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 #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 */
38 #ifdef SANDBOX_SECCOMP_FILTER
40 #include <sys/types.h>
41 #include <sys/resource.h>
42 #include <sys/prctl.h>
44 #include <linux/audit.h>
45 #include <linux/filter.h>
46 #include <linux/seccomp.h>
49 #include <asm/unistd.h>
54 #include <stddef.h> /* for offsetof */
61 #include "ssh-sandbox.h"
64 /* Linux seccomp_filter sandbox */
65 #define SECCOMP_FILTER_FAIL SECCOMP_RET_KILL
67 /* Use a signal handler to emit violations when debugging */
68 #ifdef SANDBOX_SECCOMP_FILTER_DEBUG
69 # undef SECCOMP_FILTER_FAIL
70 # define SECCOMP_FILTER_FAIL SECCOMP_RET_TRAP
71 #endif /* SANDBOX_SECCOMP_FILTER_DEBUG */
73 /* Simple helpers to avoid manual errors (but larger BPF programs). */
74 #define SC_DENY(_nr, _errno) \
75 BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, __NR_ ## _nr, 0, 1), \
76 BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_ERRNO|(_errno))
77 #define SC_ALLOW(_nr) \
78 BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, __NR_ ## _nr, 0, 1), \
79 BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_ALLOW)
81 /* Syscall filtering set for preauth. */
82 static const struct sock_filter preauth_insns[] = {
83 /* Ensure the syscall arch convention is as expected. */
84 BPF_STMT(BPF_LD+BPF_W+BPF_ABS,
85 offsetof(struct seccomp_data, arch)),
86 BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, SECCOMP_AUDIT_ARCH, 1, 0),
87 BPF_STMT(BPF_RET+BPF_K, SECCOMP_FILTER_FAIL),
88 /* Load the syscall number for checking. */
89 BPF_STMT(BPF_LD+BPF_W+BPF_ABS,
90 offsetof(struct seccomp_data, nr)),
91 SC_DENY(open, EACCES),
93 SC_ALLOW(gettimeofday),
94 SC_ALLOW(clock_gettime),
95 #ifdef __NR_time /* not defined on EABI ARM */
101 #ifdef __NR_shutdown /* not defined on archs that go via socketcall(2) */
106 #ifdef __NR__newselect
107 SC_ALLOW(_newselect),
112 #ifdef __NR_mmap2 /* EABI ARM only has mmap2() */
119 SC_ALLOW(exit_group),
120 #ifdef __NR_rt_sigprocmask
121 SC_ALLOW(rt_sigprocmask),
123 SC_ALLOW(sigprocmask),
125 BPF_STMT(BPF_RET+BPF_K, SECCOMP_FILTER_FAIL),
128 static const struct sock_fprog preauth_program = {
129 .len = (unsigned short)(sizeof(preauth_insns)/sizeof(preauth_insns[0])),
130 .filter = (struct sock_filter *)preauth_insns,
138 ssh_sandbox_init(struct monitor *monitor)
140 struct ssh_sandbox *box;
143 * Strictly, we don't need to maintain any state here but we need
144 * to return non-NULL to satisfy the API.
146 debug3("%s: preparing seccomp filter sandbox", __func__);
147 box = xcalloc(1, sizeof(*box));
153 #ifdef SANDBOX_SECCOMP_FILTER_DEBUG
154 extern struct monitor *pmonitor;
155 void mm_log_handler(LogLevel level, const char *msg, void *ctx);
158 ssh_sandbox_violation(int signum, siginfo_t *info, void *void_context)
162 snprintf(msg, sizeof(msg),
163 "%s: unexpected system call (arch:0x%x,syscall:%d @ %p)",
164 __func__, info->si_arch, info->si_syscall, info->si_call_addr);
165 mm_log_handler(SYSLOG_LEVEL_FATAL, msg, pmonitor);
170 ssh_sandbox_child_debugging(void)
172 struct sigaction act;
175 debug3("%s: installing SIGSYS handler", __func__);
176 memset(&act, 0, sizeof(act));
178 sigaddset(&mask, SIGSYS);
180 act.sa_sigaction = &ssh_sandbox_violation;
181 act.sa_flags = SA_SIGINFO;
182 if (sigaction(SIGSYS, &act, NULL) == -1)
183 fatal("%s: sigaction(SIGSYS): %s", __func__, strerror(errno));
184 if (sigprocmask(SIG_UNBLOCK, &mask, NULL) == -1)
185 fatal("%s: sigprocmask(SIGSYS): %s",
186 __func__, strerror(errno));
188 #endif /* SANDBOX_SECCOMP_FILTER_DEBUG */
191 ssh_sandbox_child(struct ssh_sandbox *box)
193 struct rlimit rl_zero;
196 /* Set rlimits for completeness if possible. */
197 rl_zero.rlim_cur = rl_zero.rlim_max = 0;
198 if (setrlimit(RLIMIT_FSIZE, &rl_zero) == -1)
199 fatal("%s: setrlimit(RLIMIT_FSIZE, { 0, 0 }): %s",
200 __func__, strerror(errno));
201 if (setrlimit(RLIMIT_NOFILE, &rl_zero) == -1)
202 fatal("%s: setrlimit(RLIMIT_NOFILE, { 0, 0 }): %s",
203 __func__, strerror(errno));
204 if (setrlimit(RLIMIT_NPROC, &rl_zero) == -1)
205 fatal("%s: setrlimit(RLIMIT_NPROC, { 0, 0 }): %s",
206 __func__, strerror(errno));
208 #ifdef SANDBOX_SECCOMP_FILTER_DEBUG
209 ssh_sandbox_child_debugging();
210 #endif /* SANDBOX_SECCOMP_FILTER_DEBUG */
212 debug3("%s: setting PR_SET_NO_NEW_PRIVS", __func__);
213 if (prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0) == -1) {
214 debug("%s: prctl(PR_SET_NO_NEW_PRIVS): %s",
215 __func__, strerror(errno));
218 debug3("%s: attaching seccomp filter program", __func__);
219 if (prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, &preauth_program) == -1)
220 debug("%s: prctl(PR_SET_SECCOMP): %s",
221 __func__, strerror(errno));
223 fatal("%s: SECCOMP_MODE_FILTER activated but "
224 "PR_SET_NO_NEW_PRIVS failed", __func__);
228 ssh_sandbox_parent_finish(struct ssh_sandbox *box)
231 debug3("%s: finished", __func__);
235 ssh_sandbox_parent_preauth(struct ssh_sandbox *box, pid_t child_pid)
237 box->child_pid = child_pid;
240 #endif /* SANDBOX_SECCOMP_FILTER */