xref: /openbmc/linux/fs/verity/open.c (revision e553d2a5)
1  // SPDX-License-Identifier: GPL-2.0
2  /*
3   * fs/verity/open.c: opening fs-verity files
4   *
5   * Copyright 2019 Google LLC
6   */
7  
8  #include "fsverity_private.h"
9  
10  #include <linux/slab.h>
11  
12  static struct kmem_cache *fsverity_info_cachep;
13  
14  /**
15   * fsverity_init_merkle_tree_params() - initialize Merkle tree parameters
16   * @params: the parameters struct to initialize
17   * @inode: the inode for which the Merkle tree is being built
18   * @hash_algorithm: number of hash algorithm to use
19   * @log_blocksize: log base 2 of block size to use
20   * @salt: pointer to salt (optional)
21   * @salt_size: size of salt, possibly 0
22   *
23   * Validate the hash algorithm and block size, then compute the tree topology
24   * (num levels, num blocks in each level, etc.) and initialize @params.
25   *
26   * Return: 0 on success, -errno on failure
27   */
28  int fsverity_init_merkle_tree_params(struct merkle_tree_params *params,
29  				     const struct inode *inode,
30  				     unsigned int hash_algorithm,
31  				     unsigned int log_blocksize,
32  				     const u8 *salt, size_t salt_size)
33  {
34  	const struct fsverity_hash_alg *hash_alg;
35  	int err;
36  	u64 blocks;
37  	u64 offset;
38  	int level;
39  
40  	memset(params, 0, sizeof(*params));
41  
42  	hash_alg = fsverity_get_hash_alg(inode, hash_algorithm);
43  	if (IS_ERR(hash_alg))
44  		return PTR_ERR(hash_alg);
45  	params->hash_alg = hash_alg;
46  	params->digest_size = hash_alg->digest_size;
47  
48  	params->hashstate = fsverity_prepare_hash_state(hash_alg, salt,
49  							salt_size);
50  	if (IS_ERR(params->hashstate)) {
51  		err = PTR_ERR(params->hashstate);
52  		params->hashstate = NULL;
53  		fsverity_err(inode, "Error %d preparing hash state", err);
54  		goto out_err;
55  	}
56  
57  	if (log_blocksize != PAGE_SHIFT) {
58  		fsverity_warn(inode, "Unsupported log_blocksize: %u",
59  			      log_blocksize);
60  		err = -EINVAL;
61  		goto out_err;
62  	}
63  	params->log_blocksize = log_blocksize;
64  	params->block_size = 1 << log_blocksize;
65  
66  	if (WARN_ON(!is_power_of_2(params->digest_size))) {
67  		err = -EINVAL;
68  		goto out_err;
69  	}
70  	if (params->block_size < 2 * params->digest_size) {
71  		fsverity_warn(inode,
72  			      "Merkle tree block size (%u) too small for hash algorithm \"%s\"",
73  			      params->block_size, hash_alg->name);
74  		err = -EINVAL;
75  		goto out_err;
76  	}
77  	params->log_arity = params->log_blocksize - ilog2(params->digest_size);
78  	params->hashes_per_block = 1 << params->log_arity;
79  
80  	pr_debug("Merkle tree uses %s with %u-byte blocks (%u hashes/block), salt=%*phN\n",
81  		 hash_alg->name, params->block_size, params->hashes_per_block,
82  		 (int)salt_size, salt);
83  
84  	/*
85  	 * Compute the number of levels in the Merkle tree and create a map from
86  	 * level to the starting block of that level.  Level 'num_levels - 1' is
87  	 * the root and is stored first.  Level 0 is the level directly "above"
88  	 * the data blocks and is stored last.
89  	 */
90  
91  	/* Compute number of levels and the number of blocks in each level */
92  	blocks = (inode->i_size + params->block_size - 1) >> log_blocksize;
93  	pr_debug("Data is %lld bytes (%llu blocks)\n", inode->i_size, blocks);
94  	while (blocks > 1) {
95  		if (params->num_levels >= FS_VERITY_MAX_LEVELS) {
96  			fsverity_err(inode, "Too many levels in Merkle tree");
97  			err = -EINVAL;
98  			goto out_err;
99  		}
100  		blocks = (blocks + params->hashes_per_block - 1) >>
101  			 params->log_arity;
102  		/* temporarily using level_start[] to store blocks in level */
103  		params->level_start[params->num_levels++] = blocks;
104  	}
105  
106  	/* Compute the starting block of each level */
107  	offset = 0;
108  	for (level = (int)params->num_levels - 1; level >= 0; level--) {
109  		blocks = params->level_start[level];
110  		params->level_start[level] = offset;
111  		pr_debug("Level %d is %llu blocks starting at index %llu\n",
112  			 level, blocks, offset);
113  		offset += blocks;
114  	}
115  
116  	params->tree_size = offset << log_blocksize;
117  	return 0;
118  
119  out_err:
120  	kfree(params->hashstate);
121  	memset(params, 0, sizeof(*params));
122  	return err;
123  }
124  
125  /*
126   * Compute the file measurement by hashing the fsverity_descriptor excluding the
127   * signature and with the sig_size field set to 0.
128   */
129  static int compute_file_measurement(const struct fsverity_hash_alg *hash_alg,
130  				    struct fsverity_descriptor *desc,
131  				    u8 *measurement)
132  {
133  	__le32 sig_size = desc->sig_size;
134  	int err;
135  
136  	desc->sig_size = 0;
137  	err = fsverity_hash_buffer(hash_alg, desc, sizeof(*desc), measurement);
138  	desc->sig_size = sig_size;
139  
140  	return err;
141  }
142  
143  /*
144   * Validate the given fsverity_descriptor and create a new fsverity_info from
145   * it.  The signature (if present) is also checked.
146   */
147  struct fsverity_info *fsverity_create_info(const struct inode *inode,
148  					   void *_desc, size_t desc_size)
149  {
150  	struct fsverity_descriptor *desc = _desc;
151  	struct fsverity_info *vi;
152  	int err;
153  
154  	if (desc_size < sizeof(*desc)) {
155  		fsverity_err(inode, "Unrecognized descriptor size: %zu bytes",
156  			     desc_size);
157  		return ERR_PTR(-EINVAL);
158  	}
159  
160  	if (desc->version != 1) {
161  		fsverity_err(inode, "Unrecognized descriptor version: %u",
162  			     desc->version);
163  		return ERR_PTR(-EINVAL);
164  	}
165  
166  	if (memchr_inv(desc->__reserved, 0, sizeof(desc->__reserved))) {
167  		fsverity_err(inode, "Reserved bits set in descriptor");
168  		return ERR_PTR(-EINVAL);
169  	}
170  
171  	if (desc->salt_size > sizeof(desc->salt)) {
172  		fsverity_err(inode, "Invalid salt_size: %u", desc->salt_size);
173  		return ERR_PTR(-EINVAL);
174  	}
175  
176  	if (le64_to_cpu(desc->data_size) != inode->i_size) {
177  		fsverity_err(inode,
178  			     "Wrong data_size: %llu (desc) != %lld (inode)",
179  			     le64_to_cpu(desc->data_size), inode->i_size);
180  		return ERR_PTR(-EINVAL);
181  	}
182  
183  	vi = kmem_cache_zalloc(fsverity_info_cachep, GFP_KERNEL);
184  	if (!vi)
185  		return ERR_PTR(-ENOMEM);
186  	vi->inode = inode;
187  
188  	err = fsverity_init_merkle_tree_params(&vi->tree_params, inode,
189  					       desc->hash_algorithm,
190  					       desc->log_blocksize,
191  					       desc->salt, desc->salt_size);
192  	if (err) {
193  		fsverity_err(inode,
194  			     "Error %d initializing Merkle tree parameters",
195  			     err);
196  		goto out;
197  	}
198  
199  	memcpy(vi->root_hash, desc->root_hash, vi->tree_params.digest_size);
200  
201  	err = compute_file_measurement(vi->tree_params.hash_alg, desc,
202  				       vi->measurement);
203  	if (err) {
204  		fsverity_err(inode, "Error %d computing file measurement", err);
205  		goto out;
206  	}
207  	pr_debug("Computed file measurement: %s:%*phN\n",
208  		 vi->tree_params.hash_alg->name,
209  		 vi->tree_params.digest_size, vi->measurement);
210  
211  	err = fsverity_verify_signature(vi, desc, desc_size);
212  out:
213  	if (err) {
214  		fsverity_free_info(vi);
215  		vi = ERR_PTR(err);
216  	}
217  	return vi;
218  }
219  
220  void fsverity_set_info(struct inode *inode, struct fsverity_info *vi)
221  {
222  	/*
223  	 * Multiple processes may race to set ->i_verity_info, so use cmpxchg.
224  	 * This pairs with the READ_ONCE() in fsverity_get_info().
225  	 */
226  	if (cmpxchg(&inode->i_verity_info, NULL, vi) != NULL)
227  		fsverity_free_info(vi);
228  }
229  
230  void fsverity_free_info(struct fsverity_info *vi)
231  {
232  	if (!vi)
233  		return;
234  	kfree(vi->tree_params.hashstate);
235  	kmem_cache_free(fsverity_info_cachep, vi);
236  }
237  
238  /* Ensure the inode has an ->i_verity_info */
239  static int ensure_verity_info(struct inode *inode)
240  {
241  	struct fsverity_info *vi = fsverity_get_info(inode);
242  	struct fsverity_descriptor *desc;
243  	int res;
244  
245  	if (vi)
246  		return 0;
247  
248  	res = inode->i_sb->s_vop->get_verity_descriptor(inode, NULL, 0);
249  	if (res < 0) {
250  		fsverity_err(inode,
251  			     "Error %d getting verity descriptor size", res);
252  		return res;
253  	}
254  	if (res > FS_VERITY_MAX_DESCRIPTOR_SIZE) {
255  		fsverity_err(inode, "Verity descriptor is too large (%d bytes)",
256  			     res);
257  		return -EMSGSIZE;
258  	}
259  	desc = kmalloc(res, GFP_KERNEL);
260  	if (!desc)
261  		return -ENOMEM;
262  	res = inode->i_sb->s_vop->get_verity_descriptor(inode, desc, res);
263  	if (res < 0) {
264  		fsverity_err(inode, "Error %d reading verity descriptor", res);
265  		goto out_free_desc;
266  	}
267  
268  	vi = fsverity_create_info(inode, desc, res);
269  	if (IS_ERR(vi)) {
270  		res = PTR_ERR(vi);
271  		goto out_free_desc;
272  	}
273  
274  	fsverity_set_info(inode, vi);
275  	res = 0;
276  out_free_desc:
277  	kfree(desc);
278  	return res;
279  }
280  
281  /**
282   * fsverity_file_open() - prepare to open a verity file
283   * @inode: the inode being opened
284   * @filp: the struct file being set up
285   *
286   * When opening a verity file, deny the open if it is for writing.  Otherwise,
287   * set up the inode's ->i_verity_info if not already done.
288   *
289   * When combined with fscrypt, this must be called after fscrypt_file_open().
290   * Otherwise, we won't have the key set up to decrypt the verity metadata.
291   *
292   * Return: 0 on success, -errno on failure
293   */
294  int fsverity_file_open(struct inode *inode, struct file *filp)
295  {
296  	if (!IS_VERITY(inode))
297  		return 0;
298  
299  	if (filp->f_mode & FMODE_WRITE) {
300  		pr_debug("Denying opening verity file (ino %lu) for write\n",
301  			 inode->i_ino);
302  		return -EPERM;
303  	}
304  
305  	return ensure_verity_info(inode);
306  }
307  EXPORT_SYMBOL_GPL(fsverity_file_open);
308  
309  /**
310   * fsverity_prepare_setattr() - prepare to change a verity inode's attributes
311   * @dentry: dentry through which the inode is being changed
312   * @attr: attributes to change
313   *
314   * Verity files are immutable, so deny truncates.  This isn't covered by the
315   * open-time check because sys_truncate() takes a path, not a file descriptor.
316   *
317   * Return: 0 on success, -errno on failure
318   */
319  int fsverity_prepare_setattr(struct dentry *dentry, struct iattr *attr)
320  {
321  	if (IS_VERITY(d_inode(dentry)) && (attr->ia_valid & ATTR_SIZE)) {
322  		pr_debug("Denying truncate of verity file (ino %lu)\n",
323  			 d_inode(dentry)->i_ino);
324  		return -EPERM;
325  	}
326  	return 0;
327  }
328  EXPORT_SYMBOL_GPL(fsverity_prepare_setattr);
329  
330  /**
331   * fsverity_cleanup_inode() - free the inode's verity info, if present
332   *
333   * Filesystems must call this on inode eviction to free ->i_verity_info.
334   */
335  void fsverity_cleanup_inode(struct inode *inode)
336  {
337  	fsverity_free_info(inode->i_verity_info);
338  	inode->i_verity_info = NULL;
339  }
340  EXPORT_SYMBOL_GPL(fsverity_cleanup_inode);
341  
342  int __init fsverity_init_info_cache(void)
343  {
344  	fsverity_info_cachep = KMEM_CACHE_USERCOPY(fsverity_info,
345  						   SLAB_RECLAIM_ACCOUNT,
346  						   measurement);
347  	if (!fsverity_info_cachep)
348  		return -ENOMEM;
349  	return 0;
350  }
351  
352  void __init fsverity_exit_info_cache(void)
353  {
354  	kmem_cache_destroy(fsverity_info_cachep);
355  	fsverity_info_cachep = NULL;
356  }
357