seccomp/seccomp_deny_open.c

This is seccomp/seccomp_deny_open.c, an example to accompany the book, The Linux Programming Interface.

This file is not printed in the book; it demonstrates Linux features that are not described in the book (typically features that have appeared since the book was published).

The source code file is copyright 2010, Michael Kerrisk, and is licensed under the GNU General Public License, version 3.

In the listing below, the names of Linux system calls and C library functions are hyperlinked to manual pages from the Linux man-pages project, and the names of functions implemented in the book are hyperlinked to the implementations of those functions.

  Cover of The Linux Programming Interface

Function list (Bold in this list means a function is not static)

/* seccomp_deny_open.c

   A simple seccomp filter example. Install a filter that kills the process
   if open() or openat() is called.
*/
#define _GNU_SOURCE
#include <stddef.h>
#include <fcntl.h>
#include <linux/audit.h>
#include <sys/syscall.h>
#include <linux/filter.h>
#include <linux/seccomp.h>
#include <sys/prctl.h>
#include "tlpi_hdr.h"

/* For the x32 ABI, all system call numbers have bit 30 set */

#define X32_SYSCALL_BIT         0x40000000

/* The following is a hack to allow for systems (pre-Linux 4.14) that don't
   provide SECCOMP_RET_KILL_PROCESS, which kills (all threads in) a process.
   On those systems, define SECCOMP_RET_KILL_PROCESS as SECCOMP_RET_KILL
   (which simply kills the calling thread). */

#ifndef SECCOMP_RET_KILL_PROCESS
#define SECCOMP_RET_KILL_PROCESS SECCOMP_RET_KILL
#endif
static int
seccomp(unsigned int operation, unsigned int flags, void *args)
{
    return syscall(__NR_seccomp, operation, flags, args);
}
static void
install_filter(void)
{
    struct sock_filter filter[] = {
        /* Load architecture */

        BPF_STMT(BPF_LD | BPF_W | BPF_ABS,
                (offsetof(struct seccomp_data, arch))),

        /* Kill process if the architecture is not what we expect */

        BPF_JUMP(BPF_JMP | BPF_JEQ | BPF_K, AUDIT_ARCH_X86_64, 1, 0),
        BPF_STMT(BPF_RET | BPF_K, SECCOMP_RET_KILL_PROCESS),

        /* Load system call number */

        BPF_STMT(BPF_LD | BPF_W | BPF_ABS,
                 (offsetof(struct seccomp_data, nr))),

        /* Kill the process if this is an x32 system call (bit 30 is set) */

        BPF_JUMP(BPF_JMP | BPF_JGE | BPF_K, X32_SYSCALL_BIT, 0, 1),
        BPF_STMT(BPF_RET | BPF_K, SECCOMP_RET_KILL_PROCESS),

        /* Allow system calls other than open() and openat() */

        BPF_JUMP(BPF_JMP | BPF_JEQ | BPF_K, __NR_open, 2, 0),
        BPF_JUMP(BPF_JMP | BPF_JEQ | BPF_K, __NR_openat, 1, 0),
        BPF_STMT(BPF_RET | BPF_K, SECCOMP_RET_ALLOW),
        BPF_STMT(BPF_RET | BPF_K, SECCOMP_RET_KILL_PROCESS)

    };

    struct sock_fprog prog = {
        .len = (unsigned short) (sizeof(filter) / sizeof(filter[0])),
        .filter = filter,
    };

    if (seccomp(SECCOMP_SET_MODE_FILTER, 0, &prog) == -1)
        errExit("seccomp");
    /* On Linux 3.16 and earlier, we must instead use:
            if (prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, &prog))
                errExit("prctl-PR_SET_SECCOMP");
    */
}
int
main(int argc, char **argv)
{
    if (prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0))
        errExit("prctl");

    install_filter();

    if (open("/tmp/a", O_RDONLY) == -1)
        errExit("open");

    printf("We shouldn't see this message\n");

    exit(EXIT_SUCCESS);
}

 

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