sandbox-seccomp-filter.c revision 248613
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#ifdef __NR_time /* not defined on EABI ARM */ 95 SC_ALLOW(time), 96#endif 97 SC_ALLOW(read), 98 SC_ALLOW(write), 99 SC_ALLOW(close), 100 SC_ALLOW(brk), 101 SC_ALLOW(poll), 102#ifdef __NR__newselect 103 SC_ALLOW(_newselect), 104#else 105 SC_ALLOW(select), 106#endif 107 SC_ALLOW(madvise), 108#ifdef __NR_mmap2 /* EABI ARM only has mmap2() */ 109 SC_ALLOW(mmap2), 110#endif 111#ifdef __NR_mmap 112 SC_ALLOW(mmap), 113#endif 114 SC_ALLOW(munmap), 115 SC_ALLOW(exit_group), 116#ifdef __NR_rt_sigprocmask 117 SC_ALLOW(rt_sigprocmask), 118#else 119 SC_ALLOW(sigprocmask), 120#endif 121 BPF_STMT(BPF_RET+BPF_K, SECCOMP_FILTER_FAIL), 122}; 123 124static const struct sock_fprog preauth_program = { 125 .len = (unsigned short)(sizeof(preauth_insns)/sizeof(preauth_insns[0])), 126 .filter = (struct sock_filter *)preauth_insns, 127}; 128 129struct ssh_sandbox { 130 pid_t child_pid; 131}; 132 133struct ssh_sandbox * 134ssh_sandbox_init(void) 135{ 136 struct ssh_sandbox *box; 137 138 /* 139 * Strictly, we don't need to maintain any state here but we need 140 * to return non-NULL to satisfy the API. 141 */ 142 debug3("%s: preparing seccomp filter sandbox", __func__); 143 box = xcalloc(1, sizeof(*box)); 144 box->child_pid = 0; 145 146 return box; 147} 148 149#ifdef SANDBOX_SECCOMP_FILTER_DEBUG 150extern struct monitor *pmonitor; 151void mm_log_handler(LogLevel level, const char *msg, void *ctx); 152 153static void 154ssh_sandbox_violation(int signum, siginfo_t *info, void *void_context) 155{ 156 char msg[256]; 157 158 snprintf(msg, sizeof(msg), 159 "%s: unexpected system call (arch:0x%x,syscall:%d @ %p)", 160 __func__, info->si_arch, info->si_syscall, info->si_call_addr); 161 mm_log_handler(SYSLOG_LEVEL_FATAL, msg, pmonitor); 162 _exit(1); 163} 164 165static void 166ssh_sandbox_child_debugging(void) 167{ 168 struct sigaction act; 169 sigset_t mask; 170 171 debug3("%s: installing SIGSYS handler", __func__); 172 memset(&act, 0, sizeof(act)); 173 sigemptyset(&mask); 174 sigaddset(&mask, SIGSYS); 175 176 act.sa_sigaction = &ssh_sandbox_violation; 177 act.sa_flags = SA_SIGINFO; 178 if (sigaction(SIGSYS, &act, NULL) == -1) 179 fatal("%s: sigaction(SIGSYS): %s", __func__, strerror(errno)); 180 if (sigprocmask(SIG_UNBLOCK, &mask, NULL) == -1) 181 fatal("%s: sigprocmask(SIGSYS): %s", 182 __func__, strerror(errno)); 183} 184#endif /* SANDBOX_SECCOMP_FILTER_DEBUG */ 185 186void 187ssh_sandbox_child(struct ssh_sandbox *box) 188{ 189 struct rlimit rl_zero; 190 int nnp_failed = 0; 191 192 /* Set rlimits for completeness if possible. */ 193 rl_zero.rlim_cur = rl_zero.rlim_max = 0; 194 if (setrlimit(RLIMIT_FSIZE, &rl_zero) == -1) 195 fatal("%s: setrlimit(RLIMIT_FSIZE, { 0, 0 }): %s", 196 __func__, strerror(errno)); 197 if (setrlimit(RLIMIT_NOFILE, &rl_zero) == -1) 198 fatal("%s: setrlimit(RLIMIT_NOFILE, { 0, 0 }): %s", 199 __func__, strerror(errno)); 200 if (setrlimit(RLIMIT_NPROC, &rl_zero) == -1) 201 fatal("%s: setrlimit(RLIMIT_NPROC, { 0, 0 }): %s", 202 __func__, strerror(errno)); 203 204#ifdef SANDBOX_SECCOMP_FILTER_DEBUG 205 ssh_sandbox_child_debugging(); 206#endif /* SANDBOX_SECCOMP_FILTER_DEBUG */ 207 208 debug3("%s: setting PR_SET_NO_NEW_PRIVS", __func__); 209 if (prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0) == -1) { 210 debug("%s: prctl(PR_SET_NO_NEW_PRIVS): %s", 211 __func__, strerror(errno)); 212 nnp_failed = 1; 213 } 214 debug3("%s: attaching seccomp filter program", __func__); 215 if (prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, &preauth_program) == -1) 216 debug("%s: prctl(PR_SET_SECCOMP): %s", 217 __func__, strerror(errno)); 218 else if (nnp_failed) 219 fatal("%s: SECCOMP_MODE_FILTER activated but " 220 "PR_SET_NO_NEW_PRIVS failed", __func__); 221} 222 223void 224ssh_sandbox_parent_finish(struct ssh_sandbox *box) 225{ 226 free(box); 227 debug3("%s: finished", __func__); 228} 229 230void 231ssh_sandbox_parent_preauth(struct ssh_sandbox *box, pid_t child_pid) 232{ 233 box->child_pid = child_pid; 234} 235 236#endif /* SANDBOX_SECCOMP_FILTER */ 237