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diff --git a/Documentation/userspace-api/seccomp_filter.rst b/Documentation/userspace-api/seccomp_filter.rst new file mode 100644 index 000000000..cff0fa7f3 --- /dev/null +++ b/Documentation/userspace-api/seccomp_filter.rst @@ -0,0 +1,376 @@ +=========================================== +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_PROCESS`` will always take precedence.) + +In precedence order, they are: + +``SECCOMP_RET_KILL_PROCESS``: + Results in the entire process exiting immediately without executing + the system call. The exit status of the task (``status & 0x7f``) + will be ``SIGSYS``, not ``SIGKILL``. + +``SECCOMP_RET_KILL_THREAD``: + 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_USER_NOTIF``: + Results in a ``struct seccomp_notif`` message sent on the userspace + notification fd, if it is attached, or ``-ENOSYS`` if it is not. See + below on discussion of how to handle user notifications. + +``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_LOG``: + Results in the system call being executed after it is logged. This + should be used by application developers to learn which syscalls their + application needs without having to iterate through multiple test and + development cycles to build the list. + + This action will only be logged if "log" is present in the + actions_logged sysctl string. + +``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. + +Userspace Notification +====================== + +The ``SECCOMP_RET_USER_NOTIF`` return code lets seccomp filters pass a +particular syscall to userspace to be handled. This may be useful for +applications like container managers, which wish to intercept particular +syscalls (``mount()``, ``finit_module()``, etc.) and change their behavior. + +To acquire a notification FD, use the ``SECCOMP_FILTER_FLAG_NEW_LISTENER`` +argument to the ``seccomp()`` syscall: + +.. code-block:: c + + fd = seccomp(SECCOMP_SET_MODE_FILTER, SECCOMP_FILTER_FLAG_NEW_LISTENER, &prog); + +which (on success) will return a listener fd for the filter, which can then be +passed around via ``SCM_RIGHTS`` or similar. Note that filter fds correspond to +a particular filter, and not a particular task. So if this task then forks, +notifications from both tasks will appear on the same filter fd. Reads and +writes to/from a filter fd are also synchronized, so a filter fd can safely +have many readers. + +The interface for a seccomp notification fd consists of two structures: + +.. code-block:: c + + struct seccomp_notif_sizes { + __u16 seccomp_notif; + __u16 seccomp_notif_resp; + __u16 seccomp_data; + }; + + struct seccomp_notif { + __u64 id; + __u32 pid; + __u32 flags; + struct seccomp_data data; + }; + + struct seccomp_notif_resp { + __u64 id; + __s64 val; + __s32 error; + __u32 flags; + }; + +The ``struct seccomp_notif_sizes`` structure can be used to determine the size +of the various structures used in seccomp notifications. The size of ``struct +seccomp_data`` may change in the future, so code should use: + +.. code-block:: c + + struct seccomp_notif_sizes sizes; + seccomp(SECCOMP_GET_NOTIF_SIZES, 0, &sizes); + +to determine the size of the various structures to allocate. See +samples/seccomp/user-trap.c for an example. + +Users can read via ``ioctl(SECCOMP_IOCTL_NOTIF_RECV)`` (or ``poll()``) on a +seccomp notification fd to receive a ``struct seccomp_notif``, which contains +five members: the input length of the structure, a unique-per-filter ``id``, +the ``pid`` of the task which triggered this request (which may be 0 if the +task is in a pid ns not visible from the listener's pid namespace). The +notification also contains the ``data`` passed to seccomp, and a filters flag. +The structure should be zeroed out prior to calling the ioctl. + +Userspace can then make a decision based on this information about what to do, +and ``ioctl(SECCOMP_IOCTL_NOTIF_SEND)`` a response, indicating what should be +returned to userspace. The ``id`` member of ``struct seccomp_notif_resp`` should +be the same ``id`` as in ``struct seccomp_notif``. + +Userspace can also add file descriptors to the notifying process via +``ioctl(SECCOMP_IOCTL_NOTIF_ADDFD)``. The ``id`` member of +``struct seccomp_notif_addfd`` should be the same ``id`` as in +``struct seccomp_notif``. The ``newfd_flags`` flag may be used to set flags +like O_CLOEXEC on the file descriptor in the notifying process. If the supervisor +wants to inject the file descriptor with a specific number, the +``SECCOMP_ADDFD_FLAG_SETFD`` flag can be used, and set the ``newfd`` member to +the specific number to use. If that file descriptor is already open in the +notifying process it will be replaced. The supervisor can also add an FD, and +respond atomically by using the ``SECCOMP_ADDFD_FLAG_SEND`` flag and the return +value will be the injected file descriptor number. + +The notifying process can be preempted, resulting in the notification being +aborted. This can be problematic when trying to take actions on behalf of the +notifying process that are long-running and typically retryable (mounting a +filesystem). Alternatively, at filter installation time, the +``SECCOMP_FILTER_FLAG_WAIT_KILLABLE_RECV`` flag can be set. This flag makes it +such that when a user notification is received by the supervisor, the notifying +process will ignore non-fatal signals until the response is sent. Signals that +are sent prior to the notification being received by userspace are handled +normally. + +It is worth noting that ``struct seccomp_data`` contains the values of register +arguments to the syscall, but does not contain pointers to memory. The task's +memory is accessible to suitably privileged traces via ``ptrace()`` or +``/proc/pid/mem``. However, care should be taken to avoid the TOCTOU mentioned +above in this document: all arguments being read from the tracee's memory +should be read into the tracer's memory before any policy decisions are made. +This allows for an atomic decision on syscall arguments. + +Sysctls +======= + +Seccomp's sysctl files can be found in the ``/proc/sys/kernel/seccomp/`` +directory. Here's a description of each file in that directory: + +``actions_avail``: + A read-only ordered list of seccomp return values (refer to the + ``SECCOMP_RET_*`` macros above) in string form. The ordering, from + left-to-right, is the least permissive return value to the most + permissive return value. + + The list represents the set of seccomp return values supported + by the kernel. A userspace program may use this list to + determine if the actions found in the ``seccomp.h``, when the + program was built, differs from the set of actions actually + supported in the current running kernel. + +``actions_logged``: + A read-write ordered list of seccomp return values (refer to the + ``SECCOMP_RET_*`` macros above) that are allowed to be logged. Writes + to the file do not need to be in ordered form but reads from the file + will be ordered in the same way as the actions_avail sysctl. + + The ``allow`` string is not accepted in the ``actions_logged`` sysctl + as it is not possible to log ``SECCOMP_RET_ALLOW`` actions. Attempting + to write ``allow`` to the sysctl will result in an EINVAL being + returned. + +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. |