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/audit.h>
47 #include <linux/filter.h>
48 #include <linux/seccomp.h>
51 #include <asm/unistd.h>
56 #include <stddef.h> /* for offsetof */
63 #include "ssh-sandbox.h"
66 /* Linux seccomp_filter sandbox */
67 #define SECCOMP_FILTER_FAIL SECCOMP_RET_KILL
69 /* Use a signal handler to emit violations when debugging */
70 #ifdef SANDBOX_SECCOMP_FILTER_DEBUG
71 # undef SECCOMP_FILTER_FAIL
72 # define SECCOMP_FILTER_FAIL SECCOMP_RET_TRAP
73 #endif /* SANDBOX_SECCOMP_FILTER_DEBUG */
75 /* Simple helpers to avoid manual errors (but larger BPF programs). */
76 #define SC_DENY(_nr, _errno) \
77 BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, __NR_ ## _nr, 0, 1), \
78 BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_ERRNO|(_errno))
79 #define SC_ALLOW(_nr) \
80 BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, __NR_ ## _nr, 0, 1), \
81 BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_ALLOW)
83 /* Syscall filtering set for preauth. */
84 static const struct sock_filter preauth_insns[] = {
85 /* Ensure the syscall arch convention is as expected. */
86 BPF_STMT(BPF_LD+BPF_W+BPF_ABS,
87 offsetof(struct seccomp_data, arch)),
88 BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, SECCOMP_AUDIT_ARCH, 1, 0),
89 BPF_STMT(BPF_RET+BPF_K, SECCOMP_FILTER_FAIL),
90 /* Load the syscall number for checking. */
91 BPF_STMT(BPF_LD+BPF_W+BPF_ABS,
92 offsetof(struct seccomp_data, nr)),
93 SC_DENY(open, EACCES),
94 SC_DENY(stat, EACCES),
96 SC_ALLOW(gettimeofday),
97 SC_ALLOW(clock_gettime),
98 #ifdef __NR_time /* not defined on EABI ARM */
104 #ifdef __NR_shutdown /* not defined on archs that go via socketcall(2) */
109 #ifdef __NR__newselect
110 SC_ALLOW(_newselect),
115 #ifdef __NR_mmap2 /* EABI ARM only has mmap2() */
126 SC_ALLOW(exit_group),
127 #ifdef __NR_rt_sigprocmask
128 SC_ALLOW(rt_sigprocmask),
130 SC_ALLOW(sigprocmask),
132 BPF_STMT(BPF_RET+BPF_K, SECCOMP_FILTER_FAIL),
135 static const struct sock_fprog preauth_program = {
136 .len = (unsigned short)(sizeof(preauth_insns)/sizeof(preauth_insns[0])),
137 .filter = (struct sock_filter *)preauth_insns,
145 ssh_sandbox_init(struct monitor *monitor)
147 struct ssh_sandbox *box;
150 * Strictly, we don't need to maintain any state here but we need
151 * to return non-NULL to satisfy the API.
153 debug3("%s: preparing seccomp filter sandbox", __func__);
154 box = xcalloc(1, sizeof(*box));
160 #ifdef SANDBOX_SECCOMP_FILTER_DEBUG
161 extern struct monitor *pmonitor;
162 void mm_log_handler(LogLevel level, const char *msg, void *ctx);
165 ssh_sandbox_violation(int signum, siginfo_t *info, void *void_context)
169 snprintf(msg, sizeof(msg),
170 "%s: unexpected system call (arch:0x%x,syscall:%d @ %p)",
171 __func__, info->si_arch, info->si_syscall, info->si_call_addr);
172 mm_log_handler(SYSLOG_LEVEL_FATAL, msg, pmonitor);
177 ssh_sandbox_child_debugging(void)
179 struct sigaction act;
182 debug3("%s: installing SIGSYS handler", __func__);
183 memset(&act, 0, sizeof(act));
185 sigaddset(&mask, SIGSYS);
187 act.sa_sigaction = &ssh_sandbox_violation;
188 act.sa_flags = SA_SIGINFO;
189 if (sigaction(SIGSYS, &act, NULL) == -1)
190 fatal("%s: sigaction(SIGSYS): %s", __func__, strerror(errno));
191 if (sigprocmask(SIG_UNBLOCK, &mask, NULL) == -1)
192 fatal("%s: sigprocmask(SIGSYS): %s",
193 __func__, strerror(errno));
195 #endif /* SANDBOX_SECCOMP_FILTER_DEBUG */
198 ssh_sandbox_child(struct ssh_sandbox *box)
200 struct rlimit rl_zero;
203 /* Set rlimits for completeness if possible. */
204 rl_zero.rlim_cur = rl_zero.rlim_max = 0;
205 if (setrlimit(RLIMIT_FSIZE, &rl_zero) == -1)
206 fatal("%s: setrlimit(RLIMIT_FSIZE, { 0, 0 }): %s",
207 __func__, strerror(errno));
208 if (setrlimit(RLIMIT_NOFILE, &rl_zero) == -1)
209 fatal("%s: setrlimit(RLIMIT_NOFILE, { 0, 0 }): %s",
210 __func__, strerror(errno));
211 if (setrlimit(RLIMIT_NPROC, &rl_zero) == -1)
212 fatal("%s: setrlimit(RLIMIT_NPROC, { 0, 0 }): %s",
213 __func__, strerror(errno));
215 #ifdef SANDBOX_SECCOMP_FILTER_DEBUG
216 ssh_sandbox_child_debugging();
217 #endif /* SANDBOX_SECCOMP_FILTER_DEBUG */
219 debug3("%s: setting PR_SET_NO_NEW_PRIVS", __func__);
220 if (prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0) == -1) {
221 debug("%s: prctl(PR_SET_NO_NEW_PRIVS): %s",
222 __func__, strerror(errno));
225 debug3("%s: attaching seccomp filter program", __func__);
226 if (prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, &preauth_program) == -1)
227 debug("%s: prctl(PR_SET_SECCOMP): %s",
228 __func__, strerror(errno));
230 fatal("%s: SECCOMP_MODE_FILTER activated but "
231 "PR_SET_NO_NEW_PRIVS failed", __func__);
235 ssh_sandbox_parent_finish(struct ssh_sandbox *box)
238 debug3("%s: finished", __func__);
242 ssh_sandbox_parent_preauth(struct ssh_sandbox *box, pid_t child_pid)
244 box->child_pid = child_pid;
247 #endif /* SANDBOX_SECCOMP_FILTER */