samples/seccomp/bpf-direct.c

   1 // SPDX-License-Identifier: GPL-2.0
   2 /*
   3  * Seccomp filter example for x86 (32-bit and 64-bit) with BPF macros
   4  *
   5  * Copyright (c) 2012 The Chromium OS Authors <chromium-os-dev@chromium.org>
   6  * Author: Will Drewry <wad@chromium.org>
   7  *
   8  * The code may be used by anyone for any purpose,
   9  * and can serve as a starting point for developing
  10  * applications using prctl(PR_SET_SECCOMP, 2, ...).
  11  */
  12 #if defined(__i386__) || defined(__x86_64__)
  13 #define SUPPORTED_ARCH 1
  14 #endif
  15
  16 #if defined(SUPPORTED_ARCH)
  17 #define __USE_GNU 1
  18 #define _GNU_SOURCE 1
  19
  20 #include <linux/types.h>
  21 #include <linux/filter.h>
  22 #include <linux/seccomp.h>
  23 #include <linux/unistd.h>
  24 #include <signal.h>
  25 #include <stdio.h>
  26 #include <stddef.h>
  27 #include <string.h>
  28 #include <sys/prctl.h>
  29 #include <unistd.h>
  30
  31 #define syscall_arg(_n) (offsetof(struct seccomp_data, args[_n]))
  32 #define syscall_nr (offsetof(struct seccomp_data, nr))
  33
  34 #if defined(__i386__)
  35 #define REG_RESULT      REG_EAX
  36 #define REG_SYSCALL     REG_EAX
  37 #define REG_ARG0        REG_EBX
  38 #define REG_ARG1        REG_ECX
  39 #define REG_ARG2        REG_EDX
  40 #define REG_ARG3        REG_ESI
  41 #define REG_ARG4        REG_EDI
  42 #define REG_ARG5        REG_EBP
  43 #elif defined(__x86_64__)
  44 #define REG_RESULT      REG_RAX
  45 #define REG_SYSCALL     REG_RAX
  46 #define REG_ARG0        REG_RDI
  47 #define REG_ARG1        REG_RSI
  48 #define REG_ARG2        REG_RDX
  49 #define REG_ARG3        REG_R10
  50 #define REG_ARG4        REG_R8
  51 #define REG_ARG5        REG_R9
  52 #endif
  53
  54 #ifndef PR_SET_NO_NEW_PRIVS
  55 #define PR_SET_NO_NEW_PRIVS 38
  56 #endif
  57
  58 #ifndef SYS_SECCOMP
  59 #define SYS_SECCOMP 1
  60 #endif
  61
  62 static void emulator(int nr, siginfo_t *info, void *void_context)
  63 {
  64         ucontext_t *ctx = (ucontext_t *)(void_context);
  65         int syscall;
  66         char *buf;
  67         ssize_t bytes;
  68         size_t len;
  69         if (info->si_code != SYS_SECCOMP)
  70                 return;
  71         if (!ctx)
  72                 return;
  73         syscall = ctx->uc_mcontext.gregs[REG_SYSCALL];
  74         buf = (char *) ctx->uc_mcontext.gregs[REG_ARG1];
  75         len = (size_t) ctx->uc_mcontext.gregs[REG_ARG2];
  76
  77         if (syscall != __NR_write)
  78                 return;
  79         if (ctx->uc_mcontext.gregs[REG_ARG0] != STDERR_FILENO)
  80                 return;
  81         /* Redirect stderr messages to stdout. Doesn't handle EINTR, etc */
  82         ctx->uc_mcontext.gregs[REG_RESULT] = -1;
  83         if (write(STDOUT_FILENO, "[ERR] ", 6) > 0) {
  84                 bytes = write(STDOUT_FILENO, buf, len);
  85                 ctx->uc_mcontext.gregs[REG_RESULT] = bytes;
  86         }
  87         return;
  88 }
  89
  90 static int install_emulator(void)
  91 {
  92         struct sigaction act;
  93         sigset_t mask;
  94         memset(&act, 0, sizeof(act));
  95         sigemptyset(&mask);
  96         sigaddset(&mask, SIGSYS);
  97
  98         act.sa_sigaction = &emulator;
  99         act.sa_flags = SA_SIGINFO;
 100         if (sigaction(SIGSYS, &act, NULL) < 0) {
 101                 perror("sigaction");
 102                 return -1;
 103         }
 104         if (sigprocmask(SIG_UNBLOCK, &mask, NULL)) {
 105                 perror("sigprocmask");
 106                 return -1;
 107         }
 108         return 0;
 109 }
 110
 111 static int install_filter(void)
 112 {
 113         struct sock_filter filter[] = {
 114                 /* Grab the system call number */
 115                 BPF_STMT(BPF_LD+BPF_W+BPF_ABS, syscall_nr),
 116                 /* Jump table for the allowed syscalls */
 117                 BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, __NR_rt_sigreturn, 0, 1),
 118                 BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_ALLOW),
 119 #ifdef __NR_sigreturn
 120                 BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, __NR_sigreturn, 0, 1),
 121                 BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_ALLOW),
 122 #endif
 123                 BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, __NR_exit_group, 0, 1),
 124                 BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_ALLOW),
 125                 BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, __NR_exit, 0, 1),
 126                 BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_ALLOW),
 127                 BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, __NR_read, 1, 0),
 128                 BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, __NR_write, 3, 2),
 129
 130                 /* Check that read is only using stdin. */
 131                 BPF_STMT(BPF_LD+BPF_W+BPF_ABS, syscall_arg(0)),
 132                 BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, STDIN_FILENO, 4, 0),
 133                 BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_KILL),
 134
 135                 /* Check that write is only using stdout */
 136                 BPF_STMT(BPF_LD+BPF_W+BPF_ABS, syscall_arg(0)),
 137                 BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, STDOUT_FILENO, 1, 0),
 138                 /* Trap attempts to write to stderr */
 139                 BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, STDERR_FILENO, 1, 2),
 140
 141                 BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_ALLOW),
 142                 BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_TRAP),
 143                 BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_KILL),
 144         };
 145         struct sock_fprog prog = {
 146                 .len = (unsigned short)(sizeof(filter)/sizeof(filter[0])),
 147                 .filter = filter,
 148         };
 149
 150         if (prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0)) {
 151                 perror("prctl(NO_NEW_PRIVS)");
 152                 return 1;
 153         }
 154
 155
 156         if (prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, &prog)) {
 157                 perror("prctl");
 158                 return 1;
 159         }
 160         return 0;
 161 }
 162
 163 #define payload(_c) (_c), sizeof((_c))
 164 int main(int argc, char **argv)
 165 {
 166         char buf[4096];
 167         ssize_t bytes = 0;
 168         if (install_emulator())
 169                 return 1;
 170         if (install_filter())
 171                 return 1;
 172         syscall(__NR_write, STDOUT_FILENO,
 173                 payload("OHAI! WHAT IS YOUR NAME? "));
 174         bytes = syscall(__NR_read, STDIN_FILENO, buf, sizeof(buf));
 175         syscall(__NR_write, STDOUT_FILENO, payload("HELLO, "));
 176         syscall(__NR_write, STDOUT_FILENO, buf, bytes);
 177         syscall(__NR_write, STDERR_FILENO,
 178                 payload("Error message going to STDERR\n"));
 179         return 0;
 180 }
 181 #else   /* SUPPORTED_ARCH */
 182 /*
 183  * This sample is x86-only.  Since kernel samples are compiled with the
 184  * host toolchain, a non-x86 host will result in using only the main()
 185  * below.
 186  */
 187 int main(void)
 188 {
 189         return 1;
 190 }
 191 #endif  /* SUPPORTED_ARCH */