1/*
2 * Naive system call dropper built on seccomp_filter.
3 *
4 * Copyright (c) 2012 The Chromium OS Authors <chromium-os-dev@chromium.org>
5 * Author: Will Drewry <wad@chromium.org>
6 *
7 * The code may be used by anyone for any purpose,
8 * and can serve as a starting point for developing
9 * applications using prctl(PR_SET_SECCOMP, 2, ...).
10 *
11 * When run, returns the specified errno for the specified
12 * system call number against the given architecture.
13 *
14 * Run this one as root as PR_SET_NO_NEW_PRIVS is not called.
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
28static int install_filter(int nr, int arch, 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 (prctl(PR_SET_SECCOMP, 2, &prog)) {
46		perror("prctl");
47		return 1;
48	}
49	return 0;
50}
51
52int main(int argc, char **argv)
53{
54	if (argc < 5) {
55		fprintf(stderr, "Usage:\n"
56			"dropper <syscall_nr> <arch> <errno> <prog> [<args>]\n"
57			"Hint:	AUDIT_ARCH_I386: 0x%X\n"
58			"	AUDIT_ARCH_X86_64: 0x%X\n"
59			"\n", AUDIT_ARCH_I386, AUDIT_ARCH_X86_64);
60		return 1;
61	}
62	if (install_filter(strtol(argv[1], NULL, 0), strtol(argv[2], NULL, 0),
63			   strtol(argv[3], NULL, 0)))
64		return 1;
65	execv(argv[4], &argv[4]);
66	printf("Failed to execv\n");
67	return 255;
68}
69