xref: /freebsd/crypto/openssh/sandbox-seccomp-filter.c (revision 907b59d76938e654f0d040a888e8dfca3de1e222)
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 /* XXX it should be possible to do logging via the log socket safely */
29 
30 #ifdef SANDBOX_SECCOMP_FILTER_DEBUG
31 /* Use the kernel headers in case of an older toolchain. */
32 # include <asm/siginfo.h>
33 # define __have_siginfo_t 1
34 # define __have_sigval_t 1
35 # define __have_sigevent_t 1
36 #endif /* SANDBOX_SECCOMP_FILTER_DEBUG */
37 
38 #include "includes.h"
39 
40 #ifdef SANDBOX_SECCOMP_FILTER
41 
42 #include <sys/types.h>
43 #include <sys/resource.h>
44 #include <sys/prctl.h>
45 
46 #include <linux/net.h>
47 #include <linux/audit.h>
48 #include <linux/filter.h>
49 #include <linux/seccomp.h>
50 #include <elf.h>
51 
52 #include <asm/unistd.h>
53 
54 #include <errno.h>
55 #include <signal.h>
56 #include <stdarg.h>
57 #include <stddef.h>  /* for offsetof */
58 #include <stdio.h>
59 #include <stdlib.h>
60 #include <string.h>
61 #include <unistd.h>
62 
63 #include "log.h"
64 #include "ssh-sandbox.h"
65 #include "xmalloc.h"
66 
67 /* Linux seccomp_filter sandbox */
68 #define SECCOMP_FILTER_FAIL SECCOMP_RET_KILL
69 
70 /* Use a signal handler to emit violations when debugging */
71 #ifdef SANDBOX_SECCOMP_FILTER_DEBUG
72 # undef SECCOMP_FILTER_FAIL
73 # define SECCOMP_FILTER_FAIL SECCOMP_RET_TRAP
74 #endif /* SANDBOX_SECCOMP_FILTER_DEBUG */
75 
76 /* Simple helpers to avoid manual errors (but larger BPF programs). */
77 #define SC_DENY(_nr, _errno) \
78 	BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, __NR_ ## _nr, 0, 1), \
79 	BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_ERRNO|(_errno))
80 #define SC_ALLOW(_nr) \
81 	BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, __NR_ ## _nr, 0, 1), \
82 	BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_ALLOW)
83 #define SC_ALLOW_ARG(_nr, _arg_nr, _arg_val) \
84 	BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, __NR_ ## _nr, 0, 4), \
85 	/* load first syscall argument */ \
86 	BPF_STMT(BPF_LD+BPF_W+BPF_ABS, \
87 	    offsetof(struct seccomp_data, args[(_arg_nr)])), \
88 	BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, (_arg_val), 0, 1), \
89 	BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_ALLOW), \
90 	/* reload syscall number; all rules expect it in accumulator */ \
91 	BPF_STMT(BPF_LD+BPF_W+BPF_ABS, \
92 		offsetof(struct seccomp_data, nr))
93 
94 /* Syscall filtering set for preauth. */
95 static const struct sock_filter preauth_insns[] = {
96 	/* Ensure the syscall arch convention is as expected. */
97 	BPF_STMT(BPF_LD+BPF_W+BPF_ABS,
98 		offsetof(struct seccomp_data, arch)),
99 	BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, SECCOMP_AUDIT_ARCH, 1, 0),
100 	BPF_STMT(BPF_RET+BPF_K, SECCOMP_FILTER_FAIL),
101 	/* Load the syscall number for checking. */
102 	BPF_STMT(BPF_LD+BPF_W+BPF_ABS,
103 		offsetof(struct seccomp_data, nr)),
104 
105 	/* Syscalls to non-fatally deny */
106 #ifdef __NR_fstat
107 	SC_DENY(fstat, EACCES),
108 #endif
109 #ifdef __NR_fstat64
110 	SC_DENY(fstat64, EACCES),
111 #endif
112 #ifdef __NR_open
113 	SC_DENY(open, EACCES),
114 #endif
115 #ifdef __NR_openat
116 	SC_DENY(openat, EACCES),
117 #endif
118 #ifdef __NR_newfstatat
119 	SC_DENY(newfstatat, EACCES),
120 #endif
121 #ifdef __NR_stat
122 	SC_DENY(stat, EACCES),
123 #endif
124 #ifdef __NR_stat64
125 	SC_DENY(stat64, EACCES),
126 #endif
127 
128 	/* Syscalls to permit */
129 #ifdef __NR_brk
130 	SC_ALLOW(brk),
131 #endif
132 #ifdef __NR_clock_gettime
133 	SC_ALLOW(clock_gettime),
134 #endif
135 #ifdef __NR_close
136 	SC_ALLOW(close),
137 #endif
138 #ifdef __NR_exit
139 	SC_ALLOW(exit),
140 #endif
141 #ifdef __NR_exit_group
142 	SC_ALLOW(exit_group),
143 #endif
144 #ifdef __NR_getpgid
145 	SC_ALLOW(getpgid),
146 #endif
147 #ifdef __NR_getpid
148 	SC_ALLOW(getpid),
149 #endif
150 #ifdef __NR_getrandom
151 	SC_ALLOW(getrandom),
152 #endif
153 #ifdef __NR_gettimeofday
154 	SC_ALLOW(gettimeofday),
155 #endif
156 #ifdef __NR_madvise
157 	SC_ALLOW(madvise),
158 #endif
159 #ifdef __NR_mmap
160 	SC_ALLOW(mmap),
161 #endif
162 #ifdef __NR_mmap2
163 	SC_ALLOW(mmap2),
164 #endif
165 #ifdef __NR_mremap
166 	SC_ALLOW(mremap),
167 #endif
168 #ifdef __NR_munmap
169 	SC_ALLOW(munmap),
170 #endif
171 #ifdef __NR__newselect
172 	SC_ALLOW(_newselect),
173 #endif
174 #ifdef __NR_poll
175 	SC_ALLOW(poll),
176 #endif
177 #ifdef __NR_pselect6
178 	SC_ALLOW(pselect6),
179 #endif
180 #ifdef __NR_read
181 	SC_ALLOW(read),
182 #endif
183 #ifdef __NR_rt_sigprocmask
184 	SC_ALLOW(rt_sigprocmask),
185 #endif
186 #ifdef __NR_select
187 	SC_ALLOW(select),
188 #endif
189 #ifdef __NR_shutdown
190 	SC_ALLOW(shutdown),
191 #endif
192 #ifdef __NR_sigprocmask
193 	SC_ALLOW(sigprocmask),
194 #endif
195 #ifdef __NR_time
196 	SC_ALLOW(time),
197 #endif
198 #ifdef __NR_write
199 	SC_ALLOW(write),
200 #endif
201 #ifdef __NR_socketcall
202 	SC_ALLOW_ARG(socketcall, 0, SYS_SHUTDOWN),
203 #endif
204 
205 	/* Default deny */
206 	BPF_STMT(BPF_RET+BPF_K, SECCOMP_FILTER_FAIL),
207 };
208 
209 static const struct sock_fprog preauth_program = {
210 	.len = (unsigned short)(sizeof(preauth_insns)/sizeof(preauth_insns[0])),
211 	.filter = (struct sock_filter *)preauth_insns,
212 };
213 
214 struct ssh_sandbox {
215 	pid_t child_pid;
216 };
217 
218 struct ssh_sandbox *
219 ssh_sandbox_init(struct monitor *monitor)
220 {
221 	struct ssh_sandbox *box;
222 
223 	/*
224 	 * Strictly, we don't need to maintain any state here but we need
225 	 * to return non-NULL to satisfy the API.
226 	 */
227 	debug3("%s: preparing seccomp filter sandbox", __func__);
228 	box = xcalloc(1, sizeof(*box));
229 	box->child_pid = 0;
230 
231 	return box;
232 }
233 
234 #ifdef SANDBOX_SECCOMP_FILTER_DEBUG
235 extern struct monitor *pmonitor;
236 void mm_log_handler(LogLevel level, const char *msg, void *ctx);
237 
238 static void
239 ssh_sandbox_violation(int signum, siginfo_t *info, void *void_context)
240 {
241 	char msg[256];
242 
243 	snprintf(msg, sizeof(msg),
244 	    "%s: unexpected system call (arch:0x%x,syscall:%d @ %p)",
245 	    __func__, info->si_arch, info->si_syscall, info->si_call_addr);
246 	mm_log_handler(SYSLOG_LEVEL_FATAL, msg, pmonitor);
247 	_exit(1);
248 }
249 
250 static void
251 ssh_sandbox_child_debugging(void)
252 {
253 	struct sigaction act;
254 	sigset_t mask;
255 
256 	debug3("%s: installing SIGSYS handler", __func__);
257 	memset(&act, 0, sizeof(act));
258 	sigemptyset(&mask);
259 	sigaddset(&mask, SIGSYS);
260 
261 	act.sa_sigaction = &ssh_sandbox_violation;
262 	act.sa_flags = SA_SIGINFO;
263 	if (sigaction(SIGSYS, &act, NULL) == -1)
264 		fatal("%s: sigaction(SIGSYS): %s", __func__, strerror(errno));
265 	if (sigprocmask(SIG_UNBLOCK, &mask, NULL) == -1)
266 		fatal("%s: sigprocmask(SIGSYS): %s",
267 		      __func__, strerror(errno));
268 }
269 #endif /* SANDBOX_SECCOMP_FILTER_DEBUG */
270 
271 void
272 ssh_sandbox_child(struct ssh_sandbox *box)
273 {
274 	struct rlimit rl_zero;
275 	int nnp_failed = 0;
276 
277 	/* Set rlimits for completeness if possible. */
278 	rl_zero.rlim_cur = rl_zero.rlim_max = 0;
279 	if (setrlimit(RLIMIT_FSIZE, &rl_zero) == -1)
280 		fatal("%s: setrlimit(RLIMIT_FSIZE, { 0, 0 }): %s",
281 			__func__, strerror(errno));
282 	if (setrlimit(RLIMIT_NOFILE, &rl_zero) == -1)
283 		fatal("%s: setrlimit(RLIMIT_NOFILE, { 0, 0 }): %s",
284 			__func__, strerror(errno));
285 	if (setrlimit(RLIMIT_NPROC, &rl_zero) == -1)
286 		fatal("%s: setrlimit(RLIMIT_NPROC, { 0, 0 }): %s",
287 			__func__, strerror(errno));
288 
289 #ifdef SANDBOX_SECCOMP_FILTER_DEBUG
290 	ssh_sandbox_child_debugging();
291 #endif /* SANDBOX_SECCOMP_FILTER_DEBUG */
292 
293 	debug3("%s: setting PR_SET_NO_NEW_PRIVS", __func__);
294 	if (prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0) == -1) {
295 		debug("%s: prctl(PR_SET_NO_NEW_PRIVS): %s",
296 		      __func__, strerror(errno));
297 		nnp_failed = 1;
298 	}
299 	debug3("%s: attaching seccomp filter program", __func__);
300 	if (prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, &preauth_program) == -1)
301 		debug("%s: prctl(PR_SET_SECCOMP): %s",
302 		      __func__, strerror(errno));
303 	else if (nnp_failed)
304 		fatal("%s: SECCOMP_MODE_FILTER activated but "
305 		    "PR_SET_NO_NEW_PRIVS failed", __func__);
306 }
307 
308 void
309 ssh_sandbox_parent_finish(struct ssh_sandbox *box)
310 {
311 	free(box);
312 	debug3("%s: finished", __func__);
313 }
314 
315 void
316 ssh_sandbox_parent_preauth(struct ssh_sandbox *box, pid_t child_pid)
317 {
318 	box->child_pid = child_pid;
319 }
320 
321 #endif /* SANDBOX_SECCOMP_FILTER */
322