xref: /openbmc/linux/security/loadpin/loadpin.c (revision 5f536ac6)
1 // SPDX-License-Identifier: GPL-2.0-only
2 /*
3  * Module and Firmware Pinning Security Module
4  *
5  * Copyright 2011-2016 Google Inc.
6  *
7  * Author: Kees Cook <keescook@chromium.org>
8  */
9 
10 #define pr_fmt(fmt) "LoadPin: " fmt
11 
12 #include <linux/module.h>
13 #include <linux/fs.h>
14 #include <linux/kernel_read_file.h>
15 #include <linux/lsm_hooks.h>
16 #include <linux/mount.h>
17 #include <linux/blkdev.h>
18 #include <linux/path.h>
19 #include <linux/sched.h>	/* current */
20 #include <linux/string_helpers.h>
21 #include <linux/dm-verity-loadpin.h>
22 #include <uapi/linux/loadpin.h>
23 
24 #define VERITY_DIGEST_FILE_HEADER "# LOADPIN_TRUSTED_VERITY_ROOT_DIGESTS"
25 
report_load(const char * origin,struct file * file,char * operation)26 static void report_load(const char *origin, struct file *file, char *operation)
27 {
28 	char *cmdline, *pathname;
29 
30 	pathname = kstrdup_quotable_file(file, GFP_KERNEL);
31 	cmdline = kstrdup_quotable_cmdline(current, GFP_KERNEL);
32 
33 	pr_notice("%s %s obj=%s%s%s pid=%d cmdline=%s%s%s\n",
34 		  origin, operation,
35 		  (pathname && pathname[0] != '<') ? "\"" : "",
36 		  pathname,
37 		  (pathname && pathname[0] != '<') ? "\"" : "",
38 		  task_pid_nr(current),
39 		  cmdline ? "\"" : "", cmdline, cmdline ? "\"" : "");
40 
41 	kfree(cmdline);
42 	kfree(pathname);
43 }
44 
45 static int enforce = IS_ENABLED(CONFIG_SECURITY_LOADPIN_ENFORCE);
46 static char *exclude_read_files[READING_MAX_ID];
47 static int ignore_read_file_id[READING_MAX_ID] __ro_after_init;
48 static struct super_block *pinned_root;
49 static DEFINE_SPINLOCK(pinned_root_spinlock);
50 #ifdef CONFIG_SECURITY_LOADPIN_VERITY
51 static bool deny_reading_verity_digests;
52 #endif
53 
54 #ifdef CONFIG_SYSCTL
55 static struct ctl_table loadpin_sysctl_table[] = {
56 	{
57 		.procname       = "enforce",
58 		.data           = &enforce,
59 		.maxlen         = sizeof(int),
60 		.mode           = 0644,
61 		.proc_handler   = proc_dointvec_minmax,
62 		.extra1         = SYSCTL_ONE,
63 		.extra2         = SYSCTL_ONE,
64 	},
65 	{ }
66 };
67 
set_sysctl(bool is_writable)68 static void set_sysctl(bool is_writable)
69 {
70 	/*
71 	 * If load pinning is not enforced via a read-only block
72 	 * device, allow sysctl to change modes for testing.
73 	 */
74 	if (is_writable)
75 		loadpin_sysctl_table[0].extra1 = SYSCTL_ZERO;
76 	else
77 		loadpin_sysctl_table[0].extra1 = SYSCTL_ONE;
78 }
79 #else
set_sysctl(bool is_writable)80 static inline void set_sysctl(bool is_writable) { }
81 #endif
82 
report_writable(struct super_block * mnt_sb,bool writable)83 static void report_writable(struct super_block *mnt_sb, bool writable)
84 {
85 	if (mnt_sb->s_bdev) {
86 		pr_info("%pg (%u:%u): %s\n", mnt_sb->s_bdev,
87 			MAJOR(mnt_sb->s_bdev->bd_dev),
88 			MINOR(mnt_sb->s_bdev->bd_dev),
89 			writable ? "writable" : "read-only");
90 	} else
91 		pr_info("mnt_sb lacks block device, treating as: writable\n");
92 
93 	if (!writable)
94 		pr_info("load pinning engaged.\n");
95 }
96 
97 /*
98  * This must be called after early kernel init, since then the rootdev
99  * is available.
100  */
sb_is_writable(struct super_block * mnt_sb)101 static bool sb_is_writable(struct super_block *mnt_sb)
102 {
103 	bool writable = true;
104 
105 	if (mnt_sb->s_bdev)
106 		writable = !bdev_read_only(mnt_sb->s_bdev);
107 
108 	return writable;
109 }
110 
loadpin_sb_free_security(struct super_block * mnt_sb)111 static void loadpin_sb_free_security(struct super_block *mnt_sb)
112 {
113 	/*
114 	 * When unmounting the filesystem we were using for load
115 	 * pinning, we acknowledge the superblock release, but make sure
116 	 * no other modules or firmware can be loaded when we are in
117 	 * enforcing mode. Otherwise, allow the root to be reestablished.
118 	 */
119 	if (!IS_ERR_OR_NULL(pinned_root) && mnt_sb == pinned_root) {
120 		if (enforce) {
121 			pinned_root = ERR_PTR(-EIO);
122 			pr_info("umount pinned fs: refusing further loads\n");
123 		} else {
124 			pinned_root = NULL;
125 		}
126 	}
127 }
128 
loadpin_check(struct file * file,enum kernel_read_file_id id)129 static int loadpin_check(struct file *file, enum kernel_read_file_id id)
130 {
131 	struct super_block *load_root;
132 	const char *origin = kernel_read_file_id_str(id);
133 	bool first_root_pin = false;
134 	bool load_root_writable;
135 
136 	/* If the file id is excluded, ignore the pinning. */
137 	if ((unsigned int)id < ARRAY_SIZE(ignore_read_file_id) &&
138 	    ignore_read_file_id[id]) {
139 		report_load(origin, file, "pinning-excluded");
140 		return 0;
141 	}
142 
143 	/* This handles the older init_module API that has a NULL file. */
144 	if (!file) {
145 		if (!enforce) {
146 			report_load(origin, NULL, "old-api-pinning-ignored");
147 			return 0;
148 		}
149 
150 		report_load(origin, NULL, "old-api-denied");
151 		return -EPERM;
152 	}
153 
154 	load_root = file->f_path.mnt->mnt_sb;
155 	load_root_writable = sb_is_writable(load_root);
156 
157 	/* First loaded module/firmware defines the root for all others. */
158 	spin_lock(&pinned_root_spinlock);
159 	/*
160 	 * pinned_root is only NULL at startup or when the pinned root has
161 	 * been unmounted while we are not in enforcing mode. Otherwise, it
162 	 * is either a valid reference, or an ERR_PTR.
163 	 */
164 	if (!pinned_root) {
165 		pinned_root = load_root;
166 		first_root_pin = true;
167 	}
168 	spin_unlock(&pinned_root_spinlock);
169 
170 	if (first_root_pin) {
171 		report_writable(pinned_root, load_root_writable);
172 		set_sysctl(load_root_writable);
173 		report_load(origin, file, "pinned");
174 	}
175 
176 	if (IS_ERR_OR_NULL(pinned_root) ||
177 	    ((load_root != pinned_root) && !dm_verity_loadpin_is_bdev_trusted(load_root->s_bdev))) {
178 		if (unlikely(!enforce)) {
179 			report_load(origin, file, "pinning-ignored");
180 			return 0;
181 		}
182 
183 		report_load(origin, file, "denied");
184 		return -EPERM;
185 	}
186 
187 	return 0;
188 }
189 
loadpin_read_file(struct file * file,enum kernel_read_file_id id,bool contents)190 static int loadpin_read_file(struct file *file, enum kernel_read_file_id id,
191 			     bool contents)
192 {
193 	/*
194 	 * LoadPin only cares about the _origin_ of a file, not its
195 	 * contents, so we can ignore the "are full contents available"
196 	 * argument here.
197 	 */
198 	return loadpin_check(file, id);
199 }
200 
loadpin_load_data(enum kernel_load_data_id id,bool contents)201 static int loadpin_load_data(enum kernel_load_data_id id, bool contents)
202 {
203 	/*
204 	 * LoadPin only cares about the _origin_ of a file, not its
205 	 * contents, so a NULL file is passed, and we can ignore the
206 	 * state of "contents".
207 	 */
208 	return loadpin_check(NULL, (enum kernel_read_file_id) id);
209 }
210 
211 static struct security_hook_list loadpin_hooks[] __ro_after_init = {
212 	LSM_HOOK_INIT(sb_free_security, loadpin_sb_free_security),
213 	LSM_HOOK_INIT(kernel_read_file, loadpin_read_file),
214 	LSM_HOOK_INIT(kernel_load_data, loadpin_load_data),
215 };
216 
parse_exclude(void)217 static void __init parse_exclude(void)
218 {
219 	int i, j;
220 	char *cur;
221 
222 	/*
223 	 * Make sure all the arrays stay within expected sizes. This
224 	 * is slightly weird because kernel_read_file_str[] includes
225 	 * READING_MAX_ID, which isn't actually meaningful here.
226 	 */
227 	BUILD_BUG_ON(ARRAY_SIZE(exclude_read_files) !=
228 		     ARRAY_SIZE(ignore_read_file_id));
229 	BUILD_BUG_ON(ARRAY_SIZE(kernel_read_file_str) <
230 		     ARRAY_SIZE(ignore_read_file_id));
231 
232 	for (i = 0; i < ARRAY_SIZE(exclude_read_files); i++) {
233 		cur = exclude_read_files[i];
234 		if (!cur)
235 			break;
236 		if (*cur == '\0')
237 			continue;
238 
239 		for (j = 0; j < ARRAY_SIZE(ignore_read_file_id); j++) {
240 			if (strcmp(cur, kernel_read_file_str[j]) == 0) {
241 				pr_info("excluding: %s\n",
242 					kernel_read_file_str[j]);
243 				ignore_read_file_id[j] = 1;
244 				/*
245 				 * Can not break, because one read_file_str
246 				 * may map to more than on read_file_id.
247 				 */
248 			}
249 		}
250 	}
251 }
252 
loadpin_init(void)253 static int __init loadpin_init(void)
254 {
255 	pr_info("ready to pin (currently %senforcing)\n",
256 		enforce ? "" : "not ");
257 	parse_exclude();
258 #ifdef CONFIG_SYSCTL
259 	if (!register_sysctl("kernel/loadpin", loadpin_sysctl_table))
260 		pr_notice("sysctl registration failed!\n");
261 #endif
262 	security_add_hooks(loadpin_hooks, ARRAY_SIZE(loadpin_hooks), "loadpin");
263 
264 	return 0;
265 }
266 
267 DEFINE_LSM(loadpin) = {
268 	.name = "loadpin",
269 	.init = loadpin_init,
270 };
271 
272 #ifdef CONFIG_SECURITY_LOADPIN_VERITY
273 
274 enum loadpin_securityfs_interface_index {
275 	LOADPIN_DM_VERITY,
276 };
277 
read_trusted_verity_root_digests(unsigned int fd)278 static int read_trusted_verity_root_digests(unsigned int fd)
279 {
280 	struct fd f;
281 	void *data;
282 	int rc;
283 	char *p, *d;
284 
285 	if (deny_reading_verity_digests)
286 		return -EPERM;
287 
288 	/* The list of trusted root digests can only be set up once */
289 	if (!list_empty(&dm_verity_loadpin_trusted_root_digests))
290 		return -EPERM;
291 
292 	f = fdget(fd);
293 	if (!f.file)
294 		return -EINVAL;
295 
296 	data = kzalloc(SZ_4K, GFP_KERNEL);
297 	if (!data) {
298 		rc = -ENOMEM;
299 		goto err;
300 	}
301 
302 	rc = kernel_read_file(f.file, 0, (void **)&data, SZ_4K - 1, NULL, READING_POLICY);
303 	if (rc < 0)
304 		goto err;
305 
306 	p = data;
307 	p[rc] = '\0';
308 	p = strim(p);
309 
310 	p = strim(data);
311 	while ((d = strsep(&p, "\n")) != NULL) {
312 		int len;
313 		struct dm_verity_loadpin_trusted_root_digest *trd;
314 
315 		if (d == data) {
316 			/* first line, validate header */
317 			if (strcmp(d, VERITY_DIGEST_FILE_HEADER)) {
318 				rc = -EPROTO;
319 				goto err;
320 			}
321 
322 			continue;
323 		}
324 
325 		len = strlen(d);
326 
327 		if (len % 2) {
328 			rc = -EPROTO;
329 			goto err;
330 		}
331 
332 		len /= 2;
333 
334 		trd = kzalloc(struct_size(trd, data, len), GFP_KERNEL);
335 		if (!trd) {
336 			rc = -ENOMEM;
337 			goto err;
338 		}
339 		trd->len = len;
340 
341 		if (hex2bin(trd->data, d, len)) {
342 			kfree(trd);
343 			rc = -EPROTO;
344 			goto err;
345 		}
346 
347 		list_add_tail(&trd->node, &dm_verity_loadpin_trusted_root_digests);
348 	}
349 
350 	if (list_empty(&dm_verity_loadpin_trusted_root_digests)) {
351 		rc = -EPROTO;
352 		goto err;
353 	}
354 
355 	kfree(data);
356 	fdput(f);
357 
358 	return 0;
359 
360 err:
361 	kfree(data);
362 
363 	/* any failure in loading/parsing invalidates the entire list */
364 	{
365 		struct dm_verity_loadpin_trusted_root_digest *trd, *tmp;
366 
367 		list_for_each_entry_safe(trd, tmp, &dm_verity_loadpin_trusted_root_digests, node) {
368 			list_del(&trd->node);
369 			kfree(trd);
370 		}
371 	}
372 
373 	/* disallow further attempts after reading a corrupt/invalid file */
374 	deny_reading_verity_digests = true;
375 
376 	fdput(f);
377 
378 	return rc;
379 }
380 
381 /******************************** securityfs ********************************/
382 
dm_verity_ioctl(struct file * filp,unsigned int cmd,unsigned long arg)383 static long dm_verity_ioctl(struct file *filp, unsigned int cmd, unsigned long arg)
384 {
385 	void __user *uarg = (void __user *)arg;
386 	unsigned int fd;
387 
388 	switch (cmd) {
389 	case LOADPIN_IOC_SET_TRUSTED_VERITY_DIGESTS:
390 		if (copy_from_user(&fd, uarg, sizeof(fd)))
391 			return -EFAULT;
392 
393 		return read_trusted_verity_root_digests(fd);
394 
395 	default:
396 		return -EINVAL;
397 	}
398 }
399 
400 static const struct file_operations loadpin_dm_verity_ops = {
401 	.unlocked_ioctl = dm_verity_ioctl,
402 	.compat_ioctl = compat_ptr_ioctl,
403 };
404 
405 /**
406  * init_loadpin_securityfs - create the securityfs directory for LoadPin
407  *
408  * We can not put this method normally under the loadpin_init() code path since
409  * the security subsystem gets initialized before the vfs caches.
410  *
411  * Returns 0 if the securityfs directory creation was successful.
412  */
init_loadpin_securityfs(void)413 static int __init init_loadpin_securityfs(void)
414 {
415 	struct dentry *loadpin_dir, *dentry;
416 
417 	loadpin_dir = securityfs_create_dir("loadpin", NULL);
418 	if (IS_ERR(loadpin_dir)) {
419 		pr_err("LoadPin: could not create securityfs dir: %ld\n",
420 		       PTR_ERR(loadpin_dir));
421 		return PTR_ERR(loadpin_dir);
422 	}
423 
424 	dentry = securityfs_create_file("dm-verity", 0600, loadpin_dir,
425 					(void *)LOADPIN_DM_VERITY, &loadpin_dm_verity_ops);
426 	if (IS_ERR(dentry)) {
427 		pr_err("LoadPin: could not create securityfs entry 'dm-verity': %ld\n",
428 		       PTR_ERR(dentry));
429 		return PTR_ERR(dentry);
430 	}
431 
432 	return 0;
433 }
434 
435 fs_initcall(init_loadpin_securityfs);
436 
437 #endif /* CONFIG_SECURITY_LOADPIN_VERITY */
438 
439 /* Should not be mutable after boot, so not listed in sysfs (perm == 0). */
440 module_param(enforce, int, 0);
441 MODULE_PARM_DESC(enforce, "Enforce module/firmware pinning");
442 module_param_array_named(exclude, exclude_read_files, charp, NULL, 0);
443 MODULE_PARM_DESC(exclude, "Exclude pinning specific read file types");
444