===========================================Seccomp BPF (SECure COMPuting with filters)=========================================== Introduction============ A large number of system calls are exposed to every userland processwith many of them going unused for the entire lifetime of the process.As system calls change and mature, bugs are found and eradicated.  Acertain subset of userland applications benefit by having a reduced setof available system calls.  The resulting set reduces the total kernelsurface exposed to the application.  System call filtering is meant foruse with those applications. Seccomp filtering provides a means for a process to specify a filter forincoming system calls.  The filter is expressed as a Berkeley PacketFilter (BPF) program, as with socket filters, except that the dataoperated on is related to the system call being made: system callnumber and the system call arguments.  This allows for expressivefiltering of system calls using a filter program language with a longhistory of being exposed to userland and a straightforward data set. Additionally, BPF makes it impossible for users of seccomp to fall preyto time-of-check-time-of-use (TOCTOU) attacks that are common in systemcall interposition frameworks.  BPF programs may not dereferencepointers which constrains all filters to solely evaluating the systemcall arguments directly. What it isn't============= System call filtering isn't a sandbox.  It provides a clearly definedmechanism for minimizing the exposed kernel surface.  It is meant to bea tool for sandbox developers to use.  Beyond that, policy for logicalbehavior and information flow should be managed with a combination ofother system hardening techniques and, potentially, an LSM of yourchoosing.  Expressive, dynamic filters provide further options down thispath (avoiding pathological sizes or selecting which of the multiplexedsystem calls in socketcall() is allowed, for instance) which could beconstrued, incorrectly, as a more complete sandboxing solution. Usage===== An additional seccomp mode is added and is enabled using the sameprctl(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 multiplefilters exist, the return value for the evaluation of a given systemcall 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 agiven system call will always use the highest precedent value. Precedence is only determined using the ``SECCOMP_RET_ACTION`` mask.  Whenmultiple filters return values of the same precedence, only the``SECCOMP_RET_DATA`` from the most recently installed filter will bereturned. Pitfalls======== The biggest pitfall to avoid during use is filtering on system callnumber without checking the architecture value.  Why?  On anyarchitecture that supports multiple system call invocation conventions,the system call numbers may vary based on the specific invocation.  Ifthe numbers in the different calling conventions overlap, then checks inthe filters may be abused.  Always check the arch value! Example======= The ``samples/seccomp/`` directory contains both an x86-specific exampleand a more generic example of a higher level macro interface for BPFprogram generation. Userspace Notification====================== The ``SECCOMP_RET_USER_NOTIF`` return code lets seccomp filters pass aparticular syscall to userspace to be handled. This may be useful forapplications like container managers, which wish to intercept particularsyscalls (``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 bepassed around via ``SCM_RIGHTS`` or similar. Note that filter fds correspond toa 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 andwrites to/from a filter fd are also synchronized, so a filter fd can safelyhave 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 sizeof the various structures used in seccomp notifications. The size of ``structseccomp_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. Seesamples/seccomp/user-trap.c for an example. Users can read via ``ioctl(SECCOMP_IOCTL_NOTIF_RECV)``  (or ``poll()``) on aseccomp notification fd to receive a ``struct seccomp_notif``, which containsfive 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 thetask is in a pid ns not visible from the listener's pid namespace). Thenotification 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 bereturned to userspace. The ``id`` member of ``struct seccomp_notif_resp`` shouldbe 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 flagslike O_CLOEXEC on the file descriptor in the notifying process. If the supervisorwants to inject the file descriptor with a specific number, the``SECCOMP_ADDFD_FLAG_SETFD`` flag can be used, and set the ``newfd`` member tothe specific number to use. If that file descriptor is already open in thenotifying process it will be replaced. The supervisor can also add an FD, andrespond atomically by using the ``SECCOMP_ADDFD_FLAG_SEND`` flag and the returnvalue will be the injected file descriptor number. The notifying process can be preempted, resulting in the notification beingaborted. This can be problematic when trying to take actions on behalf of thenotifying process that are long-running and typically retryable (mounting afilesystem). Alternatively, at filter installation time, the``SECCOMP_FILTER_FLAG_WAIT_KILLABLE_RECV`` flag can be set. This flag makes itsuch that when a user notification is received by the supervisor, the notifyingprocess will ignore non-fatal signals until the response is sent. Signals thatare sent prior to the notification being received by userspace are handlednormally. It is worth noting that ``struct seccomp_data`` contains the values of registerarguments to the syscall, but does not contain pointers to memory. The task'smemory is accessible to suitably privileged traces via ``ptrace()`` or``/proc/pid/mem``. However, care should be taken to avoid the TOCTOU mentionedabove in this document: all arguments being read from the tracee's memoryshould 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 anarchitecture supports both ptrace_event and seccomp, it will be able tosupport seccomp filter with minor fixup: ``SIGSYS`` support and seccomp returnvalue 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 thatfall back to real syscalls.  To minimize these surprises on x86, makesure you test with``/sys/devices/system/clocksource/clocksource0/current_clocksource`` set tosomething like ``acpi_pm``. On x86-64, vsyscall emulation is enabled by default.  (vsyscalls arelegacy variants on vDSO calls.)  Currently, emulated vsyscalls willhonor 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 othercondition: future kernels may improve vsyscall emulation and currentkernels in vsyscall=native mode will behave differently, but theinstructions at ``0xF...F600{0,4,8,C}00`` will not be system calls in thesecases. Note that modern systems are unlikely to use vsyscalls at all -- theyare a legacy feature and they are considerably slower than standardsyscalls.  New code will use the vDSO, and vDSO-issued system callsare indistinguishable from normal system calls.