path: root/Documentation/userspace-api
diff options
authorKees Cook <keescook@chromium.org>2017-05-13 04:51:37 -0700
committerJonathan Corbet <corbet@lwn.net>2017-05-18 10:30:01 -0600
commitc061f33f35be0ccc80f4b8e0aea5dfd2ed7e01a3 (patch)
tree591f8da7a6af2c08d53897af619f7ba369d882de /Documentation/userspace-api
parent5e33994dca0e501336b52d8aec5327a9dec6430f (diff)
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>
Diffstat (limited to 'Documentation/userspace-api')
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
.. 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)
+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.
+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:
+ 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::
+ 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:
+ Results in the task exiting immediately without executing the
+ system call. The exit status of the task (``status & 0x7f``) will
+ be ``SIGSYS``, not ``SIGKILL``.
+ 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``.
+ Results in the lower 16-bits of the return value being passed
+ to userland as the errno without executing the system call.
+ 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
+ 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.)
+ 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
+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!
+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.
+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
+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.

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