP_RET_DATA`` portion of
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the BPF program return value will be available to the tracer
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via PTRACE_GETEVENTMSG.
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via ``PTRACE_GETEVENTMSG``.
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The tracer can skip the system call by changing the syscall number
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to -1. Alternatively, the tracer can change the system call
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@ -138,19 +141,19 @@ SECCOMP_RET_TRACE:
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allow use of ptrace, even of other sandboxed processes, without
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extreme care; ptracers can use this mechanism to escape.)
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SECCOMP_RET_ALLOW:
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``SECCOMP_RET_ALLOW``:
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Results in the system call being executed.
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If multiple filters exist, the return value for the evaluation of a
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given system call will always use the highest precedent value.
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Precedence is only determined using the SECCOMP_RET_ACTION mask. When
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Precedence is only determined using the ``SECCOMP_RET_ACTION`` mask. When
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multiple filters return values of the same precedence, only the
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SECCOMP_RET_DATA from the most recently installed filter will be
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``SECCOMP_RET_DATA`` from the most recently installed filter will be
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returned.
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Pitfalls
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--------
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========
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The biggest pitfall to avoid during use is filtering on system call
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number without checking the architecture value. Why? On any
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@ -160,39 +163,40 @@ the numbers in the different calling conventions overlap, then checks in
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the filters may be abused. Always check the arch value!
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Example
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-------
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=======
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The samples/seccomp/ directory contains both an x86-specific example
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The ``samples/seccomp/`` directory contains both an x86-specific example
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and a more generic example of a higher level macro interface for BPF
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program generation.
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Adding architecture support
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-----------------------
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===========================
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See arch/Kconfig for the authoritative requirements. In general, if an
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See ``arch/Kconfig`` for the authoritative requirements. In general, if an
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architecture supports both ptrace_event and seccomp, it will be able to
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support seccomp filter with minor fixup: SIGSYS support and seccomp return
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value checking. Then it must just add CONFIG_HAVE_ARCH_SECCOMP_FILTER
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support seccomp filter with minor fixup: ``SIGSYS`` support and seccomp return
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value checking. Then it must just add ``CONFIG_HAVE_ARCH_SECCOMP_FILTER``
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to its arch-specific Kconfig.
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Caveats
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-------
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=======
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The vDSO can cause some system calls to run entirely in userspace,
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leading to surprises when you run programs on different machines that
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fall back to real syscalls. To minimize these surprises on x86, make
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sure you test with
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/sys/devices/system/clocksource/clocksource0/current_clocksource set to
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something like acpi_pm.
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``/sys/devices/system/clocksource/clocksource0/current_clocksource`` set to
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something like ``acpi_pm``.
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On x86-64, vsyscall emulation is enabled by default. (vsyscalls are
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legacy variants on vDSO calls.) Currently, emulated vsyscalls will honor seccomp, with a few oddities:
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legacy variants on vDSO calls.) Currently, emulated vsyscalls will
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honor seccomp, with a few oddities:
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- A return value of SECCOMP_RET_TRAP will set a si_call_addr pointing to
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- A return value of ``SECCOMP_RET_TRAP`` will set a ``si_call_addr`` pointing to
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the vsyscall entry for the given call and not the address after the
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'syscall' instruction. Any code which wants to restart the call
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should be aware that (a) a ret instruction has been emulated and (b)
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@ -200,7 +204,7 @@ legacy variants on vDSO calls.) Currently, emulated vsyscalls will honor seccom
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emulation security checks, making resuming the syscall mostly
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pointless.
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- A return value of SECCOMP_RET_TRACE will signal the tracer as usual,
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- A return value of ``SECCOMP_RET_TRACE`` will signal the tracer as usual,
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but the syscall may not be changed to another system call using the
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orig_rax register. It may only be changed to -1 order to skip the
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currently emulated call. Any other change MAY terminate the process.
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@ -209,14 +213,14 @@ legacy variants on vDSO calls.) Currently, emulated vsyscalls will honor seccom
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rip or rsp. (Do not rely on other changes terminating the process.
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They might work. For example, on some kernels, choosing a syscall
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that only exists in future kernels will be correctly emulated (by
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returning -ENOSYS).
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returning ``-ENOSYS``).
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To detect this quirky behavior, check for addr & ~0x0C00 ==
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0xFFFFFFFFFF600000. (For SECCOMP_RET_TRACE, use rip. For
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SECCOMP_RET_TRAP, use siginfo->si_call_addr.) Do not check any other
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To detect this quirky behavior, check for ``addr & ~0x0C00 ==
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0xFFFFFFFFFF600000``. (For ``SECCOMP_RET_TRACE``, use rip. For
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``SECCOMP_RET_TRAP``, use ``siginfo->si_call_addr``.) Do not check any other
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condition: future kernels may improve vsyscall emulation and current
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kernels in vsyscall=native mode will behave differently, but the
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instructions at 0xF...F600{0,4,8,C}00 will not be system calls in these
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instructions at ``0xF...F600{0,4,8,C}00`` will not be system calls in these
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cases.
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Note that modern systems are unlikely to use vsyscalls at all -- they
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@ -11492,6 +11492,7 @@ F: kernel/seccomp.c
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F: include/uapi/linux/seccomp.h
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F: include/linux/seccomp.h
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F: tools/testing/selftests/seccomp/*
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F: Documentation/userspace-api/seccomp_filter.rst
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K: \bsecure_computing
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K: \bTIF_SECCOMP\b
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