1// SPDX-License-Identifier: GPL-2.0
2/*
3 * This contains functions for filename crypto management
4 *
5 * Copyright (C) 2015, Google, Inc.
6 * Copyright (C) 2015, Motorola Mobility
7 *
8 * Written by Uday Savagaonkar, 2014.
9 * Modified by Jaegeuk Kim, 2015.
10 *
11 * This has not yet undergone a rigorous security audit.
12 */
13
14#include <crypto/sha2.h>
15#include <crypto/skcipher.h>
16#include <linux/export.h>
17#include <linux/namei.h>
18#include <linux/scatterlist.h>
19#include <linux/base64.h>
20
21#include "fscrypt_private.h"
22
23/*
24 * The minimum message length (input and output length), in bytes, for all
25 * filenames encryption modes. Filenames shorter than this will be zero-padded
26 * before being encrypted.
27 */
28#define FSCRYPT_FNAME_MIN_MSG_LEN 16
29
30/*
31 * struct fscrypt_nokey_name - identifier for directory entry when key is absent
32 *
33 * When userspace lists an encrypted directory without access to the key, the
34 * filesystem must present a unique "no-key name" for each filename that allows
35 * it to find the directory entry again if requested. Naively, that would just
36 * mean using the ciphertext filenames. However, since the ciphertext filenames
37 * can contain illegal characters ('\0' and '/'), they must be encoded in some
38 * way. We use base64url. But that can cause names to exceed NAME_MAX (255
39 * bytes), so we also need to use a strong hash to abbreviate long names.
40 *
41 * The filesystem may also need another kind of hash, the "dirhash", to quickly
42 * find the directory entry. Since filesystems normally compute the dirhash
43 * over the on-disk filename (i.e. the ciphertext), it's not computable from
44 * no-key names that abbreviate the ciphertext using the strong hash to fit in
45 * NAME_MAX. It's also not computable if it's a keyed hash taken over the
46 * plaintext (but it may still be available in the on-disk directory entry);
47 * casefolded directories use this type of dirhash. At least in these cases,
48 * each no-key name must include the name's dirhash too.
49 *
50 * To meet all these requirements, we base64url-encode the following
51 * variable-length structure. It contains the dirhash, or 0's if the filesystem
52 * didn't provide one; up to 149 bytes of the ciphertext name; and for
53 * ciphertexts longer than 149 bytes, also the SHA-256 of the remaining bytes.
54 *
55 * This ensures that each no-key name contains everything needed to find the
56 * directory entry again, contains only legal characters, doesn't exceed
57 * NAME_MAX, is unambiguous unless there's a SHA-256 collision, and that we only
58 * take the performance hit of SHA-256 on very long filenames (which are rare).
59 */
60struct fscrypt_nokey_name {
61 u32 dirhash[2];
62 u8 bytes[149];
63 u8 sha256[SHA256_DIGEST_SIZE];
64}; /* 189 bytes => 252 bytes base64url-encoded, which is <= NAME_MAX (255) */
65
66/*
67 * Decoded size of max-size no-key name, i.e. a name that was abbreviated using
68 * the strong hash and thus includes the 'sha256' field. This isn't simply
69 * sizeof(struct fscrypt_nokey_name), as the padding at the end isn't included.
70 */
71#define FSCRYPT_NOKEY_NAME_MAX offsetofend(struct fscrypt_nokey_name, sha256)
72
73/* Encoded size of max-size no-key name */
74#define FSCRYPT_NOKEY_NAME_MAX_ENCODED \
75 BASE64_CHARS(FSCRYPT_NOKEY_NAME_MAX)
76
77static inline bool fscrypt_is_dot_dotdot(const struct qstr *str)
78{
79 return is_dot_dotdot(name: str->name, len: str->len);
80}
81
82/**
83 * fscrypt_fname_encrypt() - encrypt a filename
84 * @inode: inode of the parent directory (for regular filenames)
85 * or of the symlink (for symlink targets). Key must already be
86 * set up.
87 * @iname: the filename to encrypt
88 * @out: (output) the encrypted filename
89 * @olen: size of the encrypted filename. It must be at least @iname->len.
90 * Any extra space is filled with NUL padding before encryption.
91 *
92 * Return: 0 on success, -errno on failure
93 */
94int fscrypt_fname_encrypt(const struct inode *inode, const struct qstr *iname,
95 u8 *out, unsigned int olen)
96{
97 const struct fscrypt_inode_info *ci = fscrypt_get_inode_info_raw(inode);
98 struct crypto_sync_skcipher *tfm = ci->ci_enc_key.tfm;
99 SYNC_SKCIPHER_REQUEST_ON_STACK(req, tfm);
100 union fscrypt_iv iv;
101 struct scatterlist sg;
102 int err;
103
104 /*
105 * Copy the filename to the output buffer for encrypting in-place and
106 * pad it with the needed number of NUL bytes.
107 */
108 if (WARN_ON_ONCE(olen < iname->len))
109 return -ENOBUFS;
110 memcpy(out, iname->name, iname->len);
111 memset(out + iname->len, 0, olen - iname->len);
112
113 fscrypt_generate_iv(iv: &iv, index: 0, ci);
114
115 skcipher_request_set_callback(
116 req, CRYPTO_TFM_REQ_MAY_BACKLOG | CRYPTO_TFM_REQ_MAY_SLEEP,
117 NULL, NULL);
118 sg_init_one(&sg, out, olen);
119 skcipher_request_set_crypt(req, src: &sg, dst: &sg, cryptlen: olen, iv: &iv);
120 err = crypto_skcipher_encrypt(req);
121 if (err)
122 fscrypt_err(inode, "Filename encryption failed: %d", err);
123 return err;
124}
125EXPORT_SYMBOL_GPL(fscrypt_fname_encrypt);
126
127/**
128 * fname_decrypt() - decrypt a filename
129 * @inode: inode of the parent directory (for regular filenames)
130 * or of the symlink (for symlink targets)
131 * @iname: the encrypted filename to decrypt
132 * @oname: (output) the decrypted filename. The caller must have allocated
133 * enough space for this, e.g. using fscrypt_fname_alloc_buffer().
134 *
135 * Return: 0 on success, -errno on failure
136 */
137static int fname_decrypt(const struct inode *inode,
138 const struct fscrypt_str *iname,
139 struct fscrypt_str *oname)
140{
141 const struct fscrypt_inode_info *ci = fscrypt_get_inode_info_raw(inode);
142 struct crypto_sync_skcipher *tfm = ci->ci_enc_key.tfm;
143 SYNC_SKCIPHER_REQUEST_ON_STACK(req, tfm);
144 union fscrypt_iv iv;
145 struct scatterlist src_sg, dst_sg;
146 int err;
147
148 fscrypt_generate_iv(iv: &iv, index: 0, ci);
149
150 skcipher_request_set_callback(
151 req, CRYPTO_TFM_REQ_MAY_BACKLOG | CRYPTO_TFM_REQ_MAY_SLEEP,
152 NULL, NULL);
153 sg_init_one(&src_sg, iname->name, iname->len);
154 sg_init_one(&dst_sg, oname->name, oname->len);
155 skcipher_request_set_crypt(req, src: &src_sg, dst: &dst_sg, cryptlen: iname->len, iv: &iv);
156 err = crypto_skcipher_decrypt(req);
157 if (err) {
158 fscrypt_err(inode, "Filename decryption failed: %d", err);
159 return err;
160 }
161
162 oname->len = strnlen(p: oname->name, maxlen: iname->len);
163 return 0;
164}
165
166bool __fscrypt_fname_encrypted_size(const union fscrypt_policy *policy,
167 u32 orig_len, u32 max_len,
168 u32 *encrypted_len_ret)
169{
170 int padding = 4 << (fscrypt_policy_flags(policy) &
171 FSCRYPT_POLICY_FLAGS_PAD_MASK);
172 u32 encrypted_len;
173
174 if (orig_len > max_len)
175 return false;
176 encrypted_len = max_t(u32, orig_len, FSCRYPT_FNAME_MIN_MSG_LEN);
177 encrypted_len = round_up(encrypted_len, padding);
178 *encrypted_len_ret = min(encrypted_len, max_len);
179 return true;
180}
181
182/**
183 * fscrypt_fname_encrypted_size() - calculate length of encrypted filename
184 * @inode: parent inode of dentry name being encrypted. Key must
185 * already be set up.
186 * @orig_len: length of the original filename
187 * @max_len: maximum length to return
188 * @encrypted_len_ret: where calculated length should be returned (on success)
189 *
190 * Filenames that are shorter than the maximum length may have their lengths
191 * increased slightly by encryption, due to padding that is applied.
192 *
193 * Return: false if the orig_len is greater than max_len. Otherwise, true and
194 * fill out encrypted_len_ret with the length (up to max_len).
195 */
196bool fscrypt_fname_encrypted_size(const struct inode *inode, u32 orig_len,
197 u32 max_len, u32 *encrypted_len_ret)
198{
199 const struct fscrypt_inode_info *ci = fscrypt_get_inode_info_raw(inode);
200
201 return __fscrypt_fname_encrypted_size(policy: &ci->ci_policy, orig_len, max_len,
202 encrypted_len_ret);
203}
204EXPORT_SYMBOL_GPL(fscrypt_fname_encrypted_size);
205
206/**
207 * fscrypt_fname_alloc_buffer() - allocate a buffer for presented filenames
208 * @max_encrypted_len: maximum length of encrypted filenames the buffer will be
209 * used to present
210 * @crypto_str: (output) buffer to allocate
211 *
212 * Allocate a buffer that is large enough to hold any decrypted or encoded
213 * filename (null-terminated), for the given maximum encrypted filename length.
214 *
215 * Return: 0 on success, -errno on failure
216 */
217int fscrypt_fname_alloc_buffer(u32 max_encrypted_len,
218 struct fscrypt_str *crypto_str)
219{
220 u32 max_presented_len = max_t(u32, FSCRYPT_NOKEY_NAME_MAX_ENCODED,
221 max_encrypted_len);
222
223 crypto_str->name = kmalloc(max_presented_len + 1, GFP_NOFS);
224 if (!crypto_str->name)
225 return -ENOMEM;
226 crypto_str->len = max_presented_len;
227 return 0;
228}
229EXPORT_SYMBOL(fscrypt_fname_alloc_buffer);
230
231/**
232 * fscrypt_fname_free_buffer() - free a buffer for presented filenames
233 * @crypto_str: the buffer to free
234 *
235 * Free a buffer that was allocated by fscrypt_fname_alloc_buffer().
236 */
237void fscrypt_fname_free_buffer(struct fscrypt_str *crypto_str)
238{
239 if (!crypto_str)
240 return;
241 kfree(objp: crypto_str->name);
242 crypto_str->name = NULL;
243}
244EXPORT_SYMBOL(fscrypt_fname_free_buffer);
245
246/**
247 * fscrypt_fname_disk_to_usr() - convert an encrypted filename to
248 * user-presentable form
249 * @inode: inode of the parent directory (for regular filenames)
250 * or of the symlink (for symlink targets)
251 * @hash: first part of the name's dirhash, if applicable. This only needs to
252 * be provided if the filename is located in an indexed directory whose
253 * encryption key may be unavailable. Not needed for symlink targets.
254 * @minor_hash: second part of the name's dirhash, if applicable
255 * @iname: encrypted filename to convert. May also be "." or "..", which
256 * aren't actually encrypted.
257 * @oname: output buffer for the user-presentable filename. The caller must
258 * have allocated enough space for this, e.g. using
259 * fscrypt_fname_alloc_buffer().
260 *
261 * If the key is available, we'll decrypt the disk name. Otherwise, we'll
262 * encode it for presentation in fscrypt_nokey_name format.
263 * See struct fscrypt_nokey_name for details.
264 *
265 * Return: 0 on success, -errno on failure
266 */
267int fscrypt_fname_disk_to_usr(const struct inode *inode,
268 u32 hash, u32 minor_hash,
269 const struct fscrypt_str *iname,
270 struct fscrypt_str *oname)
271{
272 const struct qstr qname = FSTR_TO_QSTR(iname);
273 struct fscrypt_nokey_name nokey_name;
274 u32 size; /* size of the unencoded no-key name */
275
276 if (fscrypt_is_dot_dotdot(str: &qname)) {
277 oname->name[0] = '.';
278 oname->name[iname->len - 1] = '.';
279 oname->len = iname->len;
280 return 0;
281 }
282
283 if (iname->len < FSCRYPT_FNAME_MIN_MSG_LEN)
284 return -EUCLEAN;
285
286 if (fscrypt_has_encryption_key(inode))
287 return fname_decrypt(inode, iname, oname);
288
289 /*
290 * Sanity check that struct fscrypt_nokey_name doesn't have padding
291 * between fields and that its encoded size never exceeds NAME_MAX.
292 */
293 BUILD_BUG_ON(offsetofend(struct fscrypt_nokey_name, dirhash) !=
294 offsetof(struct fscrypt_nokey_name, bytes));
295 BUILD_BUG_ON(offsetofend(struct fscrypt_nokey_name, bytes) !=
296 offsetof(struct fscrypt_nokey_name, sha256));
297 BUILD_BUG_ON(FSCRYPT_NOKEY_NAME_MAX_ENCODED > NAME_MAX);
298
299 nokey_name.dirhash[0] = hash;
300 nokey_name.dirhash[1] = minor_hash;
301
302 if (iname->len <= sizeof(nokey_name.bytes)) {
303 memcpy(nokey_name.bytes, iname->name, iname->len);
304 size = offsetof(struct fscrypt_nokey_name, bytes[iname->len]);
305 } else {
306 memcpy(nokey_name.bytes, iname->name, sizeof(nokey_name.bytes));
307 /* Compute strong hash of remaining part of name. */
308 sha256(data: &iname->name[sizeof(nokey_name.bytes)],
309 len: iname->len - sizeof(nokey_name.bytes),
310 out: nokey_name.sha256);
311 size = FSCRYPT_NOKEY_NAME_MAX;
312 }
313 oname->len = base64_encode(src: (const u8 *)&nokey_name, len: size,
314 dst: oname->name, padding: false, variant: BASE64_URLSAFE);
315 return 0;
316}
317EXPORT_SYMBOL(fscrypt_fname_disk_to_usr);
318
319/**
320 * fscrypt_setup_filename() - prepare to search a possibly encrypted directory
321 * @dir: the directory that will be searched
322 * @iname: the user-provided filename being searched for
323 * @lookup: 1 if we're allowed to proceed without the key because it's
324 * ->lookup() or we're finding the dir_entry for deletion; 0 if we cannot
325 * proceed without the key because we're going to create the dir_entry.
326 * @fname: the filename information to be filled in
327 *
328 * Given a user-provided filename @iname, this function sets @fname->disk_name
329 * to the name that would be stored in the on-disk directory entry, if possible.
330 * If the directory is unencrypted this is simply @iname. Else, if we have the
331 * directory's encryption key, then @iname is the plaintext, so we encrypt it to
332 * get the disk_name.
333 *
334 * Else, for keyless @lookup operations, @iname should be a no-key name, so we
335 * decode it to get the struct fscrypt_nokey_name. Non-@lookup operations will
336 * be impossible in this case, so we fail them with ENOKEY.
337 *
338 * If successful, fscrypt_free_filename() must be called later to clean up.
339 *
340 * Return: 0 on success, -errno on failure
341 */
342int fscrypt_setup_filename(struct inode *dir, const struct qstr *iname,
343 int lookup, struct fscrypt_name *fname)
344{
345 struct fscrypt_nokey_name *nokey_name;
346 int ret;
347
348 memset(fname, 0, sizeof(struct fscrypt_name));
349 fname->usr_fname = iname;
350
351 if (!IS_ENCRYPTED(dir) || fscrypt_is_dot_dotdot(str: iname)) {
352 fname->disk_name.name = (unsigned char *)iname->name;
353 fname->disk_name.len = iname->len;
354 return 0;
355 }
356 ret = fscrypt_get_encryption_info(inode: dir, allow_unsupported: lookup);
357 if (ret)
358 return ret;
359
360 if (fscrypt_has_encryption_key(inode: dir)) {
361 if (!fscrypt_fname_encrypted_size(dir, iname->len, NAME_MAX,
362 &fname->crypto_buf.len))
363 return -ENAMETOOLONG;
364 fname->crypto_buf.name = kmalloc(fname->crypto_buf.len,
365 GFP_NOFS);
366 if (!fname->crypto_buf.name)
367 return -ENOMEM;
368
369 ret = fscrypt_fname_encrypt(dir, iname, fname->crypto_buf.name,
370 fname->crypto_buf.len);
371 if (ret)
372 goto errout;
373 fname->disk_name.name = fname->crypto_buf.name;
374 fname->disk_name.len = fname->crypto_buf.len;
375 return 0;
376 }
377 if (!lookup)
378 return -ENOKEY;
379 fname->is_nokey_name = true;
380
381 /*
382 * We don't have the key and we are doing a lookup; decode the
383 * user-supplied name
384 */
385
386 if (iname->len > FSCRYPT_NOKEY_NAME_MAX_ENCODED)
387 return -ENOENT;
388
389 fname->crypto_buf.name = kmalloc(FSCRYPT_NOKEY_NAME_MAX, GFP_KERNEL);
390 if (fname->crypto_buf.name == NULL)
391 return -ENOMEM;
392
393 ret = base64_decode(src: iname->name, len: iname->len,
394 dst: fname->crypto_buf.name, padding: false, variant: BASE64_URLSAFE);
395 if (ret < (int)offsetof(struct fscrypt_nokey_name, bytes[1]) ||
396 (ret > offsetof(struct fscrypt_nokey_name, sha256) &&
397 ret != FSCRYPT_NOKEY_NAME_MAX)) {
398 ret = -ENOENT;
399 goto errout;
400 }
401 fname->crypto_buf.len = ret;
402
403 nokey_name = (void *)fname->crypto_buf.name;
404 fname->hash = nokey_name->dirhash[0];
405 fname->minor_hash = nokey_name->dirhash[1];
406 if (ret != FSCRYPT_NOKEY_NAME_MAX) {
407 /* The full ciphertext filename is available. */
408 fname->disk_name.name = nokey_name->bytes;
409 fname->disk_name.len =
410 ret - offsetof(struct fscrypt_nokey_name, bytes);
411 }
412 return 0;
413
414errout:
415 kfree(objp: fname->crypto_buf.name);
416 return ret;
417}
418EXPORT_SYMBOL(fscrypt_setup_filename);
419
420/**
421 * fscrypt_match_name() - test whether the given name matches a directory entry
422 * @fname: the name being searched for
423 * @de_name: the name from the directory entry
424 * @de_name_len: the length of @de_name in bytes
425 *
426 * Normally @fname->disk_name will be set, and in that case we simply compare
427 * that to the name stored in the directory entry. The only exception is that
428 * if we don't have the key for an encrypted directory and the name we're
429 * looking for is very long, then we won't have the full disk_name and instead
430 * we'll need to match against a fscrypt_nokey_name that includes a strong hash.
431 *
432 * Return: %true if the name matches, otherwise %false.
433 */
434bool fscrypt_match_name(const struct fscrypt_name *fname,
435 const u8 *de_name, u32 de_name_len)
436{
437 const struct fscrypt_nokey_name *nokey_name =
438 (const void *)fname->crypto_buf.name;
439 u8 digest[SHA256_DIGEST_SIZE];
440
441 if (likely(fname->disk_name.name)) {
442 if (de_name_len != fname->disk_name.len)
443 return false;
444 return !memcmp(p: de_name, q: fname->disk_name.name, size: de_name_len);
445 }
446 if (de_name_len <= sizeof(nokey_name->bytes))
447 return false;
448 if (memcmp(p: de_name, q: nokey_name->bytes, size: sizeof(nokey_name->bytes)))
449 return false;
450 sha256(data: &de_name[sizeof(nokey_name->bytes)],
451 len: de_name_len - sizeof(nokey_name->bytes), out: digest);
452 return !memcmp(p: digest, q: nokey_name->sha256, size: sizeof(digest));
453}
454EXPORT_SYMBOL_GPL(fscrypt_match_name);
455
456/**
457 * fscrypt_fname_siphash() - calculate the SipHash of a filename
458 * @dir: the parent directory
459 * @name: the filename to calculate the SipHash of
460 *
461 * Given a plaintext filename @name and a directory @dir which uses SipHash as
462 * its dirhash method and has had its fscrypt key set up, this function
463 * calculates the SipHash of that name using the directory's secret dirhash key.
464 *
465 * Return: the SipHash of @name using the hash key of @dir
466 */
467u64 fscrypt_fname_siphash(const struct inode *dir, const struct qstr *name)
468{
469 const struct fscrypt_inode_info *ci = fscrypt_get_inode_info_raw(inode: dir);
470
471 WARN_ON_ONCE(!ci->ci_dirhash_key_initialized);
472
473 return siphash(data: name->name, len: name->len, key: &ci->ci_dirhash_key);
474}
475EXPORT_SYMBOL_GPL(fscrypt_fname_siphash);
476
477/*
478 * Validate dentries in encrypted directories to make sure we aren't potentially
479 * caching stale dentries after a key has been added.
480 */
481int fscrypt_d_revalidate(struct inode *dir, const struct qstr *name,
482 struct dentry *dentry, unsigned int flags)
483{
484 int err;
485
486 /*
487 * Plaintext names are always valid, since fscrypt doesn't support
488 * reverting to no-key names without evicting the directory's inode
489 * -- which implies eviction of the dentries in the directory.
490 */
491 if (!(dentry->d_flags & DCACHE_NOKEY_NAME))
492 return 1;
493
494 /*
495 * No-key name; valid if the directory's key is still unavailable.
496 *
497 * Note in RCU mode we have to bail if we get here -
498 * fscrypt_get_encryption_info() may block.
499 */
500
501 if (flags & LOOKUP_RCU)
502 return -ECHILD;
503
504 /*
505 * Pass allow_unsupported=true, so that files with an unsupported
506 * encryption policy can be deleted.
507 */
508 err = fscrypt_get_encryption_info(inode: dir, allow_unsupported: true);
509 if (err < 0)
510 return err;
511
512 return !fscrypt_has_encryption_key(inode: dir);
513}
514EXPORT_SYMBOL_GPL(fscrypt_d_revalidate);
515

source code of linux/fs/crypto/fname.c