sandbox-seccomp-filter.c revision 262566
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. */ 82static 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 SC_ALLOW(brk), 102 SC_ALLOW(poll), 103#ifdef __NR__newselect 104 SC_ALLOW(_newselect), 105#else 106 SC_ALLOW(select), 107#endif 108 SC_ALLOW(madvise), 109#ifdef __NR_mmap2 /* EABI ARM only has mmap2() */ 110 SC_ALLOW(mmap2), 111#endif 112#ifdef __NR_mmap 113 SC_ALLOW(mmap), 114#endif 115 SC_ALLOW(munmap), 116 SC_ALLOW(exit_group), 117#ifdef __NR_rt_sigprocmask 118 SC_ALLOW(rt_sigprocmask), 119#else 120 SC_ALLOW(sigprocmask), 121#endif 122 BPF_STMT(BPF_RET+BPF_K, SECCOMP_FILTER_FAIL), 123}; 124 125static const struct sock_fprog preauth_program = { 126 .len = (unsigned short)(sizeof(preauth_insns)/sizeof(preauth_insns[0])), 127 .filter = (struct sock_filter *)preauth_insns, 128}; 129 130struct ssh_sandbox { 131 pid_t child_pid; 132}; 133 134struct ssh_sandbox * 135ssh_sandbox_init(struct monitor *monitor) 136{ 137 struct ssh_sandbox *box; 138 139 /* 140 * Strictly, we don't need to maintain any state here but we need 141 * to return non-NULL to satisfy the API. 142 */ 143 debug3("%s: preparing seccomp filter sandbox", __func__); 144 box = xcalloc(1, sizeof(*box)); 145 box->child_pid = 0; 146 147 return box; 148} 149 150#ifdef SANDBOX_SECCOMP_FILTER_DEBUG 151extern struct monitor *pmonitor; 152void mm_log_handler(LogLevel level, const char *msg, void *ctx); 153 154static void 155ssh_sandbox_violation(int signum, siginfo_t *info, void *void_context) 156{ 157 char msg[256]; 158 159 snprintf(msg, sizeof(msg), 160 "%s: unexpected system call (arch:0x%x,syscall:%d @ %p)", 161 __func__, info->si_arch, info->si_syscall, info->si_call_addr); 162 mm_log_handler(SYSLOG_LEVEL_FATAL, msg, pmonitor); 163 _exit(1); 164} 165 166static void 167ssh_sandbox_child_debugging(void) 168{ 169 struct sigaction act; 170 sigset_t mask; 171 172 debug3("%s: installing SIGSYS handler", __func__); 173 memset(&act, 0, sizeof(act)); 174 sigemptyset(&mask); 175 sigaddset(&mask, SIGSYS); 176 177 act.sa_sigaction = &ssh_sandbox_violation; 178 act.sa_flags = SA_SIGINFO; 179 if (sigaction(SIGSYS, &act, NULL) == -1) 180 fatal("%s: sigaction(SIGSYS): %s", __func__, strerror(errno)); 181 if (sigprocmask(SIG_UNBLOCK, &mask, NULL) == -1) 182 fatal("%s: sigprocmask(SIGSYS): %s", 183 __func__, strerror(errno)); 184} 185#endif /* SANDBOX_SECCOMP_FILTER_DEBUG */ 186 187void 188ssh_sandbox_child(struct ssh_sandbox *box) 189{ 190 struct rlimit rl_zero; 191 int nnp_failed = 0; 192 193 /* Set rlimits for completeness if possible. */ 194 rl_zero.rlim_cur = rl_zero.rlim_max = 0; 195 if (setrlimit(RLIMIT_FSIZE, &rl_zero) == -1) 196 fatal("%s: setrlimit(RLIMIT_FSIZE, { 0, 0 }): %s", 197 __func__, strerror(errno)); 198 if (setrlimit(RLIMIT_NOFILE, &rl_zero) == -1) 199 fatal("%s: setrlimit(RLIMIT_NOFILE, { 0, 0 }): %s", 200 __func__, strerror(errno)); 201 if (setrlimit(RLIMIT_NPROC, &rl_zero) == -1) 202 fatal("%s: setrlimit(RLIMIT_NPROC, { 0, 0 }): %s", 203 __func__, strerror(errno)); 204 205#ifdef SANDBOX_SECCOMP_FILTER_DEBUG 206 ssh_sandbox_child_debugging(); 207#endif /* SANDBOX_SECCOMP_FILTER_DEBUG */ 208 209 debug3("%s: setting PR_SET_NO_NEW_PRIVS", __func__); 210 if (prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0) == -1) { 211 debug("%s: prctl(PR_SET_NO_NEW_PRIVS): %s", 212 __func__, strerror(errno)); 213 nnp_failed = 1; 214 } 215 debug3("%s: attaching seccomp filter program", __func__); 216 if (prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, &preauth_program) == -1) 217 debug("%s: prctl(PR_SET_SECCOMP): %s", 218 __func__, strerror(errno)); 219 else if (nnp_failed) 220 fatal("%s: SECCOMP_MODE_FILTER activated but " 221 "PR_SET_NO_NEW_PRIVS failed", __func__); 222} 223 224void 225ssh_sandbox_parent_finish(struct ssh_sandbox *box) 226{ 227 free(box); 228 debug3("%s: finished", __func__); 229} 230 231void 232ssh_sandbox_parent_preauth(struct ssh_sandbox *box, pid_t child_pid) 233{ 234 box->child_pid = child_pid; 235} 236 237#endif /* SANDBOX_SECCOMP_FILTER */ 238