1 | /* Copyright 1998 by the Massachusetts Institute of Technology. |
---|
2 | * |
---|
3 | * Permission to use, copy, modify, and distribute this |
---|
4 | * software and its documentation for any purpose and without |
---|
5 | * fee is hereby granted, provided that the above copyright |
---|
6 | * notice appear in all copies and that both that copyright |
---|
7 | * notice and this permission notice appear in supporting |
---|
8 | * documentation, and that the name of M.I.T. not be used in |
---|
9 | * advertising or publicity pertaining to distribution of the |
---|
10 | * software without specific, written prior permission. |
---|
11 | * M.I.T. makes no representations about the suitability of |
---|
12 | * this software for any purpose. It is provided "as is" |
---|
13 | * without express or implied warranty. |
---|
14 | */ |
---|
15 | |
---|
16 | /* |
---|
17 | * saferm |
---|
18 | * |
---|
19 | * Running as root, remove and optionally zero files in /tmp that |
---|
20 | * may be owned by anybody, avoiding any possible race conditions |
---|
21 | * resulting in security problems. |
---|
22 | */ |
---|
23 | |
---|
24 | static const char rcsid[] = "$Id: saferm.c,v 1.6 1999-09-21 20:09:11 danw Exp $"; |
---|
25 | |
---|
26 | #include <sys/types.h> |
---|
27 | #include <sys/stat.h> |
---|
28 | #include <fcntl.h> |
---|
29 | #include <string.h> |
---|
30 | #include <stdio.h> |
---|
31 | #include <unistd.h> |
---|
32 | #include <stdlib.h> |
---|
33 | #include <errno.h> |
---|
34 | |
---|
35 | char *program_name; |
---|
36 | |
---|
37 | #ifndef MIN |
---|
38 | #define MIN(a, b) ((a) < (b) ? (a) : (b)) |
---|
39 | #endif |
---|
40 | |
---|
41 | static int safe_open(char *filename, struct stat *file_stat); |
---|
42 | static int zero(int file, struct stat *file_stat); |
---|
43 | static int safe_to_unlink(char *name); |
---|
44 | static char *safe_cd(char *path); |
---|
45 | static int safe_rm(char *path, int zero_file); |
---|
46 | static int safe_rmdir(char *path, int quietflag); |
---|
47 | |
---|
48 | int main(int argc, char **argv) |
---|
49 | { |
---|
50 | int c, zeroflag = 0, dirflag = 0, quietflag = 0, errflag = 0; |
---|
51 | int dir, status = 0; |
---|
52 | |
---|
53 | /* Set up our program name. */ |
---|
54 | if (argv[0] != NULL) |
---|
55 | { |
---|
56 | program_name = strrchr(argv[0], '/'); |
---|
57 | if (program_name != NULL) |
---|
58 | program_name++; |
---|
59 | else |
---|
60 | program_name = argv[0]; |
---|
61 | } |
---|
62 | else |
---|
63 | program_name = "saferm"; |
---|
64 | |
---|
65 | while ((c = getopt(argc, argv, "dqz")) != EOF) |
---|
66 | { |
---|
67 | switch(c) |
---|
68 | { |
---|
69 | case 'd': |
---|
70 | dirflag = 1; |
---|
71 | break; |
---|
72 | case 'q': |
---|
73 | quietflag = 1; |
---|
74 | break; |
---|
75 | case 'z': |
---|
76 | zeroflag = 1; |
---|
77 | break; |
---|
78 | case '?': |
---|
79 | errflag = 1; |
---|
80 | break; |
---|
81 | } |
---|
82 | } |
---|
83 | |
---|
84 | if (errflag || optind == argc || (dirflag && zeroflag)) |
---|
85 | { |
---|
86 | fprintf(stderr, "usage: %s [-q] [-d|-z] filename [filename ...]\n", |
---|
87 | program_name); |
---|
88 | exit(1); |
---|
89 | } |
---|
90 | |
---|
91 | /* Remember our current directory in case we are being passed |
---|
92 | * relative paths. |
---|
93 | */ |
---|
94 | dir = open(".", O_RDONLY); |
---|
95 | if (dir == -1) |
---|
96 | { |
---|
97 | fprintf(stderr, "%s: error opening current directory: %s\n", |
---|
98 | program_name, strerror(errno)); |
---|
99 | exit(1); |
---|
100 | } |
---|
101 | |
---|
102 | while (optind < argc) |
---|
103 | { |
---|
104 | /* If we're removing a relative path, be sure to cd to where |
---|
105 | * we started. |
---|
106 | */ |
---|
107 | if (argv[optind][0] != '/') |
---|
108 | { |
---|
109 | if (fchdir(dir) == -1) |
---|
110 | { |
---|
111 | fprintf(stderr, "%s: error cding to initial directory: %s\n", |
---|
112 | program_name, strerror(errno)); |
---|
113 | exit(1); |
---|
114 | } |
---|
115 | } |
---|
116 | |
---|
117 | if (dirflag && safe_rmdir(argv[optind], quietflag) == -1) |
---|
118 | status = 1; |
---|
119 | else if (!dirflag && safe_rm(argv[optind], zeroflag) == -1) |
---|
120 | status = 1; |
---|
121 | |
---|
122 | optind++; |
---|
123 | } |
---|
124 | |
---|
125 | exit(status); |
---|
126 | } |
---|
127 | |
---|
128 | /* Safely open a file with intent to zero. */ |
---|
129 | static int safe_open(char *filename, struct stat *file_stat) |
---|
130 | { |
---|
131 | int file; |
---|
132 | struct stat name_stat; |
---|
133 | |
---|
134 | /* Try to open the file we think we want to zero. */ |
---|
135 | file = open(filename, O_WRONLY); |
---|
136 | if (file == -1) |
---|
137 | { |
---|
138 | fprintf(stderr, "%s: open(%s) for writing failed: %s\n", |
---|
139 | program_name, filename, strerror(errno)); |
---|
140 | return -1; |
---|
141 | } |
---|
142 | |
---|
143 | /* Stat the file we actually opened. */ |
---|
144 | if (fstat(file, file_stat) == -1) |
---|
145 | { |
---|
146 | fprintf(stderr, "%s: fstat(%s) failed: %s\n", |
---|
147 | program_name, filename, strerror(errno)); |
---|
148 | close(file); |
---|
149 | return -1; |
---|
150 | } |
---|
151 | |
---|
152 | /* Make sure it's a regular file. */ |
---|
153 | if (!S_ISREG(file_stat->st_mode)) |
---|
154 | { |
---|
155 | fprintf(stderr, "%s: %s is %s\n", |
---|
156 | program_name, filename, |
---|
157 | (S_ISDIR(file_stat->st_mode) |
---|
158 | ? "a directory" |
---|
159 | : "not a regular file")); |
---|
160 | close(file); |
---|
161 | return -1; |
---|
162 | } |
---|
163 | |
---|
164 | /* Stat the file we think we want to zero. */ |
---|
165 | if (lstat(filename, &name_stat) == -1) |
---|
166 | { |
---|
167 | fprintf(stderr, "%s: lstat(%s) failed: %s\n", |
---|
168 | program_name, filename, strerror(errno)); |
---|
169 | close(file); |
---|
170 | return -1; |
---|
171 | } |
---|
172 | |
---|
173 | /* We must be sure that we are not zeroing a hard link or symbolic |
---|
174 | * link to something like the password file. To do this, we must |
---|
175 | * verify that the link count on the file we zero is equal to 1, and |
---|
176 | * that we did not follow a symbolic link to reach that file. |
---|
177 | * |
---|
178 | * Simply lstat()ing the file before or after we open it is subject to |
---|
179 | * lose in a race condition, as there is no way to know that the file we |
---|
180 | * opened is the same file as the one we lstat()ed. Simply fstat()ing |
---|
181 | * the file we actually open can't tell us whether we followed a symbolic |
---|
182 | * link to get there, and also can't give us a reliable link count also |
---|
183 | * due to race conditions. |
---|
184 | * |
---|
185 | * But if we both lstat() and fstat() and compare the inode numbers from |
---|
186 | * the two cases, we can verify that the file we have open, at the time |
---|
187 | * of our lstat(), existed in a unique location identified by the pathname |
---|
188 | * we used to open it. |
---|
189 | * |
---|
190 | * So first, verify that our two stats got the same file. |
---|
191 | */ |
---|
192 | if (file_stat->st_dev != name_stat.st_dev |
---|
193 | || file_stat->st_ino != name_stat.st_ino) |
---|
194 | { |
---|
195 | fprintf(stderr, "%s: %s and file opened are not identical\n", |
---|
196 | program_name, filename); |
---|
197 | close(file); |
---|
198 | return -1; |
---|
199 | } |
---|
200 | |
---|
201 | /* Now verify that the link count is 1. Note that file_stat may not be |
---|
202 | * used here, otherwise we could be beaten by a race condition: |
---|
203 | * |
---|
204 | * 1. The attacker creates /tmp/passwd as a symlink to /etc/passwd. |
---|
205 | * 2. We open /tmp/passwd, which follows the symlink, and so we have |
---|
206 | * fstat()ed /etc/passwd. The link count as of this stat is 1. |
---|
207 | * 3. The attacker removes the /tmp/passwd symlink and replaces it |
---|
208 | * with a hard link to /etc/passwd. |
---|
209 | * 4. We lstat() /tmp/passwd. The link count now is 2. |
---|
210 | */ |
---|
211 | if (name_stat.st_nlink != 1) |
---|
212 | { |
---|
213 | fprintf(stderr, "%s: multiple links for file %s, not zeroing\n", |
---|
214 | program_name, filename); |
---|
215 | close(file); |
---|
216 | return -1; |
---|
217 | } |
---|
218 | |
---|
219 | /* Now we know it is safe to muck with the file we opened. */ |
---|
220 | return file; |
---|
221 | } |
---|
222 | |
---|
223 | /* Zero the file represented by the passed file descriptor. Assumes |
---|
224 | * that the file pointer is already at the beginning of the file, and |
---|
225 | * file_stat points to a stat of the file. |
---|
226 | */ |
---|
227 | static int zero(int file, struct stat *file_stat) |
---|
228 | { |
---|
229 | char zeroes[BUFSIZ]; |
---|
230 | int written = 0, status; |
---|
231 | |
---|
232 | memset(zeroes, 0, BUFSIZ); |
---|
233 | |
---|
234 | while (written < file_stat->st_size) |
---|
235 | { |
---|
236 | status = write(file, zeroes, MIN(BUFSIZ, file_stat->st_size - written)); |
---|
237 | if (status == -1) |
---|
238 | { |
---|
239 | fprintf(stderr, "%s: write() failed: %s\n", |
---|
240 | program_name, strerror(errno)); |
---|
241 | return -1; |
---|
242 | } |
---|
243 | written += status; |
---|
244 | } |
---|
245 | |
---|
246 | fsync(file); |
---|
247 | return 0; |
---|
248 | } |
---|
249 | |
---|
250 | /* Return non-zero if it is safe to unlink NAME. */ |
---|
251 | static int safe_to_unlink(char *name) |
---|
252 | { |
---|
253 | struct stat st; |
---|
254 | |
---|
255 | if (lstat(name, &st) < 0) |
---|
256 | { |
---|
257 | fprintf(stderr, "%s: %s: %s\n", program_name, name, strerror(errno)); |
---|
258 | return 0; |
---|
259 | } |
---|
260 | |
---|
261 | if (S_ISDIR(st.st_mode)) |
---|
262 | { |
---|
263 | fprintf(stderr, "%s: %s is a directory\n", program_name, name); |
---|
264 | return 0; |
---|
265 | } |
---|
266 | |
---|
267 | return 1; |
---|
268 | } |
---|
269 | |
---|
270 | /* path is the full pathname to a file. We cd into the directory |
---|
271 | * containing the file, following no symbolic links. A pointer to just |
---|
272 | * the filename is returned on success (a substring of path), and NULL |
---|
273 | * is returned on failure. |
---|
274 | */ |
---|
275 | static char *safe_cd(char *path) |
---|
276 | { |
---|
277 | char *start_ptr, *end_ptr, *path_buf; |
---|
278 | struct stat above, below; |
---|
279 | |
---|
280 | if (path == NULL || *path == 0) |
---|
281 | return NULL; |
---|
282 | |
---|
283 | /* Make a copy of the path to be removed, since we're going to modify it. */ |
---|
284 | path_buf = malloc(strlen(path) + 1); |
---|
285 | if (path_buf == NULL) |
---|
286 | { |
---|
287 | fprintf(stderr, "%s: out of memory\n", program_name); |
---|
288 | return NULL; |
---|
289 | } |
---|
290 | strcpy(path_buf, path); |
---|
291 | |
---|
292 | /* Step through each path component and cd into it, verifying |
---|
293 | * that we wind up where we expect to. |
---|
294 | * |
---|
295 | * strchr()ing from start_ptr + 1 handles the absolute path case |
---|
296 | * as well as beginning to address the double-/ case. |
---|
297 | */ |
---|
298 | start_ptr = path_buf; |
---|
299 | while ((end_ptr = strchr(start_ptr + 1, '/'))) |
---|
300 | { |
---|
301 | /* Include any remaining extra /'s at the end of this component. */ |
---|
302 | while (*(end_ptr + 1) == '/') |
---|
303 | end_ptr++; |
---|
304 | |
---|
305 | *end_ptr = 0; |
---|
306 | |
---|
307 | if (lstat(start_ptr, &above) == -1) |
---|
308 | { |
---|
309 | fprintf(stderr, "%s: lstat of \"%s\" failed: %s\n", program_name, |
---|
310 | path_buf, strerror(errno)); |
---|
311 | free(path_buf); |
---|
312 | return NULL; |
---|
313 | } |
---|
314 | |
---|
315 | if (!S_ISDIR(above.st_mode)) |
---|
316 | { |
---|
317 | fprintf(stderr, "%s: final component of \"%s\" is not a directory\n", |
---|
318 | program_name, path_buf); |
---|
319 | free(path_buf); |
---|
320 | return NULL; |
---|
321 | } |
---|
322 | |
---|
323 | if (chdir(start_ptr) == -1) |
---|
324 | { |
---|
325 | fprintf(stderr, "%s: chdir to \"%s\" failed: %s\n", program_name, |
---|
326 | path_buf, strerror(errno)); |
---|
327 | free(path_buf); |
---|
328 | return NULL; |
---|
329 | } |
---|
330 | |
---|
331 | if (stat(".", &below) == -1) |
---|
332 | { |
---|
333 | fprintf(stderr, "%s: stat of \"%s\" failed: %s\n", program_name, |
---|
334 | path_buf, strerror(errno)); |
---|
335 | free(path_buf); |
---|
336 | return NULL; |
---|
337 | } |
---|
338 | |
---|
339 | if (above.st_dev != below.st_dev || above.st_ino != below.st_ino) |
---|
340 | { |
---|
341 | fprintf(stderr, "%s: final component of \"%s\" changed during " |
---|
342 | "traversal\n", program_name, path_buf); |
---|
343 | free(path_buf); |
---|
344 | return NULL; |
---|
345 | } |
---|
346 | |
---|
347 | /* Advance to next component. */ |
---|
348 | start_ptr = end_ptr + 1; |
---|
349 | |
---|
350 | /* Restore the / we nulled for diagnostics. */ |
---|
351 | *end_ptr = '/'; |
---|
352 | } |
---|
353 | |
---|
354 | free(path_buf); |
---|
355 | return (start_ptr - path_buf) + path; |
---|
356 | } |
---|
357 | |
---|
358 | /* Safely remove and optionally zero a file. -1 is returned for any |
---|
359 | * level of failure, such as zeroing failed but unlinking succeeded. |
---|
360 | */ |
---|
361 | static int safe_rm(char *path, int zero_file) |
---|
362 | { |
---|
363 | int file, status = 0; |
---|
364 | struct stat file_stat; |
---|
365 | char *name; |
---|
366 | |
---|
367 | name = safe_cd(path); |
---|
368 | if (name == NULL) |
---|
369 | return -1; |
---|
370 | |
---|
371 | /* Do the unlink before the actual zeroing so that the appearance |
---|
372 | * is atomic. |
---|
373 | */ |
---|
374 | |
---|
375 | if (zero_file) |
---|
376 | { |
---|
377 | file = safe_open(name, &file_stat); |
---|
378 | if (file == -1) |
---|
379 | return -1; |
---|
380 | } |
---|
381 | |
---|
382 | if (safe_to_unlink(name) && unlink(name) == -1) |
---|
383 | { |
---|
384 | fprintf(stderr, "%s: error unlinking %s: %s\n", program_name, |
---|
385 | path, strerror(errno)); |
---|
386 | status = -1; |
---|
387 | } |
---|
388 | |
---|
389 | if (zero_file && file != -1) |
---|
390 | { |
---|
391 | if (zero(file, &file_stat) == -1) |
---|
392 | status = -1; |
---|
393 | close(file); |
---|
394 | } |
---|
395 | |
---|
396 | return status; |
---|
397 | } |
---|
398 | |
---|
399 | /* Safely remove a directory. -1 is returned for any level of failure. */ |
---|
400 | static int safe_rmdir(char *path, int quietflag) |
---|
401 | { |
---|
402 | char *name; |
---|
403 | |
---|
404 | name = safe_cd(path); |
---|
405 | if (name == NULL) |
---|
406 | return -1; |
---|
407 | |
---|
408 | if (rmdir(name) == -1) |
---|
409 | { |
---|
410 | /* System V systems return EEXIST when a directory isn't empty |
---|
411 | * but hack the rmdir command to display a better error message. |
---|
412 | * Well, we can do that too. |
---|
413 | */ |
---|
414 | if (errno == EEXIST) |
---|
415 | errno = ENOTEMPTY; |
---|
416 | |
---|
417 | if (!quietflag || errno != ENOTEMPTY) |
---|
418 | { |
---|
419 | fprintf(stderr, "%s: error removing directory %s: %s\n", |
---|
420 | program_name, path, strerror(errno)); |
---|
421 | } |
---|
422 | return -1; |
---|
423 | } |
---|
424 | |
---|
425 | return 0; |
---|
426 | } |
---|