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| 1 | +// SPDX-License-Identifier: BSD-3-Clause |
| 2 | +/* |
| 3 | + * Very simple script interpreter that can evaluate two different commands (one |
| 4 | + * per line): |
| 5 | + * - "?" to initialize a counter from user's input; |
| 6 | + * - "+" to increment the counter (which is set to 0 by default). |
| 7 | + * |
| 8 | + * See tools/testing/selftests/exec/check-exec-tests.sh and |
| 9 | + * Documentation/userspace-api/check_exec.rst |
| 10 | + * |
| 11 | + * Copyright © 2024 Microsoft Corporation |
| 12 | + */ |
| 13 | + |
| 14 | +#define _GNU_SOURCE |
| 15 | +#include <errno.h> |
| 16 | +#include <linux/fcntl.h> |
| 17 | +#include <linux/prctl.h> |
| 18 | +#include <linux/securebits.h> |
| 19 | +#include <stdbool.h> |
| 20 | +#include <stdio.h> |
| 21 | +#include <stdlib.h> |
| 22 | +#include <string.h> |
| 23 | +#include <sys/prctl.h> |
| 24 | +#include <unistd.h> |
| 25 | + |
| 26 | +/* Returns 1 on error, 0 otherwise. */ |
| 27 | +static int interpret_buffer(char *buffer, size_t buffer_size) |
| 28 | +{ |
| 29 | + char *line, *saveptr = NULL; |
| 30 | + long long number = 0; |
| 31 | + |
| 32 | + /* Each command is the first character of a line. */ |
| 33 | + saveptr = NULL; |
| 34 | + line = strtok_r(buffer, "\n", &saveptr); |
| 35 | + while (line) { |
| 36 | + if (*line != '#' && strlen(line) != 1) { |
| 37 | + fprintf(stderr, "# ERROR: Unknown string\n"); |
| 38 | + return 1; |
| 39 | + } |
| 40 | + switch (*line) { |
| 41 | + case '#': |
| 42 | + /* Skips shebang and comments. */ |
| 43 | + break; |
| 44 | + case '+': |
| 45 | + /* Increments and prints the number. */ |
| 46 | + number++; |
| 47 | + printf("%lld\n", number); |
| 48 | + break; |
| 49 | + case '?': |
| 50 | + /* Reads integer from stdin. */ |
| 51 | + fprintf(stderr, "> Enter new number: \n"); |
| 52 | + if (scanf("%lld", &number) != 1) { |
| 53 | + fprintf(stderr, |
| 54 | + "# WARNING: Failed to read number from stdin\n"); |
| 55 | + } |
| 56 | + break; |
| 57 | + default: |
| 58 | + fprintf(stderr, "# ERROR: Unknown character '%c'\n", |
| 59 | + *line); |
| 60 | + return 1; |
| 61 | + } |
| 62 | + line = strtok_r(NULL, "\n", &saveptr); |
| 63 | + } |
| 64 | + return 0; |
| 65 | +} |
| 66 | + |
| 67 | +/* Returns 1 on error, 0 otherwise. */ |
| 68 | +static int interpret_stream(FILE *script, char *const script_name, |
| 69 | + char *const *const envp, const bool restrict_stream) |
| 70 | +{ |
| 71 | + int err; |
| 72 | + char *const script_argv[] = { script_name, NULL }; |
| 73 | + char buf[128] = {}; |
| 74 | + size_t buf_size = sizeof(buf); |
| 75 | + |
| 76 | + /* |
| 77 | + * We pass a valid argv and envp to the kernel to emulate a native |
| 78 | + * script execution. We must use the script file descriptor instead of |
| 79 | + * the script path name to avoid race conditions. |
| 80 | + */ |
| 81 | + err = execveat(fileno(script), "", script_argv, envp, |
| 82 | + AT_EMPTY_PATH | AT_EXECVE_CHECK); |
| 83 | + if (err && restrict_stream) { |
| 84 | + perror("ERROR: Script execution check"); |
| 85 | + return 1; |
| 86 | + } |
| 87 | + |
| 88 | + /* Reads script. */ |
| 89 | + buf_size = fread(buf, 1, buf_size - 1, script); |
| 90 | + return interpret_buffer(buf, buf_size); |
| 91 | +} |
| 92 | + |
| 93 | +static void print_usage(const char *argv0) |
| 94 | +{ |
| 95 | + fprintf(stderr, "usage: %s <script.inc> | -i | -c <command>\n\n", |
| 96 | + argv0); |
| 97 | + fprintf(stderr, "Example:\n"); |
| 98 | + fprintf(stderr, " ./set-exec -fi -- ./inc -i < script-exec.inc\n"); |
| 99 | +} |
| 100 | + |
| 101 | +int main(const int argc, char *const argv[], char *const *const envp) |
| 102 | +{ |
| 103 | + int opt; |
| 104 | + char *cmd = NULL; |
| 105 | + char *script_name = NULL; |
| 106 | + bool interpret_stdin = false; |
| 107 | + FILE *script_file = NULL; |
| 108 | + int secbits; |
| 109 | + bool deny_interactive, restrict_file; |
| 110 | + size_t arg_nb; |
| 111 | + |
| 112 | + secbits = prctl(PR_GET_SECUREBITS); |
| 113 | + if (secbits == -1) { |
| 114 | + /* |
| 115 | + * This should never happen, except with a buggy seccomp |
| 116 | + * filter. |
| 117 | + */ |
| 118 | + perror("ERROR: Failed to get securebits"); |
| 119 | + return 1; |
| 120 | + } |
| 121 | + |
| 122 | + deny_interactive = !!(secbits & SECBIT_EXEC_DENY_INTERACTIVE); |
| 123 | + restrict_file = !!(secbits & SECBIT_EXEC_RESTRICT_FILE); |
| 124 | + |
| 125 | + while ((opt = getopt(argc, argv, "c:i")) != -1) { |
| 126 | + switch (opt) { |
| 127 | + case 'c': |
| 128 | + if (cmd) { |
| 129 | + fprintf(stderr, "ERROR: Command already set"); |
| 130 | + return 1; |
| 131 | + } |
| 132 | + cmd = optarg; |
| 133 | + break; |
| 134 | + case 'i': |
| 135 | + interpret_stdin = true; |
| 136 | + break; |
| 137 | + default: |
| 138 | + print_usage(argv[0]); |
| 139 | + return 1; |
| 140 | + } |
| 141 | + } |
| 142 | + |
| 143 | + /* Checks that only one argument is used, or read stdin. */ |
| 144 | + arg_nb = !!cmd + !!interpret_stdin; |
| 145 | + if (arg_nb == 0 && argc == 2) { |
| 146 | + script_name = argv[1]; |
| 147 | + } else if (arg_nb != 1) { |
| 148 | + print_usage(argv[0]); |
| 149 | + return 1; |
| 150 | + } |
| 151 | + |
| 152 | + if (cmd) { |
| 153 | + /* |
| 154 | + * Other kind of interactive interpretations should be denied |
| 155 | + * as well (e.g. CLI arguments passing script snippets, |
| 156 | + * environment variables interpreted as script). However, any |
| 157 | + * way to pass script files should only be restricted according |
| 158 | + * to restrict_file. |
| 159 | + */ |
| 160 | + if (deny_interactive) { |
| 161 | + fprintf(stderr, |
| 162 | + "ERROR: Interactive interpretation denied.\n"); |
| 163 | + return 1; |
| 164 | + } |
| 165 | + |
| 166 | + return interpret_buffer(cmd, strlen(cmd)); |
| 167 | + } |
| 168 | + |
| 169 | + if (interpret_stdin && !script_name) { |
| 170 | + script_file = stdin; |
| 171 | + /* |
| 172 | + * As for any execve(2) call, this path may be logged by the |
| 173 | + * kernel. |
| 174 | + */ |
| 175 | + script_name = "/proc/self/fd/0"; |
| 176 | + /* |
| 177 | + * When stdin is used, it can point to a regular file or a |
| 178 | + * pipe. Restrict stdin execution according to |
| 179 | + * SECBIT_EXEC_DENY_INTERACTIVE but always allow executable |
| 180 | + * files (which are not considered as interactive inputs). |
| 181 | + */ |
| 182 | + return interpret_stream(script_file, script_name, envp, |
| 183 | + deny_interactive); |
| 184 | + } else if (script_name && !interpret_stdin) { |
| 185 | + /* |
| 186 | + * In this sample, we don't pass any argument to scripts, but |
| 187 | + * otherwise we would have to forge an argv with such |
| 188 | + * arguments. |
| 189 | + */ |
| 190 | + script_file = fopen(script_name, "r"); |
| 191 | + if (!script_file) { |
| 192 | + perror("ERROR: Failed to open script"); |
| 193 | + return 1; |
| 194 | + } |
| 195 | + /* |
| 196 | + * Restricts file execution according to |
| 197 | + * SECBIT_EXEC_RESTRICT_FILE. |
| 198 | + */ |
| 199 | + return interpret_stream(script_file, script_name, envp, |
| 200 | + restrict_file); |
| 201 | + } |
| 202 | + |
| 203 | + print_usage(argv[0]); |
| 204 | + return 1; |
| 205 | +} |
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