xref: /openbmc/linux/security/loadpin/loadpin.c (revision 2f190ac2) 
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 static void report_load(const char *origin, struct file *file, char *operation)
25 {
26 	char *cmdline, *pathname;
27 
28 	pathname = kstrdup_quotable_file(file, GFP_KERNEL);
29 	cmdline = kstrdup_quotable_cmdline(current, GFP_KERNEL);
30 
31 	pr_notice("%s %s obj=%s%s%s pid=%d cmdline=%s%s%s\n",
32 		  origin, operation,
33 		  (pathname && pathname[0] != '<') ? "\"" : "",
34 		  pathname,
35 		  (pathname && pathname[0] != '<') ? "\"" : "",
36 		  task_pid_nr(current),
37 		  cmdline ? "\"" : "", cmdline, cmdline ? "\"" : "");
38 
39 	kfree(cmdline);
40 	kfree(pathname);
41 }
42 
43 static int enforce = IS_ENABLED(CONFIG_SECURITY_LOADPIN_ENFORCE);
44 static char *exclude_read_files[READING_MAX_ID];
45 static int ignore_read_file_id[READING_MAX_ID] __ro_after_init;
46 static struct super_block *pinned_root;
47 static DEFINE_SPINLOCK(pinned_root_spinlock);
48 #ifdef CONFIG_SECURITY_LOADPIN_VERITY
49 static bool deny_reading_verity_digests;
50 #endif
51 
52 #ifdef CONFIG_SYSCTL
53 
54 static struct ctl_path loadpin_sysctl_path[] = {
55 	{ .procname = "kernel", },
56 	{ .procname = "loadpin", },
57 	{ }
58 };
59 
60 static struct ctl_table loadpin_sysctl_table[] = {
61 	{
62 		.procname       = "enforce",
63 		.data           = &enforce,
64 		.maxlen         = sizeof(int),
65 		.mode           = 0644,
66 		.proc_handler   = proc_dointvec_minmax,
67 		.extra1         = SYSCTL_ZERO,
68 		.extra2         = SYSCTL_ONE,
69 	},
70 	{ }
71 };
72 
73 /*
74  * This must be called after early kernel init, since then the rootdev
75  * is available.
76  */
77 static void check_pinning_enforcement(struct super_block *mnt_sb)
78 {
79 	bool ro = false;
80 
81 	/*
82 	 * If load pinning is not enforced via a read-only block
83 	 * device, allow sysctl to change modes for testing.
84 	 */
85 	if (mnt_sb->s_bdev) {
86 		ro = bdev_read_only(mnt_sb->s_bdev);
87 		pr_info("%pg (%u:%u): %s\n", mnt_sb->s_bdev,
88 			MAJOR(mnt_sb->s_bdev->bd_dev),
89 			MINOR(mnt_sb->s_bdev->bd_dev),
90 			ro ? "read-only" : "writable");
91 	} else
92 		pr_info("mnt_sb lacks block device, treating as: writable\n");
93 
94 	if (!ro) {
95 		if (!register_sysctl_paths(loadpin_sysctl_path,
96 					   loadpin_sysctl_table))
97 			pr_notice("sysctl registration failed!\n");
98 		else
99 			pr_info("enforcement can be disabled.\n");
100 	} else
101 		pr_info("load pinning engaged.\n");
102 }
103 #else
104 static void check_pinning_enforcement(struct super_block *mnt_sb)
105 {
106 	pr_info("load pinning engaged.\n");
107 }
108 #endif
109 
110 static void loadpin_sb_free_security(struct super_block *mnt_sb)
111 {
112 	/*
113 	 * When unmounting the filesystem we were using for load
114 	 * pinning, we acknowledge the superblock release, but make sure
115 	 * no other modules or firmware can be loaded.
116 	 */
117 	if (!IS_ERR_OR_NULL(pinned_root) && mnt_sb == pinned_root) {
118 		pinned_root = ERR_PTR(-EIO);
119 		pr_info("umount pinned fs: refusing further loads\n");
120 	}
121 }
122 
123 static int loadpin_read_file(struct file *file, enum kernel_read_file_id id,
124 			     bool contents)
125 {
126 	struct super_block *load_root;
127 	const char *origin = kernel_read_file_id_str(id);
128 
129 	/*
130 	 * If we will not know that we'll be seeing the full contents
131 	 * then we cannot trust a load will be complete and unchanged
132 	 * off disk. Treat all contents=false hooks as if there were
133 	 * no associated file struct.
134 	 */
135 	if (!contents)
136 		file = NULL;
137 
138 	/* If the file id is excluded, ignore the pinning. */
139 	if ((unsigned int)id < ARRAY_SIZE(ignore_read_file_id) &&
140 	    ignore_read_file_id[id]) {
141 		report_load(origin, file, "pinning-excluded");
142 		return 0;
143 	}
144 
145 	/* This handles the older init_module API that has a NULL file. */
146 	if (!file) {
147 		if (!enforce) {
148 			report_load(origin, NULL, "old-api-pinning-ignored");
149 			return 0;
150 		}
151 
152 		report_load(origin, NULL, "old-api-denied");
153 		return -EPERM;
154 	}
155 
156 	load_root = file->f_path.mnt->mnt_sb;
157 
158 	/* First loaded module/firmware defines the root for all others. */
159 	spin_lock(&pinned_root_spinlock);
160 	/*
161 	 * pinned_root is only NULL at startup. Otherwise, it is either
162 	 * a valid reference, or an ERR_PTR.
163 	 */
164 	if (!pinned_root) {
165 		pinned_root = load_root;
166 		/*
167 		 * Unlock now since it's only pinned_root we care about.
168 		 * In the worst case, we will (correctly) report pinning
169 		 * failures before we have announced that pinning is
170 		 * enforcing. This would be purely cosmetic.
171 		 */
172 		spin_unlock(&pinned_root_spinlock);
173 		check_pinning_enforcement(pinned_root);
174 		report_load(origin, file, "pinned");
175 	} else {
176 		spin_unlock(&pinned_root_spinlock);
177 	}
178 
179 	if (IS_ERR_OR_NULL(pinned_root) ||
180 	    ((load_root != pinned_root) && !dm_verity_loadpin_is_bdev_trusted(load_root->s_bdev))) {
181 		if (unlikely(!enforce)) {
182 			report_load(origin, file, "pinning-ignored");
183 			return 0;
184 		}
185 
186 		report_load(origin, file, "denied");
187 		return -EPERM;
188 	}
189 
190 	return 0;
191 }
192 
193 static int loadpin_load_data(enum kernel_load_data_id id, bool contents)
194 {
195 	return loadpin_read_file(NULL, (enum kernel_read_file_id) id, contents);
196 }
197 
198 static struct security_hook_list loadpin_hooks[] __lsm_ro_after_init = {
199 	LSM_HOOK_INIT(sb_free_security, loadpin_sb_free_security),
200 	LSM_HOOK_INIT(kernel_read_file, loadpin_read_file),
201 	LSM_HOOK_INIT(kernel_load_data, loadpin_load_data),
202 };
203 
204 static void __init parse_exclude(void)
205 {
206 	int i, j;
207 	char *cur;
208 
209 	/*
210 	 * Make sure all the arrays stay within expected sizes. This
211 	 * is slightly weird because kernel_read_file_str[] includes
212 	 * READING_MAX_ID, which isn't actually meaningful here.
213 	 */
214 	BUILD_BUG_ON(ARRAY_SIZE(exclude_read_files) !=
215 		     ARRAY_SIZE(ignore_read_file_id));
216 	BUILD_BUG_ON(ARRAY_SIZE(kernel_read_file_str) <
217 		     ARRAY_SIZE(ignore_read_file_id));
218 
219 	for (i = 0; i < ARRAY_SIZE(exclude_read_files); i++) {
220 		cur = exclude_read_files[i];
221 		if (!cur)
222 			break;
223 		if (*cur == '\0')
224 			continue;
225 
226 		for (j = 0; j < ARRAY_SIZE(ignore_read_file_id); j++) {
227 			if (strcmp(cur, kernel_read_file_str[j]) == 0) {
228 				pr_info("excluding: %s\n",
229 					kernel_read_file_str[j]);
230 				ignore_read_file_id[j] = 1;
231 				/*
232 				 * Can not break, because one read_file_str
233 				 * may map to more than on read_file_id.
234 				 */
235 			}
236 		}
237 	}
238 }
239 
240 static int __init loadpin_init(void)
241 {
242 	pr_info("ready to pin (currently %senforcing)\n",
243 		enforce ? "" : "not ");
244 	parse_exclude();
245 	security_add_hooks(loadpin_hooks, ARRAY_SIZE(loadpin_hooks), "loadpin");
246 
247 	return 0;
248 }
249 
250 DEFINE_LSM(loadpin) = {
251 	.name = "loadpin",
252 	.init = loadpin_init,
253 };
254 
255 #ifdef CONFIG_SECURITY_LOADPIN_VERITY
256 
257 enum loadpin_securityfs_interface_index {
258 	LOADPIN_DM_VERITY,
259 };
260 
261 static int read_trusted_verity_root_digests(unsigned int fd)
262 {
263 	struct fd f;
264 	void *data;
265 	int rc;
266 	char *p, *d;
267 
268 	if (deny_reading_verity_digests)
269 		return -EPERM;
270 
271 	/* The list of trusted root digests can only be set up once */
272 	if (!list_empty(&dm_verity_loadpin_trusted_root_digests))
273 		return -EPERM;
274 
275 	f = fdget(fd);
276 	if (!f.file)
277 		return -EINVAL;
278 
279 	data = kzalloc(SZ_4K, GFP_KERNEL);
280 	if (!data) {
281 		rc = -ENOMEM;
282 		goto err;
283 	}
284 
285 	rc = kernel_read_file(f.file, 0, (void **)&data, SZ_4K - 1, NULL, READING_POLICY);
286 	if (rc < 0)
287 		goto err;
288 
289 	p = data;
290 	p[rc] = '\0';
291 	p = strim(p);
292 
293 	p = strim(data);
294 	while ((d = strsep(&p, "\n")) != NULL) {
295 		int len = strlen(d);
296 		struct dm_verity_loadpin_trusted_root_digest *trd;
297 
298 		if (len % 2) {
299 			rc = -EPROTO;
300 			goto err;
301 		}
302 
303 		len /= 2;
304 
305 		trd = kzalloc(struct_size(trd, data, len), GFP_KERNEL);
306 		if (!trd) {
307 			rc = -ENOMEM;
308 			goto err;
309 		}
310 
311 		if (hex2bin(trd->data, d, len)) {
312 			kfree(trd);
313 			rc = -EPROTO;
314 			goto err;
315 		}
316 
317 		trd->len = len;
318 
319 		list_add_tail(&trd->node, &dm_verity_loadpin_trusted_root_digests);
320 	}
321 
322 	if (list_empty(&dm_verity_loadpin_trusted_root_digests)) {
323 		rc = -EPROTO;
324 		goto err;
325 	}
326 
327 	kfree(data);
328 	fdput(f);
329 
330 	return 0;
331 
332 err:
333 	kfree(data);
334 
335 	/* any failure in loading/parsing invalidates the entire list */
336 	{
337 		struct dm_verity_loadpin_trusted_root_digest *trd, *tmp;
338 
339 		list_for_each_entry_safe(trd, tmp, &dm_verity_loadpin_trusted_root_digests, node) {
340 			list_del(&trd->node);
341 			kfree(trd);
342 		}
343 	}
344 
345 	/* disallow further attempts after reading a corrupt/invalid file */
346 	deny_reading_verity_digests = true;
347 
348 	fdput(f);
349 
350 	return rc;
351 }
352 
353 /******************************** securityfs ********************************/
354 
355 static long dm_verity_ioctl(struct file *filp, unsigned int cmd, unsigned long arg)
356 {
357 	void __user *uarg = (void __user *)arg;
358 	unsigned int fd;
359 
360 	switch (cmd) {
361 	case LOADPIN_IOC_SET_TRUSTED_VERITY_DIGESTS:
362 		if (copy_from_user(&fd, uarg, sizeof(fd)))
363 			return -EFAULT;
364 
365 		return read_trusted_verity_root_digests(fd);
366 
367 	default:
368 		return -EINVAL;
369 	}
370 }
371 
372 static const struct file_operations loadpin_dm_verity_ops = {
373 	.unlocked_ioctl = dm_verity_ioctl,
374 	.compat_ioctl = compat_ptr_ioctl,
375 };
376 
377 /**
378  * init_loadpin_securityfs - create the securityfs directory for LoadPin
379  *
380  * We can not put this method normally under the loadpin_init() code path since
381  * the security subsystem gets initialized before the vfs caches.
382  *
383  * Returns 0 if the securityfs directory creation was successful.
384  */
385 static int __init init_loadpin_securityfs(void)
386 {
387 	struct dentry *loadpin_dir, *dentry;
388 
389 	loadpin_dir = securityfs_create_dir("loadpin", NULL);
390 	if (IS_ERR(loadpin_dir)) {
391 		pr_err("LoadPin: could not create securityfs dir: %ld\n",
392 		       PTR_ERR(loadpin_dir));
393 		return PTR_ERR(loadpin_dir);
394 	}
395 
396 	dentry = securityfs_create_file("dm-verity", 0600, loadpin_dir,
397 					(void *)LOADPIN_DM_VERITY, &loadpin_dm_verity_ops);
398 	if (IS_ERR(dentry)) {
399 		pr_err("LoadPin: could not create securityfs entry 'dm-verity': %ld\n",
400 		       PTR_ERR(dentry));
401 		return PTR_ERR(dentry);
402 	}
403 
404 	return 0;
405 }
406 
407 fs_initcall(init_loadpin_securityfs);
408 
409 #endif /* CONFIG_SECURITY_LOADPIN_VERITY */
410 
411 /* Should not be mutable after boot, so not listed in sysfs (perm == 0). */
412 module_param(enforce, int, 0);
413 MODULE_PARM_DESC(enforce, "Enforce module/firmware pinning");
414 module_param_array_named(exclude, exclude_read_files, charp, NULL, 0);
415 MODULE_PARM_DESC(exclude, "Exclude pinning specific read file types");
416