crypto/openssh/sandbox-seccomp-filter.c

   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 /* 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/net.h>
  47 #include <linux/audit.h>
  48 #include <linux/filter.h>
  49 #include <linux/seccomp.h>
  50 #include <elf.h>
  51
  52 #include <asm/unistd.h>
  53
  54 #include <errno.h>
  55 #include <signal.h>
  56 #include <stdarg.h>
  57 #include <stddef.h>  /* for offsetof */
  58 #include <stdio.h>
  59 #include <stdlib.h>
  60 #include <string.h>
  61 #include <unistd.h>
  62
  63 #include "log.h"
  64 #include "ssh-sandbox.h"
  65 #include "xmalloc.h"
  66
  67 /* Linux seccomp_filter sandbox */
  68 #define SECCOMP_FILTER_FAIL SECCOMP_RET_KILL
  69
  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 */
  75
  76 /* Simple helpers to avoid manual errors (but larger BPF programs). */
  77 #define SC_DENY(_nr, _errno) \
  78         BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, __NR_ ## _nr, 0, 1), \
  79         BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_ERRNO|(_errno))
  80 #define SC_ALLOW(_nr) \
  81         BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, __NR_ ## _nr, 0, 1), \
  82         BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_ALLOW)
  83 #define SC_ALLOW_ARG(_nr, _arg_nr, _arg_val) \
  84         BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, __NR_ ## _nr, 0, 4), \
  85         /* load first syscall argument */ \
  86         BPF_STMT(BPF_LD+BPF_W+BPF_ABS, \
  87             offsetof(struct seccomp_data, args[(_arg_nr)])), \
  88         BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, (_arg_val), 0, 1), \
  89         BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_ALLOW), \
  90         /* reload syscall number; all rules expect it in accumulator */ \
  91         BPF_STMT(BPF_LD+BPF_W+BPF_ABS, \
  92                 offsetof(struct seccomp_data, nr))
  93
  94 /* Syscall filtering set for preauth. */
  95 static const struct sock_filter preauth_insns[] = {
  96         /* Ensure the syscall arch convention is as expected. */
  97         BPF_STMT(BPF_LD+BPF_W+BPF_ABS,
  98                 offsetof(struct seccomp_data, arch)),
  99         BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, SECCOMP_AUDIT_ARCH, 1, 0),
 100         BPF_STMT(BPF_RET+BPF_K, SECCOMP_FILTER_FAIL),
 101         /* Load the syscall number for checking. */
 102         BPF_STMT(BPF_LD+BPF_W+BPF_ABS,
 103                 offsetof(struct seccomp_data, nr)),
 104
 105         /* Syscalls to non-fatally deny */
 106 #ifdef __NR_fstat
 107         SC_DENY(fstat, EACCES),
 108 #endif
 109 #ifdef __NR_fstat64
 110         SC_DENY(fstat64, EACCES),
 111 #endif
 112 #ifdef __NR_open
 113         SC_DENY(open, EACCES),
 114 #endif
 115 #ifdef __NR_openat
 116         SC_DENY(openat, EACCES),
 117 #endif
 118 #ifdef __NR_newfstatat
 119         SC_DENY(newfstatat, EACCES),
 120 #endif
 121 #ifdef __NR_stat
 122         SC_DENY(stat, EACCES),
 123 #endif
 124 #ifdef __NR_stat64
 125         SC_DENY(stat64, EACCES),
 126 #endif
 127
 128         /* Syscalls to permit */
 129 #ifdef __NR_brk
 130         SC_ALLOW(brk),
 131 #endif
 132 #ifdef __NR_clock_gettime
 133         SC_ALLOW(clock_gettime),
 134 #endif
 135 #ifdef __NR_close
 136         SC_ALLOW(close),
 137 #endif
 138 #ifdef __NR_exit
 139         SC_ALLOW(exit),
 140 #endif
 141 #ifdef __NR_exit_group
 142         SC_ALLOW(exit_group),
 143 #endif
 144 #ifdef __NR_getpgid
 145         SC_ALLOW(getpgid),
 146 #endif
 147 #ifdef __NR_getpid
 148         SC_ALLOW(getpid),
 149 #endif
 150 #ifdef __NR_getrandom
 151         SC_ALLOW(getrandom),
 152 #endif
 153 #ifdef __NR_gettimeofday
 154         SC_ALLOW(gettimeofday),
 155 #endif
 156 #ifdef __NR_madvise
 157         SC_ALLOW(madvise),
 158 #endif
 159 #ifdef __NR_mmap
 160         SC_ALLOW(mmap),
 161 #endif
 162 #ifdef __NR_mmap2
 163         SC_ALLOW(mmap2),
 164 #endif
 165 #ifdef __NR_mremap
 166         SC_ALLOW(mremap),
 167 #endif
 168 #ifdef __NR_munmap
 169         SC_ALLOW(munmap),
 170 #endif
 171 #ifdef __NR__newselect
 172         SC_ALLOW(_newselect),
 173 #endif
 174 #ifdef __NR_poll
 175         SC_ALLOW(poll),
 176 #endif
 177 #ifdef __NR_pselect6
 178         SC_ALLOW(pselect6),
 179 #endif
 180 #ifdef __NR_read
 181         SC_ALLOW(read),
 182 #endif
 183 #ifdef __NR_rt_sigprocmask
 184         SC_ALLOW(rt_sigprocmask),
 185 #endif
 186 #ifdef __NR_select
 187         SC_ALLOW(select),
 188 #endif
 189 #ifdef __NR_shutdown
 190         SC_ALLOW(shutdown),
 191 #endif
 192 #ifdef __NR_sigprocmask
 193         SC_ALLOW(sigprocmask),
 194 #endif
 195 #ifdef __NR_time
 196         SC_ALLOW(time),
 197 #endif
 198 #ifdef __NR_write
 199         SC_ALLOW(write),
 200 #endif
 201 #ifdef __NR_socketcall
 202         SC_ALLOW_ARG(socketcall, 0, SYS_SHUTDOWN),
 203 #endif
 204
 205         /* Default deny */
 206         BPF_STMT(BPF_RET+BPF_K, SECCOMP_FILTER_FAIL),
 207 };
 208
 209 static const struct sock_fprog preauth_program = {
 210         .len = (unsigned short)(sizeof(preauth_insns)/sizeof(preauth_insns[0])),
 211         .filter = (struct sock_filter *)preauth_insns,
 212 };
 213
 214 struct ssh_sandbox {
 215         pid_t child_pid;
 216 };
 217
 218 struct ssh_sandbox *
 219 ssh_sandbox_init(struct monitor *monitor)
 220 {
 221         struct ssh_sandbox *box;
 222
 223         /*
 224          * Strictly, we don't need to maintain any state here but we need
 225          * to return non-NULL to satisfy the API.
 226          */
 227         debug3("%s: preparing seccomp filter sandbox", __func__);
 228         box = xcalloc(1, sizeof(*box));
 229         box->child_pid = 0;
 230
 231         return box;
 232 }
 233
 234 #ifdef SANDBOX_SECCOMP_FILTER_DEBUG
 235 extern struct monitor *pmonitor;
 236 void mm_log_handler(LogLevel level, const char *msg, void *ctx);
 237
 238 static void
 239 ssh_sandbox_violation(int signum, siginfo_t *info, void *void_context)
 240 {
 241         char msg[256];
 242
 243         snprintf(msg, sizeof(msg),
 244             "%s: unexpected system call (arch:0x%x,syscall:%d @ %p)",
 245             __func__, info->si_arch, info->si_syscall, info->si_call_addr);
 246         mm_log_handler(SYSLOG_LEVEL_FATAL, msg, pmonitor);
 247         _exit(1);
 248 }
 249
 250 static void
 251 ssh_sandbox_child_debugging(void)
 252 {
 253         struct sigaction act;
 254         sigset_t mask;
 255
 256         debug3("%s: installing SIGSYS handler", __func__);
 257         memset(&act, 0, sizeof(act));
 258         sigemptyset(&mask);
 259         sigaddset(&mask, SIGSYS);
 260
 261         act.sa_sigaction = &ssh_sandbox_violation;
 262         act.sa_flags = SA_SIGINFO;
 263         if (sigaction(SIGSYS, &act, NULL) == -1)
 264                 fatal("%s: sigaction(SIGSYS): %s", __func__, strerror(errno));
 265         if (sigprocmask(SIG_UNBLOCK, &mask, NULL) == -1)
 266                 fatal("%s: sigprocmask(SIGSYS): %s",
 267                       __func__, strerror(errno));
 268 }
 269 #endif /* SANDBOX_SECCOMP_FILTER_DEBUG */
 270
 271 void
 272 ssh_sandbox_child(struct ssh_sandbox *box)
 273 {
 274         struct rlimit rl_zero;
 275         int nnp_failed = 0;
 276
 277         /* Set rlimits for completeness if possible. */
 278         rl_zero.rlim_cur = rl_zero.rlim_max = 0;
 279         if (setrlimit(RLIMIT_FSIZE, &rl_zero) == -1)
 280                 fatal("%s: setrlimit(RLIMIT_FSIZE, { 0, 0 }): %s",
 281                         __func__, strerror(errno));
 282         if (setrlimit(RLIMIT_NOFILE, &rl_zero) == -1)
 283                 fatal("%s: setrlimit(RLIMIT_NOFILE, { 0, 0 }): %s",
 284                         __func__, strerror(errno));
 285         if (setrlimit(RLIMIT_NPROC, &rl_zero) == -1)
 286                 fatal("%s: setrlimit(RLIMIT_NPROC, { 0, 0 }): %s",
 287                         __func__, strerror(errno));
 288
 289 #ifdef SANDBOX_SECCOMP_FILTER_DEBUG
 290         ssh_sandbox_child_debugging();
 291 #endif /* SANDBOX_SECCOMP_FILTER_DEBUG */
 292
 293         debug3("%s: setting PR_SET_NO_NEW_PRIVS", __func__);
 294         if (prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0) == -1) {
 295                 debug("%s: prctl(PR_SET_NO_NEW_PRIVS): %s",
 296                       __func__, strerror(errno));
 297                 nnp_failed = 1;
 298         }
 299         debug3("%s: attaching seccomp filter program", __func__);
 300         if (prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, &preauth_program) == -1)
 301                 debug("%s: prctl(PR_SET_SECCOMP): %s",
 302                       __func__, strerror(errno));
 303         else if (nnp_failed)
 304                 fatal("%s: SECCOMP_MODE_FILTER activated but "
 305                     "PR_SET_NO_NEW_PRIVS failed", __func__);
 306 }
 307
 308 void
 309 ssh_sandbox_parent_finish(struct ssh_sandbox *box)
 310 {
 311         free(box);
 312         debug3("%s: finished", __func__);
 313 }
 314
 315 void
 316 ssh_sandbox_parent_preauth(struct ssh_sandbox *box, pid_t child_pid)
 317 {
 318         box->child_pid = child_pid;
 319 }
 320
 321 #endif /* SANDBOX_SECCOMP_FILTER */