2 * Copyright (c) 2012 Will Drewry <wad@dataspill.org>
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.
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.
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.
22 * Instead, live systems the auditctl(8) may be used to monitor failures.
24 * auditctl -a task,always -F uid=<privsep uid>
26 /* #define SANDBOX_SECCOMP_FILTER_DEBUG 1 */
28 /* XXX it should be possible to do logging via the log socket safely */
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 */
40 #ifdef SANDBOX_SECCOMP_FILTER
42 #include <sys/types.h>
43 #include <sys/resource.h>
44 #include <sys/prctl.h>
46 #include <linux/net.h>
47 #include <linux/audit.h>
48 #include <linux/filter.h>
49 #include <linux/seccomp.h>
52 #include <asm/unistd.h>
57 #include <stddef.h> /* for offsetof */
64 #include "ssh-sandbox.h"
67 /* Linux seccomp_filter sandbox */
68 #define SECCOMP_FILTER_FAIL SECCOMP_RET_KILL
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 */
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
83 #error "Unknown endianness"
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))
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)),
121 /* Syscalls to non-fatally deny */
123 SC_DENY(__NR_lstat, EACCES),
126 SC_DENY(__NR_lstat64, EACCES),
129 SC_DENY(__NR_fstat, EACCES),
132 SC_DENY(__NR_fstat64, EACCES),
135 SC_DENY(__NR_open, EACCES),
138 SC_DENY(__NR_openat, EACCES),
140 #ifdef __NR_newfstatat
141 SC_DENY(__NR_newfstatat, EACCES),
144 SC_DENY(__NR_stat, EACCES),
147 SC_DENY(__NR_stat64, EACCES),
150 /* Syscalls to permit */
154 #ifdef __NR_clock_gettime
155 SC_ALLOW(__NR_clock_gettime),
158 SC_ALLOW(__NR_close),
163 #ifdef __NR_exit_group
164 SC_ALLOW(__NR_exit_group),
167 SC_ALLOW(__NR_getpgid),
170 SC_ALLOW(__NR_getpid),
172 #ifdef __NR_getrandom
173 SC_ALLOW(__NR_getrandom),
175 #ifdef __NR_gettimeofday
176 SC_ALLOW(__NR_gettimeofday),
179 SC_ALLOW(__NR_madvise),
185 SC_ALLOW(__NR_mmap2),
188 SC_ALLOW(__NR_mremap),
191 SC_ALLOW(__NR_munmap),
193 #ifdef __NR__newselect
194 SC_ALLOW(__NR__newselect),
200 SC_ALLOW(__NR_pselect6),
205 #ifdef __NR_rt_sigprocmask
206 SC_ALLOW(__NR_rt_sigprocmask),
209 SC_ALLOW(__NR_select),
212 SC_ALLOW(__NR_shutdown),
214 #ifdef __NR_sigprocmask
215 SC_ALLOW(__NR_sigprocmask),
221 SC_ALLOW(__NR_write),
223 #ifdef __NR_socketcall
224 SC_ALLOW_ARG(__NR_socketcall, 0, SYS_SHUTDOWN),
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),
232 #if defined(__x86_64__) && defined(__ILP32__) && defined(__X32_SYSCALL_BIT)
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
238 SC_ALLOW(__NR_clock_gettime & ~__X32_SYSCALL_BIT);
242 BPF_STMT(BPF_RET+BPF_K, SECCOMP_FILTER_FAIL),
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,
255 ssh_sandbox_init(struct monitor *monitor)
257 struct ssh_sandbox *box;
260 * Strictly, we don't need to maintain any state here but we need
261 * to return non-NULL to satisfy the API.
263 debug3("%s: preparing seccomp filter sandbox", __func__);
264 box = xcalloc(1, sizeof(*box));
270 #ifdef SANDBOX_SECCOMP_FILTER_DEBUG
271 extern struct monitor *pmonitor;
272 void mm_log_handler(LogLevel level, const char *msg, void *ctx);
275 ssh_sandbox_violation(int signum, siginfo_t *info, void *void_context)
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);
287 ssh_sandbox_child_debugging(void)
289 struct sigaction act;
292 debug3("%s: installing SIGSYS handler", __func__);
293 memset(&act, 0, sizeof(act));
295 sigaddset(&mask, SIGSYS);
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));
305 #endif /* SANDBOX_SECCOMP_FILTER_DEBUG */
308 ssh_sandbox_child(struct ssh_sandbox *box)
310 struct rlimit rl_zero;
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));
325 #ifdef SANDBOX_SECCOMP_FILTER_DEBUG
326 ssh_sandbox_child_debugging();
327 #endif /* SANDBOX_SECCOMP_FILTER_DEBUG */
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));
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));
340 fatal("%s: SECCOMP_MODE_FILTER activated but "
341 "PR_SET_NO_NEW_PRIVS failed", __func__);
345 ssh_sandbox_parent_finish(struct ssh_sandbox *box)
348 debug3("%s: finished", __func__);
352 ssh_sandbox_parent_preauth(struct ssh_sandbox *box, pid_t child_pid)
354 box->child_pid = child_pid;
357 #endif /* SANDBOX_SECCOMP_FILTER */