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
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_NO_NEW_PRIVS, 1, 0, 0, 0)) {
46		perror("prctl(NO_NEW_PRIVS)");
47		return 1;
48	}
49	if (prctl(PR_SET_SECCOMP, 2, &prog)) {
50		perror("prctl(PR_SET_SECCOMP)");
51		return 1;
52	}
53	return 0;
54}
55
56int main(int argc, char **argv)
57{
58	if (argc < 5) {
59		fprintf(stderr, "Usage:\n"
60			"dropper <syscall_nr> <arch> <errno> <prog> [<args>]\n"
61			"Hint:	AUDIT_ARCH_I386: 0x%X\n"
62			"	AUDIT_ARCH_X86_64: 0x%X\n"
63			"\n", AUDIT_ARCH_I386, AUDIT_ARCH_X86_64);
64		return 1;
65	}
66	if (install_filter(strtol(argv[1], NULL, 0), strtol(argv[2], NULL, 0),
67			   strtol(argv[3], NULL, 0)))
68		return 1;
69	execv(argv[4], &argv[4]);
70	printf("Failed to execv\n");
71	return 255;
72}
73