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 #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 */
35
36 #include "includes.h"
37
38 #ifdef SANDBOX_SECCOMP_FILTER
39
40 #include <sys/types.h>
41 #include <sys/resource.h>
42 #include <sys/prctl.h>
43
44 #include <linux/audit.h>
45 #include <linux/filter.h>
46 #include <linux/seccomp.h>
47 #include <elf.h>
48
49 #include <asm/unistd.h>
50
51 #include <errno.h>
52 #include <signal.h>
53 #include <stdarg.h>
54 #include <stddef.h> /* for offsetof */
55 #include <stdio.h>
56 #include <stdlib.h>
57 #include <string.h>
58 #include <unistd.h>
59
60 #include "log.h"
61 #include "ssh-sandbox.h"
62 #include "xmalloc.h"
63
64 /* Linux seccomp_filter sandbox */
65 #define SECCOMP_FILTER_FAIL SECCOMP_RET_KILL
66
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 */
72
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)
80
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),
92 SC_ALLOW(getpid),
93 SC_ALLOW(gettimeofday),
94 SC_ALLOW(clock_gettime),
95 #ifdef __NR_time /* not defined on EABI ARM */
96 SC_ALLOW(time),
97 #endif
98 SC_ALLOW(read),
99 SC_ALLOW(write),
100 SC_ALLOW(close),
101 #ifdef __NR_shutdown /* not defined on archs that go via socketcall(2) */
102 SC_ALLOW(shutdown),
103 #endif
104 SC_ALLOW(brk),
105 SC_ALLOW(poll),
106 #ifdef __NR__newselect
107 SC_ALLOW(_newselect),
108 #else
109 SC_ALLOW(select),
110 #endif
111 SC_ALLOW(madvise),
112 #ifdef __NR_mmap2 /* EABI ARM only has mmap2() */
113 SC_ALLOW(mmap2),
114 #endif
115 #ifdef __NR_mmap
116 SC_ALLOW(mmap),
117 #endif
118 SC_ALLOW(munmap),
119 SC_ALLOW(exit_group),
120 #ifdef __NR_rt_sigprocmask
121 SC_ALLOW(rt_sigprocmask),
122 #else
123 SC_ALLOW(sigprocmask),
124 #endif
125 BPF_STMT(BPF_RET+BPF_K, SECCOMP_FILTER_FAIL),
126 };
127
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,
131 };
132
133 struct ssh_sandbox {
134 pid_t child_pid;
135 };
136
137 struct ssh_sandbox *
ssh_sandbox_init(struct monitor * monitor)138 ssh_sandbox_init(struct monitor *monitor)
139 {
140 struct ssh_sandbox *box;
141
142 /*
143 * Strictly, we don't need to maintain any state here but we need
144 * to return non-NULL to satisfy the API.
145 */
146 debug3("%s: preparing seccomp filter sandbox", __func__);
147 box = xcalloc(1, sizeof(*box));
148 box->child_pid = 0;
149
150 return box;
151 }
152
153 #ifdef SANDBOX_SECCOMP_FILTER_DEBUG
154 extern struct monitor *pmonitor;
155 void mm_log_handler(LogLevel level, const char *msg, void *ctx);
156
157 static void
ssh_sandbox_violation(int signum,siginfo_t * info,void * void_context)158 ssh_sandbox_violation(int signum, siginfo_t *info, void *void_context)
159 {
160 char msg[256];
161
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);
166 _exit(1);
167 }
168
169 static void
ssh_sandbox_child_debugging(void)170 ssh_sandbox_child_debugging(void)
171 {
172 struct sigaction act;
173 sigset_t mask;
174
175 debug3("%s: installing SIGSYS handler", __func__);
176 memset(&act, 0, sizeof(act));
177 sigemptyset(&mask);
178 sigaddset(&mask, SIGSYS);
179
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));
187 }
188 #endif /* SANDBOX_SECCOMP_FILTER_DEBUG */
189
190 void
ssh_sandbox_child(struct ssh_sandbox * box)191 ssh_sandbox_child(struct ssh_sandbox *box)
192 {
193 struct rlimit rl_zero;
194 int nnp_failed = 0;
195
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));
207
208 #ifdef SANDBOX_SECCOMP_FILTER_DEBUG
209 ssh_sandbox_child_debugging();
210 #endif /* SANDBOX_SECCOMP_FILTER_DEBUG */
211
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));
216 nnp_failed = 1;
217 }
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));
222 else if (nnp_failed)
223 fatal("%s: SECCOMP_MODE_FILTER activated but "
224 "PR_SET_NO_NEW_PRIVS failed", __func__);
225 }
226
227 void
ssh_sandbox_parent_finish(struct ssh_sandbox * box)228 ssh_sandbox_parent_finish(struct ssh_sandbox *box)
229 {
230 free(box);
231 debug3("%s: finished", __func__);
232 }
233
234 void
ssh_sandbox_parent_preauth(struct ssh_sandbox * box,pid_t child_pid)235 ssh_sandbox_parent_preauth(struct ssh_sandbox *box, pid_t child_pid)
236 {
237 box->child_pid = child_pid;
238 }
239
240 #endif /* SANDBOX_SECCOMP_FILTER */
241