[PATCH 01/17] doc: ReSTify seccomp_filter.txt

From: Kees Cook
Date: Sat May 13 2017 - 07:52:15 EST

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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@xxxxxxxxxxxx>
---
Documentation/userspace-api/index.rst | 1 +
.../seccomp_filter.rst} | 116 +++++++++++----------
MAINTAINERS | 1 +
3 files changed, 62 insertions(+), 56 deletions(-)
rename Documentation/{prctl/seccomp_filter.txt => userspace-api/seccomp_filter.rst} (71%)

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/prctl/seccomp_filter.txt b/Documentation/userspace-api/seccomp_filter.rst
similarity index 71%
rename from Documentation/prctl/seccomp_filter.txt
rename to Documentation/userspace-api/seccomp_filter.rst
index 1e469ef75778..f71eb5ef1f2d 100644
--- a/Documentation/prctl/seccomp_filter.txt
+++ b/Documentation/userspace-api/seccomp_filter.rst
@@ -1,8 +1,9 @@
- SECure COMPuting with filters
- =============================
+===========================================
+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.
@@ -27,7 +28,7 @@ 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
@@ -40,13 +41,13 @@ 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:
+``CONFIG_HAVE_ARCH_SECCOMP_FILTER``, then filters may be added as below:

-PR_SET_SECCOMP:
+``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
@@ -55,24 +56,25 @@ PR_SET_SECCOMP:
acceptable values to inform the kernel which action should be
taken.

- Usage:
+ 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.
+ call will return -1 and set errno to ``EINVAL``.

- If fork/clone and execve are allowed by @prog, any child
+ 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
+ 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,
+ 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.
@@ -80,51 +82,52 @@ PR_SET_SECCOMP:
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.)
+``SECCOMP_RET_KILL`` will always take precedence.)

In precedence order, they are:

-SECCOMP_RET_KILL:
+``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.
+ 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
+``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
+ ``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.)
+ it with ``-ENOSYS`` could overwrite some useful information.)

- The SECCOMP_RET_DATA portion of the return value will be passed
- as si_errno.
+ 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.
+ ``SIGSYS`` triggered by seccomp will have a si_code of ``SYS_SECCOMP``.

-SECCOMP_RET_ERRNO:
+``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:
+``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
+ 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
+ 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.
+ 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
@@ -138,19 +141,19 @@ SECCOMP_RET_TRACE:
allow use of ptrace, even of other sandboxed processes, without
extreme care; ptracers can use this mechanism to escape.)

-SECCOMP_RET_ALLOW:
+``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
+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
+``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
@@ -160,39 +163,40 @@ 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
+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
+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
+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.
+``/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:
+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
+- 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)
@@ -200,7 +204,7 @@ legacy variants on vDSO calls.) Currently, emulated vsyscalls will honor seccom
emulation security checks, making resuming the syscall mostly
pointless.

-- A return value of SECCOMP_RET_TRACE will signal the tracer as usual,
+- 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.
@@ -209,14 +213,14 @@ legacy variants on vDSO calls.) Currently, emulated vsyscalls will honor seccom
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).
+ 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
+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
+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
diff --git a/MAINTAINERS b/MAINTAINERS
index 6b36037cbe8f..f2261713043c 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -11492,6 +11492,7 @@ F: kernel/seccomp.c
F: include/uapi/linux/seccomp.h
F: include/linux/seccomp.h
F: tools/testing/selftests/seccomp/*
+F: Documentation/userspace-api/seccomp_filter.rst
K: \bsecure_computing
K: \bTIF_SECCOMP\b

--
2.7.4

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