+++ /dev/null
- 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.
--- /dev/null
+===========================================
+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.
projects / mirror_ubuntu-artful-kernel.git / commitdiff