xref: /openbmc/linux/fs/verity/open.c (revision 6a87e0f0ce1ae8d70566935215430e718ea776ff)
1  // SPDX-License-Identifier: GPL-2.0
2  /*
3   * Opening fs-verity files
4   *
5   * Copyright 2019 Google LLC
6   */
7  
8  #include "fsverity_private.h"
9  
10  #include <linux/mm.h>
11  #include <linux/slab.h>
12  
13  static struct kmem_cache *fsverity_info_cachep;
14  
15  /**
16   * fsverity_init_merkle_tree_params() - initialize Merkle tree parameters
17   * @params: the parameters struct to initialize
18   * @inode: the inode for which the Merkle tree is being built
19   * @hash_algorithm: number of hash algorithm to use
20   * @log_blocksize: log base 2 of block size to use
21   * @salt: pointer to salt (optional)
22   * @salt_size: size of salt, possibly 0
23   *
24   * Validate the hash algorithm and block size, then compute the tree topology
25   * (num levels, num blocks in each level, etc.) and initialize @params.
26   *
27   * Return: 0 on success, -errno on failure
28   */
29  int fsverity_init_merkle_tree_params(struct merkle_tree_params *params,
30  				     const struct inode *inode,
31  				     unsigned int hash_algorithm,
32  				     unsigned int log_blocksize,
33  				     const u8 *salt, size_t salt_size)
34  {
35  	const struct fsverity_hash_alg *hash_alg;
36  	int err;
37  	u64 blocks;
38  	u64 blocks_in_level[FS_VERITY_MAX_LEVELS];
39  	u64 offset;
40  	int level;
41  
42  	memset(params, 0, sizeof(*params));
43  
44  	hash_alg = fsverity_get_hash_alg(inode, hash_algorithm);
45  	if (IS_ERR(hash_alg))
46  		return PTR_ERR(hash_alg);
47  	params->hash_alg = hash_alg;
48  	params->digest_size = hash_alg->digest_size;
49  
50  	params->hashstate = fsverity_prepare_hash_state(hash_alg, salt,
51  							salt_size);
52  	if (IS_ERR(params->hashstate)) {
53  		err = PTR_ERR(params->hashstate);
54  		params->hashstate = NULL;
55  		fsverity_err(inode, "Error %d preparing hash state", err);
56  		goto out_err;
57  	}
58  
59  	/*
60  	 * fs/verity/ directly assumes that the Merkle tree block size is a
61  	 * power of 2 less than or equal to PAGE_SIZE.  Another restriction
62  	 * arises from the interaction between fs/verity/ and the filesystems
63  	 * themselves: filesystems expect to be able to verify a single
64  	 * filesystem block of data at a time.  Therefore, the Merkle tree block
65  	 * size must also be less than or equal to the filesystem block size.
66  	 *
67  	 * The above are the only hard limitations, so in theory the Merkle tree
68  	 * block size could be as small as twice the digest size.  However,
69  	 * that's not useful, and it would result in some unusually deep and
70  	 * large Merkle trees.  So we currently require that the Merkle tree
71  	 * block size be at least 1024 bytes.  That's small enough to test the
72  	 * sub-page block case on systems with 4K pages, but not too small.
73  	 */
74  	if (log_blocksize < 10 || log_blocksize > PAGE_SHIFT ||
75  	    log_blocksize > inode->i_blkbits) {
76  		fsverity_warn(inode, "Unsupported log_blocksize: %u",
77  			      log_blocksize);
78  		err = -EINVAL;
79  		goto out_err;
80  	}
81  	params->log_blocksize = log_blocksize;
82  	params->block_size = 1 << log_blocksize;
83  	params->log_blocks_per_page = PAGE_SHIFT - log_blocksize;
84  	params->blocks_per_page = 1 << params->log_blocks_per_page;
85  
86  	if (WARN_ON_ONCE(!is_power_of_2(params->digest_size))) {
87  		err = -EINVAL;
88  		goto out_err;
89  	}
90  	if (params->block_size < 2 * params->digest_size) {
91  		fsverity_warn(inode,
92  			      "Merkle tree block size (%u) too small for hash algorithm \"%s\"",
93  			      params->block_size, hash_alg->name);
94  		err = -EINVAL;
95  		goto out_err;
96  	}
97  	params->log_digestsize = ilog2(params->digest_size);
98  	params->log_arity = log_blocksize - params->log_digestsize;
99  	params->hashes_per_block = 1 << params->log_arity;
100  
101  	/*
102  	 * Compute the number of levels in the Merkle tree and create a map from
103  	 * level to the starting block of that level.  Level 'num_levels - 1' is
104  	 * the root and is stored first.  Level 0 is the level directly "above"
105  	 * the data blocks and is stored last.
106  	 */
107  
108  	/* Compute number of levels and the number of blocks in each level */
109  	blocks = ((u64)inode->i_size + params->block_size - 1) >> log_blocksize;
110  	while (blocks > 1) {
111  		if (params->num_levels >= FS_VERITY_MAX_LEVELS) {
112  			fsverity_err(inode, "Too many levels in Merkle tree");
113  			err = -EFBIG;
114  			goto out_err;
115  		}
116  		blocks = (blocks + params->hashes_per_block - 1) >>
117  			 params->log_arity;
118  		blocks_in_level[params->num_levels++] = blocks;
119  	}
120  
121  	/* Compute the starting block of each level */
122  	offset = 0;
123  	for (level = (int)params->num_levels - 1; level >= 0; level--) {
124  		params->level_start[level] = offset;
125  		offset += blocks_in_level[level];
126  	}
127  
128  	/*
129  	 * With block_size != PAGE_SIZE, an in-memory bitmap will need to be
130  	 * allocated to track the "verified" status of hash blocks.  Don't allow
131  	 * this bitmap to get too large.  For now, limit it to 1 MiB, which
132  	 * limits the file size to about 4.4 TB with SHA-256 and 4K blocks.
133  	 *
134  	 * Together with the fact that the data, and thus also the Merkle tree,
135  	 * cannot have more than ULONG_MAX pages, this implies that hash block
136  	 * indices can always fit in an 'unsigned long'.  But to be safe, we
137  	 * explicitly check for that too.  Note, this is only for hash block
138  	 * indices; data block indices might not fit in an 'unsigned long'.
139  	 */
140  	if ((params->block_size != PAGE_SIZE && offset > 1 << 23) ||
141  	    offset > ULONG_MAX) {
142  		fsverity_err(inode, "Too many blocks in Merkle tree");
143  		err = -EFBIG;
144  		goto out_err;
145  	}
146  
147  	params->tree_size = offset << log_blocksize;
148  	params->tree_pages = PAGE_ALIGN(params->tree_size) >> PAGE_SHIFT;
149  	return 0;
150  
151  out_err:
152  	kfree(params->hashstate);
153  	memset(params, 0, sizeof(*params));
154  	return err;
155  }
156  
157  /*
158   * Compute the file digest by hashing the fsverity_descriptor excluding the
159   * builtin signature and with the sig_size field set to 0.
160   */
161  static int compute_file_digest(const struct fsverity_hash_alg *hash_alg,
162  			       struct fsverity_descriptor *desc,
163  			       u8 *file_digest)
164  {
165  	__le32 sig_size = desc->sig_size;
166  	int err;
167  
168  	desc->sig_size = 0;
169  	err = fsverity_hash_buffer(hash_alg, desc, sizeof(*desc), file_digest);
170  	desc->sig_size = sig_size;
171  
172  	return err;
173  }
174  
175  /*
176   * Create a new fsverity_info from the given fsverity_descriptor (with optional
177   * appended builtin signature), and check the signature if present.  The
178   * fsverity_descriptor must have already undergone basic validation.
179   */
180  struct fsverity_info *fsverity_create_info(const struct inode *inode,
181  					   struct fsverity_descriptor *desc)
182  {
183  	struct fsverity_info *vi;
184  	int err;
185  
186  	vi = kmem_cache_zalloc(fsverity_info_cachep, GFP_KERNEL);
187  	if (!vi)
188  		return ERR_PTR(-ENOMEM);
189  	vi->inode = inode;
190  
191  	err = fsverity_init_merkle_tree_params(&vi->tree_params, inode,
192  					       desc->hash_algorithm,
193  					       desc->log_blocksize,
194  					       desc->salt, desc->salt_size);
195  	if (err) {
196  		fsverity_err(inode,
197  			     "Error %d initializing Merkle tree parameters",
198  			     err);
199  		goto fail;
200  	}
201  
202  	memcpy(vi->root_hash, desc->root_hash, vi->tree_params.digest_size);
203  
204  	err = compute_file_digest(vi->tree_params.hash_alg, desc,
205  				  vi->file_digest);
206  	if (err) {
207  		fsverity_err(inode, "Error %d computing file digest", err);
208  		goto fail;
209  	}
210  
211  	err = fsverity_verify_signature(vi, desc->signature,
212  					le32_to_cpu(desc->sig_size));
213  	if (err)
214  		goto fail;
215  
216  	if (vi->tree_params.block_size != PAGE_SIZE) {
217  		/*
218  		 * When the Merkle tree block size and page size differ, we use
219  		 * a bitmap to keep track of which hash blocks have been
220  		 * verified.  This bitmap must contain one bit per hash block,
221  		 * including alignment to a page boundary at the end.
222  		 *
223  		 * Eventually, to support extremely large files in an efficient
224  		 * way, it might be necessary to make pages of this bitmap
225  		 * reclaimable.  But for now, simply allocating the whole bitmap
226  		 * is a simple solution that works well on the files on which
227  		 * fsverity is realistically used.  E.g., with SHA-256 and 4K
228  		 * blocks, a 100MB file only needs a 24-byte bitmap, and the
229  		 * bitmap for any file under 17GB fits in a 4K page.
230  		 */
231  		unsigned long num_bits =
232  			vi->tree_params.tree_pages <<
233  			vi->tree_params.log_blocks_per_page;
234  
235  		vi->hash_block_verified = kvcalloc(BITS_TO_LONGS(num_bits),
236  						   sizeof(unsigned long),
237  						   GFP_KERNEL);
238  		if (!vi->hash_block_verified) {
239  			err = -ENOMEM;
240  			goto fail;
241  		}
242  		spin_lock_init(&vi->hash_page_init_lock);
243  	}
244  
245  	return vi;
246  
247  fail:
248  	fsverity_free_info(vi);
249  	return ERR_PTR(err);
250  }
251  
252  void fsverity_set_info(struct inode *inode, struct fsverity_info *vi)
253  {
254  	/*
255  	 * Multiple tasks may race to set ->i_verity_info, so use
256  	 * cmpxchg_release().  This pairs with the smp_load_acquire() in
257  	 * fsverity_get_info().  I.e., here we publish ->i_verity_info with a
258  	 * RELEASE barrier so that other tasks can ACQUIRE it.
259  	 */
260  	if (cmpxchg_release(&inode->i_verity_info, NULL, vi) != NULL) {
261  		/* Lost the race, so free the fsverity_info we allocated. */
262  		fsverity_free_info(vi);
263  		/*
264  		 * Afterwards, the caller may access ->i_verity_info directly,
265  		 * so make sure to ACQUIRE the winning fsverity_info.
266  		 */
267  		(void)fsverity_get_info(inode);
268  	}
269  }
270  
271  void fsverity_free_info(struct fsverity_info *vi)
272  {
273  	if (!vi)
274  		return;
275  	kfree(vi->tree_params.hashstate);
276  	kvfree(vi->hash_block_verified);
277  	kmem_cache_free(fsverity_info_cachep, vi);
278  }
279  
280  static bool validate_fsverity_descriptor(struct inode *inode,
281  					 const struct fsverity_descriptor *desc,
282  					 size_t desc_size)
283  {
284  	if (desc_size < sizeof(*desc)) {
285  		fsverity_err(inode, "Unrecognized descriptor size: %zu bytes",
286  			     desc_size);
287  		return false;
288  	}
289  
290  	if (desc->version != 1) {
291  		fsverity_err(inode, "Unrecognized descriptor version: %u",
292  			     desc->version);
293  		return false;
294  	}
295  
296  	if (memchr_inv(desc->__reserved, 0, sizeof(desc->__reserved))) {
297  		fsverity_err(inode, "Reserved bits set in descriptor");
298  		return false;
299  	}
300  
301  	if (desc->salt_size > sizeof(desc->salt)) {
302  		fsverity_err(inode, "Invalid salt_size: %u", desc->salt_size);
303  		return false;
304  	}
305  
306  	if (le64_to_cpu(desc->data_size) != inode->i_size) {
307  		fsverity_err(inode,
308  			     "Wrong data_size: %llu (desc) != %lld (inode)",
309  			     le64_to_cpu(desc->data_size), inode->i_size);
310  		return false;
311  	}
312  
313  	if (le32_to_cpu(desc->sig_size) > desc_size - sizeof(*desc)) {
314  		fsverity_err(inode, "Signature overflows verity descriptor");
315  		return false;
316  	}
317  
318  	return true;
319  }
320  
321  /*
322   * Read the inode's fsverity_descriptor (with optional appended builtin
323   * signature) from the filesystem, and do basic validation of it.
324   */
325  int fsverity_get_descriptor(struct inode *inode,
326  			    struct fsverity_descriptor **desc_ret)
327  {
328  	int res;
329  	struct fsverity_descriptor *desc;
330  
331  	res = inode->i_sb->s_vop->get_verity_descriptor(inode, NULL, 0);
332  	if (res < 0) {
333  		fsverity_err(inode,
334  			     "Error %d getting verity descriptor size", res);
335  		return res;
336  	}
337  	if (res > FS_VERITY_MAX_DESCRIPTOR_SIZE) {
338  		fsverity_err(inode, "Verity descriptor is too large (%d bytes)",
339  			     res);
340  		return -EMSGSIZE;
341  	}
342  	desc = kmalloc(res, GFP_KERNEL);
343  	if (!desc)
344  		return -ENOMEM;
345  	res = inode->i_sb->s_vop->get_verity_descriptor(inode, desc, res);
346  	if (res < 0) {
347  		fsverity_err(inode, "Error %d reading verity descriptor", res);
348  		kfree(desc);
349  		return res;
350  	}
351  
352  	if (!validate_fsverity_descriptor(inode, desc, res)) {
353  		kfree(desc);
354  		return -EINVAL;
355  	}
356  
357  	*desc_ret = desc;
358  	return 0;
359  }
360  
361  /* Ensure the inode has an ->i_verity_info */
362  static int ensure_verity_info(struct inode *inode)
363  {
364  	struct fsverity_info *vi = fsverity_get_info(inode);
365  	struct fsverity_descriptor *desc;
366  	int err;
367  
368  	if (vi)
369  		return 0;
370  
371  	err = fsverity_get_descriptor(inode, &desc);
372  	if (err)
373  		return err;
374  
375  	vi = fsverity_create_info(inode, desc);
376  	if (IS_ERR(vi)) {
377  		err = PTR_ERR(vi);
378  		goto out_free_desc;
379  	}
380  
381  	fsverity_set_info(inode, vi);
382  	err = 0;
383  out_free_desc:
384  	kfree(desc);
385  	return err;
386  }
387  
388  int __fsverity_file_open(struct inode *inode, struct file *filp)
389  {
390  	if (filp->f_mode & FMODE_WRITE)
391  		return -EPERM;
392  	return ensure_verity_info(inode);
393  }
394  EXPORT_SYMBOL_GPL(__fsverity_file_open);
395  
396  int __fsverity_prepare_setattr(struct dentry *dentry, struct iattr *attr)
397  {
398  	if (attr->ia_valid & ATTR_SIZE)
399  		return -EPERM;
400  	return 0;
401  }
402  EXPORT_SYMBOL_GPL(__fsverity_prepare_setattr);
403  
404  void __fsverity_cleanup_inode(struct inode *inode)
405  {
406  	fsverity_free_info(inode->i_verity_info);
407  	inode->i_verity_info = NULL;
408  }
409  EXPORT_SYMBOL_GPL(__fsverity_cleanup_inode);
410  
411  int __init fsverity_init_info_cache(void)
412  {
413  	fsverity_info_cachep = KMEM_CACHE_USERCOPY(fsverity_info,
414  						   SLAB_RECLAIM_ACCOUNT,
415  						   file_digest);
416  	if (!fsverity_info_cachep)
417  		return -ENOMEM;
418  	return 0;
419  }
420  
421  void __init fsverity_exit_info_cache(void)
422  {
423  	kmem_cache_destroy(fsverity_info_cachep);
424  	fsverity_info_cachep = NULL;
425  }
426