samples/seccomp/bpf-direct.c

   1 /*
   2  * Seccomp filter example for x86 (32-bit and 64-bit) with BPF macros
   3  *
   4  * Copyright (c) 2012 The Chromium OS Authors <chromium-os-dev@chromium.org>
   5  * Author: Will Drewry <wad@chromium.org>
   6  *
   7  * The code may be used by anyone for any purpose,
   8  * and can serve as a starting point for developing
   9  * applications using prctl(PR_SET_SECCOMP, 2, ...).
  10  */
  11 #if defined(__i386__) || defined(__x86_64__)
  12 #define SUPPORTED_ARCH 1
  13 #endif
  14
  15 #if defined(SUPPORTED_ARCH)
  16 #define __USE_GNU 1
  17 #define _GNU_SOURCE 1
  18
  19 #include <linux/types.h>
  20 #include <linux/filter.h>
  21 #include <linux/seccomp.h>
  22 #include <linux/unistd.h>
  23 #include <signal.h>
  24 #include <stdio.h>
  25 #include <stddef.h>
  26 #include <string.h>
  27 #include <sys/prctl.h>
  28 #include <unistd.h>
  29
  30 #define syscall_arg(_n) (offsetof(struct seccomp_data, args[_n]))
  31 #define syscall_nr (offsetof(struct seccomp_data, nr))
  32
  33 #if defined(__i386__)
  34 #define REG_RESULT      REG_EAX
  35 #define REG_SYSCALL     REG_EAX
  36 #define REG_ARG0        REG_EBX
  37 #define REG_ARG1        REG_ECX
  38 #define REG_ARG2        REG_EDX
  39 #define REG_ARG3        REG_ESI
  40 #define REG_ARG4        REG_EDI
  41 #define REG_ARG5        REG_EBP
  42 #elif defined(__x86_64__)
  43 #define REG_RESULT      REG_RAX
  44 #define REG_SYSCALL     REG_RAX
  45 #define REG_ARG0        REG_RDI
  46 #define REG_ARG1        REG_RSI
  47 #define REG_ARG2        REG_RDX
  48 #define REG_ARG3        REG_R10
  49 #define REG_ARG4        REG_R8
  50 #define REG_ARG5        REG_R9
  51 #endif
  52
  53 #ifndef PR_SET_NO_NEW_PRIVS
  54 #define PR_SET_NO_NEW_PRIVS 38
  55 #endif
  56
  57 #ifndef SYS_SECCOMP
  58 #define SYS_SECCOMP 1
  59 #endif
  60
  61 static void emulator(int nr, siginfo_t *info, void *void_context)
  62 {
  63         ucontext_t *ctx = (ucontext_t *)(void_context);
  64         int syscall;
  65         char *buf;
  66         ssize_t bytes;
  67         size_t len;
  68         if (info->si_code != SYS_SECCOMP)
  69                 return;
  70         if (!ctx)
  71                 return;
  72         syscall = ctx->uc_mcontext.gregs[REG_SYSCALL];
  73         buf = (char *) ctx->uc_mcontext.gregs[REG_ARG1];
  74         len = (size_t) ctx->uc_mcontext.gregs[REG_ARG2];
  75
  76         if (syscall != __NR_write)
  77                 return;
  78         if (ctx->uc_mcontext.gregs[REG_ARG0] != STDERR_FILENO)
  79                 return;
  80         /* Redirect stderr messages to stdout. Doesn't handle EINTR, etc */
  81         ctx->uc_mcontext.gregs[REG_RESULT] = -1;
  82         if (write(STDOUT_FILENO, "[ERR] ", 6) > 0) {
  83                 bytes = write(STDOUT_FILENO, buf, len);
  84                 ctx->uc_mcontext.gregs[REG_RESULT] = bytes;
  85         }
  86         return;
  87 }
  88
  89 static int install_emulator(void)
  90 {
  91         struct sigaction act;
  92         sigset_t mask;
  93         memset(&act, 0, sizeof(act));
  94         sigemptyset(&mask);
  95         sigaddset(&mask, SIGSYS);
  96
  97         act.sa_sigaction = &emulator;
  98         act.sa_flags = SA_SIGINFO;
  99         if (sigaction(SIGSYS, &act, NULL) < 0) {
 100                 perror("sigaction");
 101                 return -1;
 102         }
 103         if (sigprocmask(SIG_UNBLOCK, &mask, NULL)) {
 104                 perror("sigprocmask");
 105                 return -1;
 106         }
 107         return 0;
 108 }
 109
 110 static int install_filter(void)
 111 {
 112         struct sock_filter filter[] = {
 113                 /* Grab the system call number */
 114                 BPF_STMT(BPF_LD+BPF_W+BPF_ABS, syscall_nr),
 115                 /* Jump table for the allowed syscalls */
 116                 BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, __NR_rt_sigreturn, 0, 1),
 117                 BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_ALLOW),
 118 #ifdef __NR_sigreturn
 119                 BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, __NR_sigreturn, 0, 1),
 120                 BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_ALLOW),
 121 #endif
 122                 BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, __NR_exit_group, 0, 1),
 123                 BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_ALLOW),
 124                 BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, __NR_exit, 0, 1),
 125                 BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_ALLOW),
 126                 BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, __NR_read, 1, 0),
 127                 BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, __NR_write, 3, 2),
 128
 129                 /* Check that read is only using stdin. */
 130                 BPF_STMT(BPF_LD+BPF_W+BPF_ABS, syscall_arg(0)),
 131                 BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, STDIN_FILENO, 4, 0),
 132                 BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_KILL),
 133
 134                 /* Check that write is only using stdout */
 135                 BPF_STMT(BPF_LD+BPF_W+BPF_ABS, syscall_arg(0)),
 136                 BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, STDOUT_FILENO, 1, 0),
 137                 /* Trap attempts to write to stderr */
 138                 BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, STDERR_FILENO, 1, 2),
 139
 140                 BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_ALLOW),
 141                 BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_TRAP),
 142                 BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_KILL),
 143         };
 144         struct sock_fprog prog = {
 145                 .len = (unsigned short)(sizeof(filter)/sizeof(filter[0])),
 146                 .filter = filter,
 147         };
 148
 149         if (prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0)) {
 150                 perror("prctl(NO_NEW_PRIVS)");
 151                 return 1;
 152         }
 153
 154
 155         if (prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, &prog)) {
 156                 perror("prctl");
 157                 return 1;
 158         }
 159         return 0;
 160 }
 161
 162 #define payload(_c) (_c), sizeof((_c))
 163 int main(int argc, char **argv)
 164 {
 165         char buf[4096];
 166         ssize_t bytes = 0;
 167         if (install_emulator())
 168                 return 1;
 169         if (install_filter())
 170                 return 1;
 171         syscall(__NR_write, STDOUT_FILENO,
 172                 payload("OHAI! WHAT IS YOUR NAME? "));
 173         bytes = syscall(__NR_read, STDIN_FILENO, buf, sizeof(buf));
 174         syscall(__NR_write, STDOUT_FILENO, payload("HELLO, "));
 175         syscall(__NR_write, STDOUT_FILENO, buf, bytes);
 176         syscall(__NR_write, STDERR_FILENO,
 177                 payload("Error message going to STDERR\n"));
 178         return 0;
 179 }
 180 #else   /* SUPPORTED_ARCH */
 181 /*
 182  * This sample is x86-only.  Since kernel samples are compiled with the
 183  * host toolchain, a non-x86 host will result in using only the main()
 184  * below.
 185  */
 186 int main(void)
 187 {
 188         return 1;
 189 }
 190 #endif  /* SUPPORTED_ARCH */