1 /* 2 * Copyright (c) 2012 Will Drewry <wad (at) 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/audit.h> 47 #include <linux/filter.h> 48 #include <linux/seccomp.h> 49 #include <elf.h> 50 51 #include <asm/unistd.h> 52 53 #include <errno.h> 54 #include <signal.h> 55 #include <stdarg.h> 56 #include <stddef.h> /* for offsetof */ 57 #include <stdio.h> 58 #include <stdlib.h> 59 #include <string.h> 60 #include <unistd.h> 61 62 #include "log.h" 63 #include "ssh-sandbox.h" 64 #include "xmalloc.h" 65 66 /* Linux seccomp_filter sandbox */ 67 #define SECCOMP_FILTER_FAIL SECCOMP_RET_KILL 68 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 */ 74 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) 82 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), 95 SC_ALLOW(getpid), 96 SC_ALLOW(gettimeofday), 97 SC_ALLOW(clock_gettime), 98 #ifdef __NR_time /* not defined on EABI ARM */ 99 SC_ALLOW(time), 100 #endif 101 SC_ALLOW(read), 102 SC_ALLOW(write), 103 SC_ALLOW(close), 104 #ifdef __NR_shutdown /* not defined on archs that go via socketcall(2) */ 105 SC_ALLOW(shutdown), 106 #endif 107 SC_ALLOW(brk), 108 SC_ALLOW(poll), 109 #ifdef __NR__newselect 110 SC_ALLOW(_newselect), 111 #else 112 SC_ALLOW(select), 113 #endif 114 SC_ALLOW(madvise), 115 #ifdef __NR_mmap2 /* EABI ARM only has mmap2() */ 116 SC_ALLOW(mmap2), 117 #endif 118 #ifdef __NR_mmap 119 SC_ALLOW(mmap), 120 #endif 121 #ifdef __dietlibc__ 122 SC_ALLOW(mremap), 123 SC_ALLOW(exit), 124 #endif 125 SC_ALLOW(munmap), 126 SC_ALLOW(exit_group), 127 #ifdef __NR_rt_sigprocmask 128 SC_ALLOW(rt_sigprocmask), 129 #else 130 SC_ALLOW(sigprocmask), 131 #endif 132 BPF_STMT(BPF_RET+BPF_K, SECCOMP_FILTER_FAIL), 133 }; 134 135 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, 138 }; 139 140 struct ssh_sandbox { 141 pid_t child_pid; 142 }; 143 144 struct ssh_sandbox * 145 ssh_sandbox_init(struct monitor *monitor) 146 { 147 struct ssh_sandbox *box; 148 149 /* 150 * Strictly, we don't need to maintain any state here but we need 151 * to return non-NULL to satisfy the API. 152 */ 153 debug3("%s: preparing seccomp filter sandbox", __func__); 154 box = xcalloc(1, sizeof(*box)); 155 box->child_pid = 0; 156 157 return box; 158 } 159 160 #ifdef SANDBOX_SECCOMP_FILTER_DEBUG 161 extern struct monitor *pmonitor; 162 void mm_log_handler(LogLevel level, const char *msg, void *ctx); 163 164 static void 165 ssh_sandbox_violation(int signum, siginfo_t *info, void *void_context) 166 { 167 char msg[256]; 168 169 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); 173 _exit(1); 174 } 175 176 static void 177 ssh_sandbox_child_debugging(void) 178 { 179 struct sigaction act; 180 sigset_t mask; 181 182 debug3("%s: installing SIGSYS handler", __func__); 183 memset(&act, 0, sizeof(act)); 184 sigemptyset(&mask); 185 sigaddset(&mask, SIGSYS); 186 187 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)); 194 } 195 #endif /* SANDBOX_SECCOMP_FILTER_DEBUG */ 196 197 void 198 ssh_sandbox_child(struct ssh_sandbox *box) 199 { 200 struct rlimit rl_zero; 201 int nnp_failed = 0; 202 203 /* 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)); 214 215 #ifdef SANDBOX_SECCOMP_FILTER_DEBUG 216 ssh_sandbox_child_debugging(); 217 #endif /* SANDBOX_SECCOMP_FILTER_DEBUG */ 218 219 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)); 223 nnp_failed = 1; 224 } 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)); 229 else if (nnp_failed) 230 fatal("%s: SECCOMP_MODE_FILTER activated but " 231 "PR_SET_NO_NEW_PRIVS failed", __func__); 232 } 233 234 void 235 ssh_sandbox_parent_finish(struct ssh_sandbox *box) 236 { 237 free(box); 238 debug3("%s: finished", __func__); 239 } 240 241 void 242 ssh_sandbox_parent_preauth(struct ssh_sandbox *box, pid_t child_pid) 243 { 244 box->child_pid = child_pid; 245 } 246 247 #endif /* SANDBOX_SECCOMP_FILTER */ 248
