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
5e33994dca
commit
c061f33f35
3 changed files with 62 additions and 56 deletions
|
|
@ -16,6 +16,7 @@ place where this information is gathered.
|
|||
.. toctree::
|
||||
:maxdepth: 2
|
||||
|
||||
seccomp_filter
|
||||
unshare
|
||||
|
||||
.. only:: subproject and html
|
||||
|
|
|
|||
229
Documentation/userspace-api/seccomp_filter.rst
Normal file
229
Documentation/userspace-api/seccomp_filter.rst
Normal file
|
|
@ -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.
|
||||
Loading…
Add table
Add a link
Reference in a new issue