From c061f33f35be0ccc80f4b8e0aea5dfd2ed7e01a3 Mon Sep 17 00:00:00 2001 From: Kees Cook Date: Sat, 13 May 2017 04:51:37 -0700 Subject: 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 Signed-off-by: Jonathan Corbet --- Documentation/prctl/seccomp_filter.txt | 225 ------------------------ Documentation/userspace-api/index.rst | 1 + Documentation/userspace-api/seccomp_filter.rst | 229 +++++++++++++++++++++++++ MAINTAINERS | 1 + 4 files changed, 231 insertions(+), 225 deletions(-) delete mode 100644 Documentation/prctl/seccomp_filter.txt create mode 100644 Documentation/userspace-api/seccomp_filter.rst diff --git a/Documentation/prctl/seccomp_filter.txt b/Documentation/prctl/seccomp_filter.txt deleted file mode 100644 index 1e469ef..0000000 --- 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. diff --git a/Documentation/userspace-api/index.rst b/Documentation/userspace-api/index.rst index a9d01b4..15ff123 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/userspace-api/seccomp_filter.rst b/Documentation/userspace-api/seccomp_filter.rst new file mode 100644 index 0000000..f71eb5e --- /dev/null +++ b/Documentation/userspace-api/seccomp_filter.rst @@ -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. diff --git a/MAINTAINERS b/MAINTAINERS index f7d568b..752916d 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 -- cgit v1.1