Filters

When adding filters via SECCOMP_SET_MODE_FILTER, args points to a filter program:

Each program must contain one or more BPF instructions:

When executing the instructions, the BPF program operates on the system call information made available (i.e., use the BPF_ABS addressing mode) as a (read-only) buffer of the following form:

Because numbering of system calls varies between architectures and some architectures (e.g., x86-64) allow user-space code to use the calling conventions of multiple architectures (and the convention being used may vary over the life of a process that uses execve(2) to execute binaries that employ the different conventions), it is usually necessary to verify the value of the arch field.

It is strongly recommended to use an allow-list approach whenever possible because such an approach is more robust and simple. A deny-list will have to be updated whenever a potentially dangerous system call is added (or a dangerous flag or option if those are deny-listed), and it is often possible to alter the representation of a value without altering its meaning, leading to a deny-list bypass. See also Caveats below.

The arch field is not unique for all calling conventions. The x86-64 ABI and the x32 ABI both use AUDIT_ARCH_X86_64 as arch, and they run on the same processors. Instead, the mask __X32_SYSCALL_BIT is used on the system call number to tell the two ABIs apart.

This means that a policy must either deny all syscalls with __X32_SYSCALL_BIT or it must recognize syscalls with and without __X32_SYSCALL_BIT set. A list of system calls to be denied based on nr that does not also contain nr values with __X32_SYSCALL_BIT set can be bypassed by a malicious program that sets __X32_SYSCALL_BIT.

Additionally, kernels prior to Linux 5.4 incorrectly permitted nr in the ranges 512-547 as well as the corresponding non-x32 syscalls ORed with __X32_SYSCALL_BIT. For example, nr == 521 and nr == (101 | __X32_SYSCALL_BIT) would result in invocations of ptrace(2) with potentially confused x32-vs-x86_64 semantics in the kernel. Policies intended to work on kernels before Linux 5.4 must ensure that they deny or otherwise correctly handle these system calls. On Linux 5.4 and newer, such system calls will fail with the error ENOSYS, without doing anything.

The instruction_pointer field provides the address of the machine-language instruction that performed the system call. This might be useful in conjunction with the use of /proc/[pid]/maps to perform checks based on which region (mapping) of the program made the system call. (Probably, it is wise to lock down the mmap(2) and mprotect(2) system calls to prevent the program from subverting such checks.)

When checking values from args, keep in mind that arguments are often silently truncated before being processed, but after the seccomp check. For example, this happens if the i386 ABI is used on an x86-64 kernel: although the kernel will normally not look beyond the 32 lowest bits of the arguments, the values of the full 64-bit registers will be present in the seccomp data. A less surprising example is that if the x86-64 ABI is used to perform a system call that takes an argument of type int, the more-significant half of the argument register is ignored by the system call, but visible in the seccomp data.

A seccomp filter returns a 32-bit value consisting of two parts: the most significant 16 bits (corresponding to the mask defined by the constant SECCOMP_RET_ACTION_FULL) contain one of the "action" values listed below; the least significant 16-bits (defined by the constant SECCOMP_RET_DATA) are "data" to be associated with this return value.

If multiple filters exist, they are all executed, in reverse order of their addition to the filter tree—that is, the most recently installed filter is executed first. (Note that all filters will be called even if one of the earlier filters returns SECCOMP_RET_KILL. This is done to simplify the kernel code and to provide a tiny speed-up in the execution of sets of filters by avoiding a check for this uncommon case.) The return value for the evaluation of a given system call is the first-seen action value of highest precedence (along with its accompanying data) returned by execution of all of the filters.

In decreasing order of precedence, the action values that may be returned by a seccomp filter are:

If an action value other than one of the above is specified, then the filter action is treated as either SECCOMP_RET_KILL_PROCESS (since Linux 4.14) or SECCOMP_RET_KILL_THREAD (in Linux 4.13 and earlier).

/proc interfaces

The files in the directory /proc/sys/kernel/seccomp provide additional seccomp information and configuration:

Audit logging of seccomp actions

Since Linux 4.14, the kernel provides the facility to log the actions returned by seccomp filters in the audit log. The kernel makes the decision to log an action based on the action type, whether or not the action is present in the actions_logged file, and whether kernel auditing is enabled (e.g., via the kernel boot option audit=1). The rules are as follows:

Architecture support for seccomp BPF

Architecture support for seccomp BPF filtering is available on the following architectures:

Glibc does not provide a wrapper for this system call; call it using syscall(2).

Caveats

There are various subtleties to consider when applying seccomp filters to a program, including the following:

The consequence of the above points is that it may be necessary to filter for a system call other than might be expected. Various manual pages in Section 2 provide helpful details about the differences between wrapper functions and the underlying system calls in subsections entitled C library/kernel differences.

Furthermore, note that the application of seccomp filters even risks causing bugs in an application, when the filters cause unexpected failures for legitimate operations that the application might need to perform. Such bugs may not easily be discovered when testing the seccomp filters if the bugs occur in rarely used application code paths.

Seccomp-specific BPF details

Note the following BPF details specific to seccomp filters:

Program source

#include <errno.h>
#include <stddef.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <linux/audit.h>
#include <linux/filter.h>
#include <linux/seccomp.h>
#include <sys/prctl.h>

#define X32_SYSCALL_BIT 0x40000000
#define ARRAY_SIZE(arr) (sizeof(arr) / sizeof((arr)[0]))

static int
install_filter(int syscall_nr, int t_arch, int f_errno)
{
    unsigned int upper_nr_limit = 0xffffffff;

    /* Assume that AUDIT_ARCH_X86_64 means the normal x86−64 ABI
       (in the x32 ABI, all system calls have bit 30 set in the
       'nr' field, meaning the numbers are >= X32_SYSCALL_BIT). */
    if (t_arch == AUDIT_ARCH_X86_64)
        upper_nr_limit = X32_SYSCALL_BIT − 1;

    struct sock_filter filter[] = {
        /* [0] Load architecture from 'seccomp_data' buffer into
               accumulator. */
        BPF_STMT(BPF_LD | BPF_W | BPF_ABS,
                 (offsetof(struct seccomp_data, arch))),

        /* [1] Jump forward 5 instructions if architecture does not
               match 't_arch'. */
        BPF_JUMP(BPF_JMP | BPF_JEQ | BPF_K, t_arch, 0, 5),

        /* [2] Load system call number from 'seccomp_data' buffer into
               accumulator. */
        BPF_STMT(BPF_LD | BPF_W | BPF_ABS,
                 (offsetof(struct seccomp_data, nr))),

        /* [3] Check ABI − only needed for x86−64 in deny−list use
               cases.  Use BPF_JGT instead of checking against the bit
               mask to avoid having to reload the syscall number. */
        BPF_JUMP(BPF_JMP | BPF_JGT | BPF_K, upper_nr_limit, 3, 0),

        /* [4] Jump forward 1 instruction if system call number
               does not match 'syscall_nr'. */
        BPF_JUMP(BPF_JMP | BPF_JEQ | BPF_K, syscall_nr, 0, 1),

        /* [5] Matching architecture and system call: don't execute
           the system call, and return 'f_errno' in 'errno'. */
        BPF_STMT(BPF_RET | BPF_K,
                 SECCOMP_RET_ERRNO | (f_errno & SECCOMP_RET_DATA)),

        /* [6] Destination of system call number mismatch: allow other
               system calls. */
        BPF_STMT(BPF_RET | BPF_K, SECCOMP_RET_ALLOW),

        /* [7] Destination of architecture mismatch: kill process. */
        BPF_STMT(BPF_RET | BPF_K, SECCOMP_RET_KILL_PROCESS),
    };

    struct sock_fprog prog = {
        .len = ARRAY_SIZE(filter),
        .filter = filter,
    };

    if (seccomp(SECCOMP_SET_MODE_FILTER, 0, &prog)) {
        perror("seccomp");
        return 1;
    }

    return 0;
}

int
main(int argc, char **argv)
{
    if (argc < 5) {
        fprintf(stderr, "Usage: "
                "%s <syscall_nr> <arch> <errno> <prog> [<args>]\n"
                "Hint for <arch>: AUDIT_ARCH_I386: 0x%X\n"
                "                 AUDIT_ARCH_X86_64: 0x%X\n"
                "\n", argv[0], AUDIT_ARCH_I386, AUDIT_ARCH_X86_64);
        exit(EXIT_FAILURE);
    }

    if (prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0)) {
        perror("prctl");
        exit(EXIT_FAILURE);
    }

    if (install_filter(strtol(argv[1], NULL, 0),
                       strtol(argv[2], NULL, 0),
                       strtol(argv[3], NULL, 0)))
        exit(EXIT_FAILURE);

    execv(argv[4], &argv[4]);
    perror("execv");
    exit(EXIT_FAILURE);
}