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 <linux/namei.h>
15#include <linux/scatterlist.h>
16#include <crypto/hash.h>
17#include <crypto/sha2.h>
18#include <crypto/skcipher.h>
19#include "fscrypt_private.h"
20
21/*
22 * The minimum message length (input and output length), in bytes, for all
23 * filenames encryption modes. Filenames shorter than this will be zero-padded
24 * before being encrypted.
25 */
26#define FSCRYPT_FNAME_MIN_MSG_LEN 16
27
28/*
29 * struct fscrypt_nokey_name - identifier for directory entry when key is absent
30 *
31 * When userspace lists an encrypted directory without access to the key, the
32 * filesystem must present a unique "no-key name" for each filename that allows
33 * it to find the directory entry again if requested. Naively, that would just
34 * mean using the ciphertext filenames. However, since the ciphertext filenames
35 * can contain illegal characters ('\0' and '/'), they must be encoded in some
36 * way. We use base64url. But that can cause names to exceed NAME_MAX (255
37 * bytes), so we also need to use a strong hash to abbreviate long names.
38 *
39 * The filesystem may also need another kind of hash, the "dirhash", to quickly
40 * find the directory entry. Since filesystems normally compute the dirhash
41 * over the on-disk filename (i.e. the ciphertext), it's not computable from
42 * no-key names that abbreviate the ciphertext using the strong hash to fit in
43 * NAME_MAX. It's also not computable if it's a keyed hash taken over the
44 * plaintext (but it may still be available in the on-disk directory entry);
45 * casefolded directories use this type of dirhash. At least in these cases,
46 * each no-key name must include the name's dirhash too.
47 *
48 * To meet all these requirements, we base64url-encode the following
49 * variable-length structure. It contains the dirhash, or 0's if the filesystem
50 * didn't provide one; up to 149 bytes of the ciphertext name; and for
51 * ciphertexts longer than 149 bytes, also the SHA-256 of the remaining bytes.
52 *
53 * This ensures that each no-key name contains everything needed to find the
54 * directory entry again, contains only legal characters, doesn't exceed
55 * NAME_MAX, is unambiguous unless there's a SHA-256 collision, and that we only
56 * take the performance hit of SHA-256 on very long filenames (which are rare).
57 */
58struct fscrypt_nokey_name {
59 u32 dirhash[2];
60 u8 bytes[149];
61 u8 sha256[SHA256_DIGEST_SIZE];
62}; /* 189 bytes => 252 bytes base64url-encoded, which is <= NAME_MAX (255) */
63
64/*
65 * Decoded size of max-size no-key name, i.e. a name that was abbreviated using
66 * the strong hash and thus includes the 'sha256' field. This isn't simply
67 * sizeof(struct fscrypt_nokey_name), as the padding at the end isn't included.
68 */
69#define FSCRYPT_NOKEY_NAME_MAX offsetofend(struct fscrypt_nokey_name, sha256)
70
71/* Encoded size of max-size no-key name */
72#define FSCRYPT_NOKEY_NAME_MAX_ENCODED \
73 FSCRYPT_BASE64URL_CHARS(FSCRYPT_NOKEY_NAME_MAX)
74
75static inline bool fscrypt_is_dot_dotdot(const struct qstr *str)
76{
77 if (str->len == 1 && str->name[0] == '.')
78 return true;
79
80 if (str->len == 2 && str->name[0] == '.' && str->name[1] == '.')
81 return true;
82
83 return false;
84}
85
86/**
87 * fscrypt_fname_encrypt() - encrypt a filename
88 * @inode: inode of the parent directory (for regular filenames)
89 * or of the symlink (for symlink targets). Key must already be
90 * set up.
91 * @iname: the filename to encrypt
92 * @out: (output) the encrypted filename
93 * @olen: size of the encrypted filename. It must be at least @iname->len.
94 * Any extra space is filled with NUL padding before encryption.
95 *
96 * Return: 0 on success, -errno on failure
97 */
98int fscrypt_fname_encrypt(const struct inode *inode, const struct qstr *iname,
99 u8 *out, unsigned int olen)
100{
101 struct skcipher_request *req = NULL;
102 DECLARE_CRYPTO_WAIT(wait);
103 const struct fscrypt_inode_info *ci = inode->i_crypt_info;
104 struct crypto_skcipher *tfm = ci->ci_enc_key.tfm;
105 union fscrypt_iv iv;
106 struct scatterlist sg;
107 int res;
108
109 /*
110 * Copy the filename to the output buffer for encrypting in-place and
111 * pad it with the needed number of NUL bytes.
112 */
113 if (WARN_ON_ONCE(olen < iname->len))
114 return -ENOBUFS;
115 memcpy(out, iname->name, iname->len);
116 memset(out + iname->len, 0, olen - iname->len);
117
118 /* Initialize the IV */
119 fscrypt_generate_iv(iv: &iv, index: 0, ci);
120
121 /* Set up the encryption request */
122 req = skcipher_request_alloc(tfm, GFP_NOFS);
123 if (!req)
124 return -ENOMEM;
125 skcipher_request_set_callback(req,
126 CRYPTO_TFM_REQ_MAY_BACKLOG | CRYPTO_TFM_REQ_MAY_SLEEP,
127 compl: crypto_req_done, data: &wait);
128 sg_init_one(&sg, out, olen);
129 skcipher_request_set_crypt(req, src: &sg, dst: &sg, cryptlen: olen, iv: &iv);
130
131 /* Do the encryption */
132 res = crypto_wait_req(err: crypto_skcipher_encrypt(req), wait: &wait);
133 skcipher_request_free(req);
134 if (res < 0) {
135 fscrypt_err(inode, "Filename encryption failed: %d", res);
136 return res;
137 }
138
139 return 0;
140}
141EXPORT_SYMBOL_GPL(fscrypt_fname_encrypt);
142
143/**
144 * fname_decrypt() - decrypt a filename
145 * @inode: inode of the parent directory (for regular filenames)
146 * or of the symlink (for symlink targets)
147 * @iname: the encrypted filename to decrypt
148 * @oname: (output) the decrypted filename. The caller must have allocated
149 * enough space for this, e.g. using fscrypt_fname_alloc_buffer().
150 *
151 * Return: 0 on success, -errno on failure
152 */
153static int fname_decrypt(const struct inode *inode,
154 const struct fscrypt_str *iname,
155 struct fscrypt_str *oname)
156{
157 struct skcipher_request *req = NULL;
158 DECLARE_CRYPTO_WAIT(wait);
159 struct scatterlist src_sg, dst_sg;
160 const struct fscrypt_inode_info *ci = inode->i_crypt_info;
161 struct crypto_skcipher *tfm = ci->ci_enc_key.tfm;
162 union fscrypt_iv iv;
163 int res;
164
165 /* Allocate request */
166 req = skcipher_request_alloc(tfm, GFP_NOFS);
167 if (!req)
168 return -ENOMEM;
169 skcipher_request_set_callback(req,
170 CRYPTO_TFM_REQ_MAY_BACKLOG | CRYPTO_TFM_REQ_MAY_SLEEP,
171 compl: crypto_req_done, data: &wait);
172
173 /* Initialize IV */
174 fscrypt_generate_iv(iv: &iv, index: 0, ci);
175
176 /* Create decryption request */
177 sg_init_one(&src_sg, iname->name, iname->len);
178 sg_init_one(&dst_sg, oname->name, oname->len);
179 skcipher_request_set_crypt(req, src: &src_sg, dst: &dst_sg, cryptlen: iname->len, iv: &iv);
180 res = crypto_wait_req(err: crypto_skcipher_decrypt(req), wait: &wait);
181 skcipher_request_free(req);
182 if (res < 0) {
183 fscrypt_err(inode, "Filename decryption failed: %d", res);
184 return res;
185 }
186
187 oname->len = strnlen(p: oname->name, maxlen: iname->len);
188 return 0;
189}
190
191static const char base64url_table[65] =
192 "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789-_";
193
194#define FSCRYPT_BASE64URL_CHARS(nbytes) DIV_ROUND_UP((nbytes) * 4, 3)
195
196/**
197 * fscrypt_base64url_encode() - base64url-encode some binary data
198 * @src: the binary data to encode
199 * @srclen: the length of @src in bytes
200 * @dst: (output) the base64url-encoded string. Not NUL-terminated.
201 *
202 * Encodes data using base64url encoding, i.e. the "Base 64 Encoding with URL
203 * and Filename Safe Alphabet" specified by RFC 4648. '='-padding isn't used,
204 * as it's unneeded and not required by the RFC. base64url is used instead of
205 * base64 to avoid the '/' character, which isn't allowed in filenames.
206 *
207 * Return: the length of the resulting base64url-encoded string in bytes.
208 * This will be equal to FSCRYPT_BASE64URL_CHARS(srclen).
209 */
210static int fscrypt_base64url_encode(const u8 *src, int srclen, char *dst)
211{
212 u32 ac = 0;
213 int bits = 0;
214 int i;
215 char *cp = dst;
216
217 for (i = 0; i < srclen; i++) {
218 ac = (ac << 8) | src[i];
219 bits += 8;
220 do {
221 bits -= 6;
222 *cp++ = base64url_table[(ac >> bits) & 0x3f];
223 } while (bits >= 6);
224 }
225 if (bits)
226 *cp++ = base64url_table[(ac << (6 - bits)) & 0x3f];
227 return cp - dst;
228}
229
230/**
231 * fscrypt_base64url_decode() - base64url-decode a string
232 * @src: the string to decode. Doesn't need to be NUL-terminated.
233 * @srclen: the length of @src in bytes
234 * @dst: (output) the decoded binary data
235 *
236 * Decodes a string using base64url encoding, i.e. the "Base 64 Encoding with
237 * URL and Filename Safe Alphabet" specified by RFC 4648. '='-padding isn't
238 * accepted, nor are non-encoding characters such as whitespace.
239 *
240 * This implementation hasn't been optimized for performance.
241 *
242 * Return: the length of the resulting decoded binary data in bytes,
243 * or -1 if the string isn't a valid base64url string.
244 */
245static int fscrypt_base64url_decode(const char *src, int srclen, u8 *dst)
246{
247 u32 ac = 0;
248 int bits = 0;
249 int i;
250 u8 *bp = dst;
251
252 for (i = 0; i < srclen; i++) {
253 const char *p = strchr(base64url_table, src[i]);
254
255 if (p == NULL || src[i] == 0)
256 return -1;
257 ac = (ac << 6) | (p - base64url_table);
258 bits += 6;
259 if (bits >= 8) {
260 bits -= 8;
261 *bp++ = (u8)(ac >> bits);
262 }
263 }
264 if (ac & ((1 << bits) - 1))
265 return -1;
266 return bp - dst;
267}
268
269bool __fscrypt_fname_encrypted_size(const union fscrypt_policy *policy,
270 u32 orig_len, u32 max_len,
271 u32 *encrypted_len_ret)
272{
273 int padding = 4 << (fscrypt_policy_flags(policy) &
274 FSCRYPT_POLICY_FLAGS_PAD_MASK);
275 u32 encrypted_len;
276
277 if (orig_len > max_len)
278 return false;
279 encrypted_len = max_t(u32, orig_len, FSCRYPT_FNAME_MIN_MSG_LEN);
280 encrypted_len = round_up(encrypted_len, padding);
281 *encrypted_len_ret = min(encrypted_len, max_len);
282 return true;
283}
284
285/**
286 * fscrypt_fname_encrypted_size() - calculate length of encrypted filename
287 * @inode: parent inode of dentry name being encrypted. Key must
288 * already be set up.
289 * @orig_len: length of the original filename
290 * @max_len: maximum length to return
291 * @encrypted_len_ret: where calculated length should be returned (on success)
292 *
293 * Filenames that are shorter than the maximum length may have their lengths
294 * increased slightly by encryption, due to padding that is applied.
295 *
296 * Return: false if the orig_len is greater than max_len. Otherwise, true and
297 * fill out encrypted_len_ret with the length (up to max_len).
298 */
299bool fscrypt_fname_encrypted_size(const struct inode *inode, u32 orig_len,
300 u32 max_len, u32 *encrypted_len_ret)
301{
302 return __fscrypt_fname_encrypted_size(policy: &inode->i_crypt_info->ci_policy,
303 orig_len, max_len,
304 encrypted_len_ret);
305}
306EXPORT_SYMBOL_GPL(fscrypt_fname_encrypted_size);
307
308/**
309 * fscrypt_fname_alloc_buffer() - allocate a buffer for presented filenames
310 * @max_encrypted_len: maximum length of encrypted filenames the buffer will be
311 * used to present
312 * @crypto_str: (output) buffer to allocate
313 *
314 * Allocate a buffer that is large enough to hold any decrypted or encoded
315 * filename (null-terminated), for the given maximum encrypted filename length.
316 *
317 * Return: 0 on success, -errno on failure
318 */
319int fscrypt_fname_alloc_buffer(u32 max_encrypted_len,
320 struct fscrypt_str *crypto_str)
321{
322 u32 max_presented_len = max_t(u32, FSCRYPT_NOKEY_NAME_MAX_ENCODED,
323 max_encrypted_len);
324
325 crypto_str->name = kmalloc(size: max_presented_len + 1, GFP_NOFS);
326 if (!crypto_str->name)
327 return -ENOMEM;
328 crypto_str->len = max_presented_len;
329 return 0;
330}
331EXPORT_SYMBOL(fscrypt_fname_alloc_buffer);
332
333/**
334 * fscrypt_fname_free_buffer() - free a buffer for presented filenames
335 * @crypto_str: the buffer to free
336 *
337 * Free a buffer that was allocated by fscrypt_fname_alloc_buffer().
338 */
339void fscrypt_fname_free_buffer(struct fscrypt_str *crypto_str)
340{
341 if (!crypto_str)
342 return;
343 kfree(objp: crypto_str->name);
344 crypto_str->name = NULL;
345}
346EXPORT_SYMBOL(fscrypt_fname_free_buffer);
347
348/**
349 * fscrypt_fname_disk_to_usr() - convert an encrypted filename to
350 * user-presentable form
351 * @inode: inode of the parent directory (for regular filenames)
352 * or of the symlink (for symlink targets)
353 * @hash: first part of the name's dirhash, if applicable. This only needs to
354 * be provided if the filename is located in an indexed directory whose
355 * encryption key may be unavailable. Not needed for symlink targets.
356 * @minor_hash: second part of the name's dirhash, if applicable
357 * @iname: encrypted filename to convert. May also be "." or "..", which
358 * aren't actually encrypted.
359 * @oname: output buffer for the user-presentable filename. The caller must
360 * have allocated enough space for this, e.g. using
361 * fscrypt_fname_alloc_buffer().
362 *
363 * If the key is available, we'll decrypt the disk name. Otherwise, we'll
364 * encode it for presentation in fscrypt_nokey_name format.
365 * See struct fscrypt_nokey_name for details.
366 *
367 * Return: 0 on success, -errno on failure
368 */
369int fscrypt_fname_disk_to_usr(const struct inode *inode,
370 u32 hash, u32 minor_hash,
371 const struct fscrypt_str *iname,
372 struct fscrypt_str *oname)
373{
374 const struct qstr qname = FSTR_TO_QSTR(iname);
375 struct fscrypt_nokey_name nokey_name;
376 u32 size; /* size of the unencoded no-key name */
377
378 if (fscrypt_is_dot_dotdot(str: &qname)) {
379 oname->name[0] = '.';
380 oname->name[iname->len - 1] = '.';
381 oname->len = iname->len;
382 return 0;
383 }
384
385 if (iname->len < FSCRYPT_FNAME_MIN_MSG_LEN)
386 return -EUCLEAN;
387
388 if (fscrypt_has_encryption_key(inode))
389 return fname_decrypt(inode, iname, oname);
390
391 /*
392 * Sanity check that struct fscrypt_nokey_name doesn't have padding
393 * between fields and that its encoded size never exceeds NAME_MAX.
394 */
395 BUILD_BUG_ON(offsetofend(struct fscrypt_nokey_name, dirhash) !=
396 offsetof(struct fscrypt_nokey_name, bytes));
397 BUILD_BUG_ON(offsetofend(struct fscrypt_nokey_name, bytes) !=
398 offsetof(struct fscrypt_nokey_name, sha256));
399 BUILD_BUG_ON(FSCRYPT_NOKEY_NAME_MAX_ENCODED > NAME_MAX);
400
401 nokey_name.dirhash[0] = hash;
402 nokey_name.dirhash[1] = minor_hash;
403
404 if (iname->len <= sizeof(nokey_name.bytes)) {
405 memcpy(nokey_name.bytes, iname->name, iname->len);
406 size = offsetof(struct fscrypt_nokey_name, bytes[iname->len]);
407 } else {
408 memcpy(nokey_name.bytes, iname->name, sizeof(nokey_name.bytes));
409 /* Compute strong hash of remaining part of name. */
410 sha256(data: &iname->name[sizeof(nokey_name.bytes)],
411 len: iname->len - sizeof(nokey_name.bytes),
412 out: nokey_name.sha256);
413 size = FSCRYPT_NOKEY_NAME_MAX;
414 }
415 oname->len = fscrypt_base64url_encode(src: (const u8 *)&nokey_name, srclen: size,
416 dst: oname->name);
417 return 0;
418}
419EXPORT_SYMBOL(fscrypt_fname_disk_to_usr);
420
421/**
422 * fscrypt_setup_filename() - prepare to search a possibly encrypted directory
423 * @dir: the directory that will be searched
424 * @iname: the user-provided filename being searched for
425 * @lookup: 1 if we're allowed to proceed without the key because it's
426 * ->lookup() or we're finding the dir_entry for deletion; 0 if we cannot
427 * proceed without the key because we're going to create the dir_entry.
428 * @fname: the filename information to be filled in
429 *
430 * Given a user-provided filename @iname, this function sets @fname->disk_name
431 * to the name that would be stored in the on-disk directory entry, if possible.
432 * If the directory is unencrypted this is simply @iname. Else, if we have the
433 * directory's encryption key, then @iname is the plaintext, so we encrypt it to
434 * get the disk_name.
435 *
436 * Else, for keyless @lookup operations, @iname should be a no-key name, so we
437 * decode it to get the struct fscrypt_nokey_name. Non-@lookup operations will
438 * be impossible in this case, so we fail them with ENOKEY.
439 *
440 * If successful, fscrypt_free_filename() must be called later to clean up.
441 *
442 * Return: 0 on success, -errno on failure
443 */
444int fscrypt_setup_filename(struct inode *dir, const struct qstr *iname,
445 int lookup, struct fscrypt_name *fname)
446{
447 struct fscrypt_nokey_name *nokey_name;
448 int ret;
449
450 memset(fname, 0, sizeof(struct fscrypt_name));
451 fname->usr_fname = iname;
452
453 if (!IS_ENCRYPTED(dir) || fscrypt_is_dot_dotdot(str: iname)) {
454 fname->disk_name.name = (unsigned char *)iname->name;
455 fname->disk_name.len = iname->len;
456 return 0;
457 }
458 ret = fscrypt_get_encryption_info(inode: dir, allow_unsupported: lookup);
459 if (ret)
460 return ret;
461
462 if (fscrypt_has_encryption_key(inode: dir)) {
463 if (!fscrypt_fname_encrypted_size(dir, iname->len, NAME_MAX,
464 &fname->crypto_buf.len))
465 return -ENAMETOOLONG;
466 fname->crypto_buf.name = kmalloc(size: fname->crypto_buf.len,
467 GFP_NOFS);
468 if (!fname->crypto_buf.name)
469 return -ENOMEM;
470
471 ret = fscrypt_fname_encrypt(dir, iname, fname->crypto_buf.name,
472 fname->crypto_buf.len);
473 if (ret)
474 goto errout;
475 fname->disk_name.name = fname->crypto_buf.name;
476 fname->disk_name.len = fname->crypto_buf.len;
477 return 0;
478 }
479 if (!lookup)
480 return -ENOKEY;
481 fname->is_nokey_name = true;
482
483 /*
484 * We don't have the key and we are doing a lookup; decode the
485 * user-supplied name
486 */
487
488 if (iname->len > FSCRYPT_NOKEY_NAME_MAX_ENCODED)
489 return -ENOENT;
490
491 fname->crypto_buf.name = kmalloc(FSCRYPT_NOKEY_NAME_MAX, GFP_KERNEL);
492 if (fname->crypto_buf.name == NULL)
493 return -ENOMEM;
494
495 ret = fscrypt_base64url_decode(src: iname->name, srclen: iname->len,
496 dst: fname->crypto_buf.name);
497 if (ret < (int)offsetof(struct fscrypt_nokey_name, bytes[1]) ||
498 (ret > offsetof(struct fscrypt_nokey_name, sha256) &&
499 ret != FSCRYPT_NOKEY_NAME_MAX)) {
500 ret = -ENOENT;
501 goto errout;
502 }
503 fname->crypto_buf.len = ret;
504
505 nokey_name = (void *)fname->crypto_buf.name;
506 fname->hash = nokey_name->dirhash[0];
507 fname->minor_hash = nokey_name->dirhash[1];
508 if (ret != FSCRYPT_NOKEY_NAME_MAX) {
509 /* The full ciphertext filename is available. */
510 fname->disk_name.name = nokey_name->bytes;
511 fname->disk_name.len =
512 ret - offsetof(struct fscrypt_nokey_name, bytes);
513 }
514 return 0;
515
516errout:
517 kfree(objp: fname->crypto_buf.name);
518 return ret;
519}
520EXPORT_SYMBOL(fscrypt_setup_filename);
521
522/**
523 * fscrypt_match_name() - test whether the given name matches a directory entry
524 * @fname: the name being searched for
525 * @de_name: the name from the directory entry
526 * @de_name_len: the length of @de_name in bytes
527 *
528 * Normally @fname->disk_name will be set, and in that case we simply compare
529 * that to the name stored in the directory entry. The only exception is that
530 * if we don't have the key for an encrypted directory and the name we're
531 * looking for is very long, then we won't have the full disk_name and instead
532 * we'll need to match against a fscrypt_nokey_name that includes a strong hash.
533 *
534 * Return: %true if the name matches, otherwise %false.
535 */
536bool fscrypt_match_name(const struct fscrypt_name *fname,
537 const u8 *de_name, u32 de_name_len)
538{
539 const struct fscrypt_nokey_name *nokey_name =
540 (const void *)fname->crypto_buf.name;
541 u8 digest[SHA256_DIGEST_SIZE];
542
543 if (likely(fname->disk_name.name)) {
544 if (de_name_len != fname->disk_name.len)
545 return false;
546 return !memcmp(p: de_name, q: fname->disk_name.name, size: de_name_len);
547 }
548 if (de_name_len <= sizeof(nokey_name->bytes))
549 return false;
550 if (memcmp(p: de_name, q: nokey_name->bytes, size: sizeof(nokey_name->bytes)))
551 return false;
552 sha256(data: &de_name[sizeof(nokey_name->bytes)],
553 len: de_name_len - sizeof(nokey_name->bytes), out: digest);
554 return !memcmp(p: digest, q: nokey_name->sha256, size: sizeof(digest));
555}
556EXPORT_SYMBOL_GPL(fscrypt_match_name);
557
558/**
559 * fscrypt_fname_siphash() - calculate the SipHash of a filename
560 * @dir: the parent directory
561 * @name: the filename to calculate the SipHash of
562 *
563 * Given a plaintext filename @name and a directory @dir which uses SipHash as
564 * its dirhash method and has had its fscrypt key set up, this function
565 * calculates the SipHash of that name using the directory's secret dirhash key.
566 *
567 * Return: the SipHash of @name using the hash key of @dir
568 */
569u64 fscrypt_fname_siphash(const struct inode *dir, const struct qstr *name)
570{
571 const struct fscrypt_inode_info *ci = dir->i_crypt_info;
572
573 WARN_ON_ONCE(!ci->ci_dirhash_key_initialized);
574
575 return siphash(data: name->name, len: name->len, key: &ci->ci_dirhash_key);
576}
577EXPORT_SYMBOL_GPL(fscrypt_fname_siphash);
578
579/*
580 * Validate dentries in encrypted directories to make sure we aren't potentially
581 * caching stale dentries after a key has been added.
582 */
583int fscrypt_d_revalidate(struct dentry *dentry, unsigned int flags)
584{
585 struct dentry *dir;
586 int err;
587 int valid;
588
589 /*
590 * Plaintext names are always valid, since fscrypt doesn't support
591 * reverting to no-key names without evicting the directory's inode
592 * -- which implies eviction of the dentries in the directory.
593 */
594 if (!(dentry->d_flags & DCACHE_NOKEY_NAME))
595 return 1;
596
597 /*
598 * No-key name; valid if the directory's key is still unavailable.
599 *
600 * Although fscrypt forbids rename() on no-key names, we still must use
601 * dget_parent() here rather than use ->d_parent directly. That's
602 * because a corrupted fs image may contain directory hard links, which
603 * the VFS handles by moving the directory's dentry tree in the dcache
604 * each time ->lookup() finds the directory and it already has a dentry
605 * elsewhere. Thus ->d_parent can be changing, and we must safely grab
606 * a reference to some ->d_parent to prevent it from being freed.
607 */
608
609 if (flags & LOOKUP_RCU)
610 return -ECHILD;
611
612 dir = dget_parent(dentry);
613 /*
614 * Pass allow_unsupported=true, so that files with an unsupported
615 * encryption policy can be deleted.
616 */
617 err = fscrypt_get_encryption_info(inode: d_inode(dentry: dir), allow_unsupported: true);
618 valid = !fscrypt_has_encryption_key(inode: d_inode(dentry: dir));
619 dput(dir);
620
621 if (err < 0)
622 return err;
623
624 return valid;
625}
626EXPORT_SYMBOL_GPL(fscrypt_d_revalidate);
627

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