sandbox-seccomp-filter.c revision 239846 
1/*
2 * Copyright (c) 2012 Will Drewry <wad@dataspill.org>
3 *
4 * Permission to use, copy, modify, and distribute this software for any
5 * purpose with or without fee is hereby granted, provided that the above
6 * copyright notice and this permission notice appear in all copies.
7 *
8 * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
9 * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
10 * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
11 * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
12 * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
13 * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
14 * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
15 */
16
17/*
18 * Uncomment the SANDBOX_SECCOMP_FILTER_DEBUG macro below to help diagnose
19 * filter breakage during development. *Do not* use this in production,
20 * as it relies on making library calls that are unsafe in signal context.
21 *
22 * Instead, live systems the auditctl(8) may be used to monitor failures.
23 * E.g.
24 *   auditctl -a task,always -F uid=<privsep uid>
25 */
26/* #define SANDBOX_SECCOMP_FILTER_DEBUG 1 */
27
28#ifdef SANDBOX_SECCOMP_FILTER_DEBUG
29/* Use the kernel headers in case of an older toolchain. */
30# include <asm/siginfo.h>
31# define __have_siginfo_t 1
32# define __have_sigval_t 1
33# define __have_sigevent_t 1
34#endif /* SANDBOX_SECCOMP_FILTER_DEBUG */
35
36#include "includes.h"
37
38#ifdef SANDBOX_SECCOMP_FILTER
39
40#include <sys/types.h>
41#include <sys/resource.h>
42#include <sys/prctl.h>
43
44#include <linux/audit.h>
45#include <linux/filter.h>
46#include <linux/seccomp.h>
47
48#include <asm/unistd.h>
49
50#include <errno.h>
51#include <signal.h>
52#include <stdarg.h>
53#include <stddef.h>  /* for offsetof */
54#include <stdio.h>
55#include <stdlib.h>
56#include <string.h>
57#include <unistd.h>
58
59#include "log.h"
60#include "ssh-sandbox.h"
61#include "xmalloc.h"
62
63/* Linux seccomp_filter sandbox */
64#define SECCOMP_FILTER_FAIL SECCOMP_RET_KILL
65
66/* Use a signal handler to emit violations when debugging */
67#ifdef SANDBOX_SECCOMP_FILTER_DEBUG
68# undef SECCOMP_FILTER_FAIL
69# define SECCOMP_FILTER_FAIL SECCOMP_RET_TRAP
70#endif /* SANDBOX_SECCOMP_FILTER_DEBUG */
71
72/* Simple helpers to avoid manual errors (but larger BPF programs). */
73#define SC_DENY(_nr, _errno) \
74	BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, __NR_ ## _nr, 0, 1), \
75	BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_ERRNO|(_errno))
76#define SC_ALLOW(_nr) \
77	BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, __NR_ ## _nr, 0, 1), \
78	BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_ALLOW)
79
80/* Syscall filtering set for preauth. */
81static const struct sock_filter preauth_insns[] = {
82	/* Ensure the syscall arch convention is as expected. */
83	BPF_STMT(BPF_LD+BPF_W+BPF_ABS,
84		offsetof(struct seccomp_data, arch)),
85	BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, SECCOMP_AUDIT_ARCH, 1, 0),
86	BPF_STMT(BPF_RET+BPF_K, SECCOMP_FILTER_FAIL),
87	/* Load the syscall number for checking. */
88	BPF_STMT(BPF_LD+BPF_W+BPF_ABS,
89		offsetof(struct seccomp_data, nr)),
90	SC_DENY(open, EACCES),
91	SC_ALLOW(getpid),
92	SC_ALLOW(gettimeofday),
93	SC_ALLOW(time),
94	SC_ALLOW(read),
95	SC_ALLOW(write),
96	SC_ALLOW(close),
97	SC_ALLOW(brk),
98	SC_ALLOW(poll),
99#ifdef __NR__newselect
100	SC_ALLOW(_newselect),
101#else
102	SC_ALLOW(select),
103#endif
104	SC_ALLOW(madvise),
105	SC_ALLOW(mmap),
106	SC_ALLOW(munmap),
107	SC_ALLOW(exit_group),
108#ifdef __NR_rt_sigprocmask
109	SC_ALLOW(rt_sigprocmask),
110#else
111	SC_ALLOW(sigprocmask),
112#endif
113	BPF_STMT(BPF_RET+BPF_K, SECCOMP_FILTER_FAIL),
114};
115
116static const struct sock_fprog preauth_program = {
117	.len = (unsigned short)(sizeof(preauth_insns)/sizeof(preauth_insns[0])),
118	.filter = (struct sock_filter *)preauth_insns,
119};
120
121struct ssh_sandbox {
122	pid_t child_pid;
123};
124
125struct ssh_sandbox *
126ssh_sandbox_init(void)
127{
128	struct ssh_sandbox *box;
129
130	/*
131	 * Strictly, we don't need to maintain any state here but we need
132	 * to return non-NULL to satisfy the API.
133	 */
134	debug3("%s: preparing seccomp filter sandbox", __func__);
135	box = xcalloc(1, sizeof(*box));
136	box->child_pid = 0;
137
138	return box;
139}
140
141#ifdef SANDBOX_SECCOMP_FILTER_DEBUG
142extern struct monitor *pmonitor;
143void mm_log_handler(LogLevel level, const char *msg, void *ctx);
144
145static void
146ssh_sandbox_violation(int signum, siginfo_t *info, void *void_context)
147{
148	char msg[256];
149
150	snprintf(msg, sizeof(msg),
151	    "%s: unexpected system call (arch:0x%x,syscall:%d @ %p)",
152	    __func__, info->si_arch, info->si_syscall, info->si_call_addr);
153	mm_log_handler(SYSLOG_LEVEL_FATAL, msg, pmonitor);
154	_exit(1);
155}
156
157static void
158ssh_sandbox_child_debugging(void)
159{
160	struct sigaction act;
161	sigset_t mask;
162
163	debug3("%s: installing SIGSYS handler", __func__);
164	memset(&act, 0, sizeof(act));
165	sigemptyset(&mask);
166	sigaddset(&mask, SIGSYS);
167
168	act.sa_sigaction = &ssh_sandbox_violation;
169	act.sa_flags = SA_SIGINFO;
170	if (sigaction(SIGSYS, &act, NULL) == -1)
171		fatal("%s: sigaction(SIGSYS): %s", __func__, strerror(errno));
172	if (sigprocmask(SIG_UNBLOCK, &mask, NULL) == -1)
173		fatal("%s: sigprocmask(SIGSYS): %s",
174		      __func__, strerror(errno));
175}
176#endif /* SANDBOX_SECCOMP_FILTER_DEBUG */
177
178void
179ssh_sandbox_child(struct ssh_sandbox *box)
180{
181	struct rlimit rl_zero;
182
183	/* Set rlimits for completeness if possible. */
184	rl_zero.rlim_cur = rl_zero.rlim_max = 0;
185	if (setrlimit(RLIMIT_FSIZE, &rl_zero) == -1)
186		fatal("%s: setrlimit(RLIMIT_FSIZE, { 0, 0 }): %s",
187			__func__, strerror(errno));
188	if (setrlimit(RLIMIT_NOFILE, &rl_zero) == -1)
189		fatal("%s: setrlimit(RLIMIT_NOFILE, { 0, 0 }): %s",
190			__func__, strerror(errno));
191	if (setrlimit(RLIMIT_NPROC, &rl_zero) == -1)
192		fatal("%s: setrlimit(RLIMIT_NPROC, { 0, 0 }): %s",
193			__func__, strerror(errno));
194
195#ifdef SANDBOX_SECCOMP_FILTER_DEBUG
196	ssh_sandbox_child_debugging();
197#endif /* SANDBOX_SECCOMP_FILTER_DEBUG */
198
199	debug3("%s: setting PR_SET_NO_NEW_PRIVS", __func__);
200	if (prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0) == -1)
201		fatal("%s: prctl(PR_SET_NO_NEW_PRIVS): %s",
202		      __func__, strerror(errno));
203	debug3("%s: attaching seccomp filter program", __func__);
204	if (prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, &preauth_program) == -1)
205		fatal("%s: prctl(PR_SET_SECCOMP): %s",
206		      __func__, strerror(errno));
207}
208
209void
210ssh_sandbox_parent_finish(struct ssh_sandbox *box)
211{
212	free(box);
213	debug3("%s: finished", __func__);
214}
215
216void
217ssh_sandbox_parent_preauth(struct ssh_sandbox *box, pid_t child_pid)
218{
219	box->child_pid = child_pid;
220}
221
222#endif /* SANDBOX_SECCOMP_FILTER */
223