samples/seccomp/dropper.c

   1 // SPDX-License-Identifier: GPL-2.0
   2 /*
   3  * Naive system call dropper built on seccomp_filter.
   4  *
   5  * Copyright (c) 2012 The Chromium OS Authors <chromium-os-dev@chromium.org>
   6  * Author: Will Drewry <wad@chromium.org>
   7  *
   8  * The code may be used by anyone for any purpose,
   9  * and can serve as a starting point for developing
  10  * applications using prctl(PR_SET_SECCOMP, 2, ...).
  11  *
  12  * When run, returns the specified errno for the specified
  13  * system call number against the given architecture.
  14  *
  15  */
  16
  17 #include <errno.h>
  18 #include <linux/audit.h>
  19 #include <linux/filter.h>
  20 #include <linux/seccomp.h>
  21 #include <linux/unistd.h>
  22 #include <stdio.h>
  23 #include <stddef.h>
  24 #include <stdlib.h>
  25 #include <sys/prctl.h>
  26 #include <unistd.h>
  27
  28 static int install_filter(int arch, int nr, int error)
  29 {
  30         struct sock_filter filter[] = {
  31                 BPF_STMT(BPF_LD+BPF_W+BPF_ABS,
  32                          (offsetof(struct seccomp_data, arch))),
  33                 BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, arch, 0, 3),
  34                 BPF_STMT(BPF_LD+BPF_W+BPF_ABS,
  35                          (offsetof(struct seccomp_data, nr))),
  36                 BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, nr, 0, 1),
  37                 BPF_STMT(BPF_RET+BPF_K,
  38                          SECCOMP_RET_ERRNO|(error & SECCOMP_RET_DATA)),
  39                 BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_ALLOW),
  40         };
  41         struct sock_fprog prog = {
  42                 .len = (unsigned short)(sizeof(filter)/sizeof(filter[0])),
  43                 .filter = filter,
  44         };
  45         if (error == -1) {
  46                 struct sock_filter kill = BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_KILL);
  47                 filter[4] = kill;
  48         }
  49         if (prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0)) {
  50                 perror("prctl(NO_NEW_PRIVS)");
  51                 return 1;
  52         }
  53         if (prctl(PR_SET_SECCOMP, 2, &prog)) {
  54                 perror("prctl(PR_SET_SECCOMP)");
  55                 return 1;
  56         }
  57         return 0;
  58 }
  59
  60 int main(int argc, char **argv)
  61 {
  62         if (argc < 5) {
  63                 fprintf(stderr, "Usage:\n"
  64                         "dropper <arch> <syscall_nr> <errno> <prog> [<args>]\n"
  65                         "Hint:  AUDIT_ARCH_I386: 0x%X\n"
  66                         "       AUDIT_ARCH_X86_64: 0x%X\n"
  67                         "       errno == -1 means SECCOMP_RET_KILL\n"
  68                         "\n", AUDIT_ARCH_I386, AUDIT_ARCH_X86_64);
  69                 return 1;
  70         }
  71         if (install_filter(strtol(argv[1], NULL, 0), strtol(argv[2], NULL, 0),
  72                            strtol(argv[3], NULL, 0)))
  73                 return 1;
  74         execv(argv[4], &argv[4]);
  75         printf("Failed to execv\n");
  76         return 255;
  77 }