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Diffstat (limited to 'Documentation/prctl/seccomp_filter.txt')
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diff --git a/Documentation/prctl/seccomp_filter.txt b/Documentation/prctl/seccomp_filter.txt new file mode 100644 index 00000000..1e469ef7 --- /dev/null +++ b/Documentation/prctl/seccomp_filter.txt @@ -0,0 +1,225 @@ + 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. |