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 #if __BYTE_ORDER == __LITTLE_ENDIAN
77 # define ARG_LO_OFFSET 0
78 # define ARG_HI_OFFSET sizeof(uint32_t)
79 #elif __BYTE_ORDER == __BIG_ENDIAN
80 # define ARG_LO_OFFSET sizeof(uint32_t)
81 # define ARG_HI_OFFSET 0
82 #else
83 #error "Unknown endianness"
84 #endif
85
86 /* Simple helpers to avoid manual errors (but larger BPF programs). */
87 #define SC_DENY(_nr, _errno) \
88 BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, (_nr), 0, 1), \
89 BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_ERRNO|(_errno))
90 #define SC_ALLOW(_nr) \
91 BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, (_nr), 0, 1), \
92 BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_ALLOW)
93 #define SC_ALLOW_ARG(_nr, _arg_nr, _arg_val) \
94 BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, (_nr), 0, 6), \
95 /* load and test first syscall argument, low word */ \
96 BPF_STMT(BPF_LD+BPF_W+BPF_ABS, \
97 offsetof(struct seccomp_data, args[(_arg_nr)]) + ARG_LO_OFFSET), \
98 BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, \
99 ((_arg_val) & 0xFFFFFFFF), 0, 3), \
100 /* load and test first syscall argument, high word */ \
101 BPF_STMT(BPF_LD+BPF_W+BPF_ABS, \
102 offsetof(struct seccomp_data, args[(_arg_nr)]) + ARG_HI_OFFSET), \
103 BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, \
104 (((uint32_t)((uint64_t)(_arg_val) >> 32)) & 0xFFFFFFFF), 0, 1), \
105 BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_ALLOW), \
106 /* reload syscall number; all rules expect it in accumulator */ \
107 BPF_STMT(BPF_LD+BPF_W+BPF_ABS, \
108 offsetof(struct seccomp_data, nr))
109
110 /* Syscall filtering set for preauth. */
111 static const struct sock_filter preauth_insns[] = {
112 /* Ensure the syscall arch convention is as expected. */
113 BPF_STMT(BPF_LD+BPF_W+BPF_ABS,
114 offsetof(struct seccomp_data, arch)),
115 BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, SECCOMP_AUDIT_ARCH, 1, 0),
116 BPF_STMT(BPF_RET+BPF_K, SECCOMP_FILTER_FAIL),
117 /* Load the syscall number for checking. */
118 BPF_STMT(BPF_LD+BPF_W+BPF_ABS,
119 offsetof(struct seccomp_data, nr)),
120
121 /* Syscalls to non-fatally deny */
122 #ifdef __NR_lstat
123 SC_DENY(__NR_lstat, EACCES),
124 #endif
125 #ifdef __NR_lstat64
126 SC_DENY(__NR_lstat64, EACCES),
127 #endif
128 #ifdef __NR_fstat
129 SC_DENY(__NR_fstat, EACCES),
130 #endif
131 #ifdef __NR_fstat64
132 SC_DENY(__NR_fstat64, EACCES),
133 #endif
134 #ifdef __NR_open
135 SC_DENY(__NR_open, EACCES),
136 #endif
137 #ifdef __NR_openat
138 SC_DENY(__NR_openat, EACCES),
139 #endif
140 #ifdef __NR_newfstatat
141 SC_DENY(__NR_newfstatat, EACCES),
142 #endif
143 #ifdef __NR_stat
144 SC_DENY(__NR_stat, EACCES),
145 #endif
146 #ifdef __NR_stat64
147 SC_DENY(__NR_stat64, EACCES),
148 #endif
149
150 /* Syscalls to permit */
151 #ifdef __NR_brk
152 SC_ALLOW(__NR_brk),
153 #endif
154 #ifdef __NR_clock_gettime
155 SC_ALLOW(__NR_clock_gettime),
156 #endif
157 #ifdef __NR_close
158 SC_ALLOW(__NR_close),
159 #endif
160 #ifdef __NR_exit
161 SC_ALLOW(__NR_exit),
162 #endif
163 #ifdef __NR_exit_group
164 SC_ALLOW(__NR_exit_group),
165 #endif
166 #ifdef __NR_getpgid
167 SC_ALLOW(__NR_getpgid),
168 #endif
169 #ifdef __NR_getpid
170 SC_ALLOW(__NR_getpid),
171 #endif
172 #ifdef __NR_getrandom
173 SC_ALLOW(__NR_getrandom),
174 #endif
175 #ifdef __NR_gettimeofday
176 SC_ALLOW(__NR_gettimeofday),
177 #endif
178 #ifdef __NR_madvise
179 SC_ALLOW(__NR_madvise),
180 #endif
181 #ifdef __NR_mmap
182 SC_ALLOW(__NR_mmap),
183 #endif
184 #ifdef __NR_mmap2
185 SC_ALLOW(__NR_mmap2),
186 #endif
187 #ifdef __NR_mremap
188 SC_ALLOW(__NR_mremap),
189 #endif
190 #ifdef __NR_munmap
191 SC_ALLOW(__NR_munmap),
192 #endif
193 #ifdef __NR__newselect
194 SC_ALLOW(__NR__newselect),
195 #endif
196 #ifdef __NR_poll
197 SC_ALLOW(__NR_poll),
198 #endif
199 #ifdef __NR_pselect6
200 SC_ALLOW(__NR_pselect6),
201 #endif
202 #ifdef __NR_read
203 SC_ALLOW(__NR_read),
204 #endif
205 #ifdef __NR_rt_sigprocmask
206 SC_ALLOW(__NR_rt_sigprocmask),
207 #endif
208 #ifdef __NR_select
209 SC_ALLOW(__NR_select),
210 #endif
211 #ifdef __NR_shutdown
212 SC_ALLOW(__NR_shutdown),
213 #endif
214 #ifdef __NR_sigprocmask
215 SC_ALLOW(__NR_sigprocmask),
216 #endif
217 #ifdef __NR_time
218 SC_ALLOW(__NR_time),
219 #endif
220 #ifdef __NR_write
221 SC_ALLOW(__NR_write),
222 #endif
223 #ifdef __NR_socketcall
224 SC_ALLOW_ARG(__NR_socketcall, 0, SYS_SHUTDOWN),
225 #endif
226 #if defined(__NR_ioctl) && defined(__s390__)
227 /* Allow ioctls for ICA crypto card on s390 */
228 SC_ALLOW_ARG(__NR_ioctl, 1, Z90STAT_STATUS_MASK),
229 SC_ALLOW_ARG(__NR_ioctl, 1, ICARSAMODEXPO),
230 SC_ALLOW_ARG(__NR_ioctl, 1, ICARSACRT),
231 #endif
232 #if defined(__x86_64__) && defined(__ILP32__) && defined(__X32_SYSCALL_BIT)
233 /*
234 * On Linux x32, the clock_gettime VDSO falls back to the
235 * x86-64 syscall under some circumstances, e.g.
236 * https://bugs.debian.org/849923
237 */
238 SC_ALLOW(__NR_clock_gettime & ~__X32_SYSCALL_BIT);
239 #endif
240
241 /* Default deny */
242 BPF_STMT(BPF_RET+BPF_K, SECCOMP_FILTER_FAIL),
243 };
244
245 static const struct sock_fprog preauth_program = {
246 .len = (unsigned short)(sizeof(preauth_insns)/sizeof(preauth_insns[0])),
247 .filter = (struct sock_filter *)preauth_insns,
248 };
249
250 struct ssh_sandbox {
251 pid_t child_pid;
252 };
253
254 struct ssh_sandbox *
ssh_sandbox_init(struct monitor * monitor)255 ssh_sandbox_init(struct monitor *monitor)
256 {
257 struct ssh_sandbox *box;
258
259 /*
260 * Strictly, we don't need to maintain any state here but we need
261 * to return non-NULL to satisfy the API.
262 */
263 debug3("%s: preparing seccomp filter sandbox", __func__);
264 box = xcalloc(1, sizeof(*box));
265 box->child_pid = 0;
266
267 return box;
268 }
269
270 #ifdef SANDBOX_SECCOMP_FILTER_DEBUG
271 extern struct monitor *pmonitor;
272 void mm_log_handler(LogLevel level, const char *msg, void *ctx);
273
274 static void
ssh_sandbox_violation(int signum,siginfo_t * info,void * void_context)275 ssh_sandbox_violation(int signum, siginfo_t *info, void *void_context)
276 {
277 char msg[256];
278
279 snprintf(msg, sizeof(msg),
280 "%s: unexpected system call (arch:0x%x,syscall:%d @ %p)",
281 __func__, info->si_arch, info->si_syscall, info->si_call_addr);
282 mm_log_handler(SYSLOG_LEVEL_FATAL, msg, pmonitor);
283 _exit(1);
284 }
285
286 static void
ssh_sandbox_child_debugging(void)287 ssh_sandbox_child_debugging(void)
288 {
289 struct sigaction act;
290 sigset_t mask;
291
292 debug3("%s: installing SIGSYS handler", __func__);
293 memset(&act, 0, sizeof(act));
294 sigemptyset(&mask);
295 sigaddset(&mask, SIGSYS);
296
297 act.sa_sigaction = &ssh_sandbox_violation;
298 act.sa_flags = SA_SIGINFO;
299 if (sigaction(SIGSYS, &act, NULL) == -1)
300 fatal("%s: sigaction(SIGSYS): %s", __func__, strerror(errno));
301 if (sigprocmask(SIG_UNBLOCK, &mask, NULL) == -1)
302 fatal("%s: sigprocmask(SIGSYS): %s",
303 __func__, strerror(errno));
304 }
305 #endif /* SANDBOX_SECCOMP_FILTER_DEBUG */
306
307 void
ssh_sandbox_child(struct ssh_sandbox * box)308 ssh_sandbox_child(struct ssh_sandbox *box)
309 {
310 struct rlimit rl_zero;
311 int nnp_failed = 0;
312
313 /* Set rlimits for completeness if possible. */
314 rl_zero.rlim_cur = rl_zero.rlim_max = 0;
315 if (setrlimit(RLIMIT_FSIZE, &rl_zero) == -1)
316 fatal("%s: setrlimit(RLIMIT_FSIZE, { 0, 0 }): %s",
317 __func__, strerror(errno));
318 if (setrlimit(RLIMIT_NOFILE, &rl_zero) == -1)
319 fatal("%s: setrlimit(RLIMIT_NOFILE, { 0, 0 }): %s",
320 __func__, strerror(errno));
321 if (setrlimit(RLIMIT_NPROC, &rl_zero) == -1)
322 fatal("%s: setrlimit(RLIMIT_NPROC, { 0, 0 }): %s",
323 __func__, strerror(errno));
324
325 #ifdef SANDBOX_SECCOMP_FILTER_DEBUG
326 ssh_sandbox_child_debugging();
327 #endif /* SANDBOX_SECCOMP_FILTER_DEBUG */
328
329 debug3("%s: setting PR_SET_NO_NEW_PRIVS", __func__);
330 if (prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0) == -1) {
331 debug("%s: prctl(PR_SET_NO_NEW_PRIVS): %s",
332 __func__, strerror(errno));
333 nnp_failed = 1;
334 }
335 debug3("%s: attaching seccomp filter program", __func__);
336 if (prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, &preauth_program) == -1)
337 debug("%s: prctl(PR_SET_SECCOMP): %s",
338 __func__, strerror(errno));
339 else if (nnp_failed)
340 fatal("%s: SECCOMP_MODE_FILTER activated but "
341 "PR_SET_NO_NEW_PRIVS failed", __func__);
342 }
343
344 void
ssh_sandbox_parent_finish(struct ssh_sandbox * box)345 ssh_sandbox_parent_finish(struct ssh_sandbox *box)
346 {
347 free(box);
348 debug3("%s: finished", __func__);
349 }
350
351 void
ssh_sandbox_parent_preauth(struct ssh_sandbox * box,pid_t child_pid)352 ssh_sandbox_parent_preauth(struct ssh_sandbox *box, pid_t child_pid)
353 {
354 box->child_pid = child_pid;
355 }
356
357 #endif /* SANDBOX_SECCOMP_FILTER */
358