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>

1 files changed, 0 insertions, 225 deletions

diff --git a/Documentation/prctl/seccomp_filter.txt b/Documentation/prctl/seccomp_filter.txt
deleted file mode 100644
index 1e469ef75778..000000000000
--- a/Documentation/prctl/seccomp_filter.txt
+++ /dev/null
@@ -1,225 +0,0 @@
-                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_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.
-
-        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.