1 // SPDX-License-Identifier: GPL-2.0
3 * This contains functions for filename crypto management
5 * Copyright (C) 2015, Google, Inc.
6 * Copyright (C) 2015, Motorola Mobility
8 * Written by Uday Savagaonkar, 2014.
9 * Modified by Jaegeuk Kim, 2015.
11 * This has not yet undergone a rigorous security audit.
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"
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.
26 #define FSCRYPT_FNAME_MIN_MSG_LEN 16
29 * struct fscrypt_nokey_name - identifier for directory entry when key is absent
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.
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.
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.
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).
58 struct fscrypt_nokey_name {
61 u8 sha256[SHA256_DIGEST_SIZE];
62 }; /* 189 bytes => 252 bytes base64url-encoded, which is <= NAME_MAX (255) */
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.
69 #define FSCRYPT_NOKEY_NAME_MAX offsetofend(struct fscrypt_nokey_name, sha256)
71 /* Encoded size of max-size no-key name */
72 #define FSCRYPT_NOKEY_NAME_MAX_ENCODED \
73 FSCRYPT_BASE64URL_CHARS(FSCRYPT_NOKEY_NAME_MAX)
75 static inline bool fscrypt_is_dot_dotdot(const struct qstr *str)
77 if (str->len == 1 && str->name[0] == '.')
80 if (str->len == 2 && str->name[0] == '.' && str->name[1] == '.')
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)
90 * @iname: the filename to encrypt
91 * @out: (output) the encrypted filename
92 * @olen: size of the encrypted filename. It must be at least @iname->len.
93 * Any extra space is filled with NUL padding before encryption.
95 * Return: 0 on success, -errno on failure
97 int fscrypt_fname_encrypt(const struct inode *inode, const struct qstr *iname,
98 u8 *out, unsigned int olen)
100 struct skcipher_request *req = NULL;
101 DECLARE_CRYPTO_WAIT(wait);
102 const struct fscrypt_info *ci = inode->i_crypt_info;
103 struct crypto_skcipher *tfm = ci->ci_enc_key.tfm;
105 struct scatterlist sg;
109 * Copy the filename to the output buffer for encrypting in-place and
110 * pad it with the needed number of NUL bytes.
112 if (WARN_ON(olen < iname->len))
114 memcpy(out, iname->name, iname->len);
115 memset(out + iname->len, 0, olen - iname->len);
117 /* Initialize the IV */
118 fscrypt_generate_iv(&iv, 0, ci);
120 /* Set up the encryption request */
121 req = skcipher_request_alloc(tfm, GFP_NOFS);
124 skcipher_request_set_callback(req,
125 CRYPTO_TFM_REQ_MAY_BACKLOG | CRYPTO_TFM_REQ_MAY_SLEEP,
126 crypto_req_done, &wait);
127 sg_init_one(&sg, out, olen);
128 skcipher_request_set_crypt(req, &sg, &sg, olen, &iv);
130 /* Do the encryption */
131 res = crypto_wait_req(crypto_skcipher_encrypt(req), &wait);
132 skcipher_request_free(req);
134 fscrypt_err(inode, "Filename encryption failed: %d", res);
142 * fname_decrypt() - decrypt a filename
143 * @inode: inode of the parent directory (for regular filenames)
144 * or of the symlink (for symlink targets)
145 * @iname: the encrypted filename to decrypt
146 * @oname: (output) the decrypted filename. The caller must have allocated
147 * enough space for this, e.g. using fscrypt_fname_alloc_buffer().
149 * Return: 0 on success, -errno on failure
151 static int fname_decrypt(const struct inode *inode,
152 const struct fscrypt_str *iname,
153 struct fscrypt_str *oname)
155 struct skcipher_request *req = NULL;
156 DECLARE_CRYPTO_WAIT(wait);
157 struct scatterlist src_sg, dst_sg;
158 const struct fscrypt_info *ci = inode->i_crypt_info;
159 struct crypto_skcipher *tfm = ci->ci_enc_key.tfm;
163 /* Allocate request */
164 req = skcipher_request_alloc(tfm, GFP_NOFS);
167 skcipher_request_set_callback(req,
168 CRYPTO_TFM_REQ_MAY_BACKLOG | CRYPTO_TFM_REQ_MAY_SLEEP,
169 crypto_req_done, &wait);
172 fscrypt_generate_iv(&iv, 0, ci);
174 /* Create decryption request */
175 sg_init_one(&src_sg, iname->name, iname->len);
176 sg_init_one(&dst_sg, oname->name, oname->len);
177 skcipher_request_set_crypt(req, &src_sg, &dst_sg, iname->len, &iv);
178 res = crypto_wait_req(crypto_skcipher_decrypt(req), &wait);
179 skcipher_request_free(req);
181 fscrypt_err(inode, "Filename decryption failed: %d", res);
185 oname->len = strnlen(oname->name, iname->len);
189 static const char base64url_table[65] =
190 "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789-_";
192 #define FSCRYPT_BASE64URL_CHARS(nbytes) DIV_ROUND_UP((nbytes) * 4, 3)
195 * fscrypt_base64url_encode() - base64url-encode some binary data
196 * @src: the binary data to encode
197 * @srclen: the length of @src in bytes
198 * @dst: (output) the base64url-encoded string. Not NUL-terminated.
200 * Encodes data using base64url encoding, i.e. the "Base 64 Encoding with URL
201 * and Filename Safe Alphabet" specified by RFC 4648. '='-padding isn't used,
202 * as it's unneeded and not required by the RFC. base64url is used instead of
203 * base64 to avoid the '/' character, which isn't allowed in filenames.
205 * Return: the length of the resulting base64url-encoded string in bytes.
206 * This will be equal to FSCRYPT_BASE64URL_CHARS(srclen).
208 static int fscrypt_base64url_encode(const u8 *src, int srclen, char *dst)
215 for (i = 0; i < srclen; i++) {
216 ac = (ac << 8) | src[i];
220 *cp++ = base64url_table[(ac >> bits) & 0x3f];
224 *cp++ = base64url_table[(ac << (6 - bits)) & 0x3f];
229 * fscrypt_base64url_decode() - base64url-decode a string
230 * @src: the string to decode. Doesn't need to be NUL-terminated.
231 * @srclen: the length of @src in bytes
232 * @dst: (output) the decoded binary data
234 * Decodes a string using base64url encoding, i.e. the "Base 64 Encoding with
235 * URL and Filename Safe Alphabet" specified by RFC 4648. '='-padding isn't
236 * accepted, nor are non-encoding characters such as whitespace.
238 * This implementation hasn't been optimized for performance.
240 * Return: the length of the resulting decoded binary data in bytes,
241 * or -1 if the string isn't a valid base64url string.
243 static int fscrypt_base64url_decode(const char *src, int srclen, u8 *dst)
250 for (i = 0; i < srclen; i++) {
251 const char *p = strchr(base64url_table, src[i]);
253 if (p == NULL || src[i] == 0)
255 ac = (ac << 6) | (p - base64url_table);
259 *bp++ = (u8)(ac >> bits);
262 if (ac & ((1 << bits) - 1))
267 bool fscrypt_fname_encrypted_size(const union fscrypt_policy *policy,
268 u32 orig_len, u32 max_len,
269 u32 *encrypted_len_ret)
271 int padding = 4 << (fscrypt_policy_flags(policy) &
272 FSCRYPT_POLICY_FLAGS_PAD_MASK);
275 if (orig_len > max_len)
277 encrypted_len = max_t(u32, orig_len, FSCRYPT_FNAME_MIN_MSG_LEN);
278 encrypted_len = round_up(encrypted_len, padding);
279 *encrypted_len_ret = min(encrypted_len, max_len);
284 * fscrypt_fname_alloc_buffer() - allocate a buffer for presented filenames
285 * @max_encrypted_len: maximum length of encrypted filenames the buffer will be
287 * @crypto_str: (output) buffer to allocate
289 * Allocate a buffer that is large enough to hold any decrypted or encoded
290 * filename (null-terminated), for the given maximum encrypted filename length.
292 * Return: 0 on success, -errno on failure
294 int fscrypt_fname_alloc_buffer(u32 max_encrypted_len,
295 struct fscrypt_str *crypto_str)
297 u32 max_presented_len = max_t(u32, FSCRYPT_NOKEY_NAME_MAX_ENCODED,
300 crypto_str->name = kmalloc(max_presented_len + 1, GFP_NOFS);
301 if (!crypto_str->name)
303 crypto_str->len = max_presented_len;
306 EXPORT_SYMBOL(fscrypt_fname_alloc_buffer);
309 * fscrypt_fname_free_buffer() - free a buffer for presented filenames
310 * @crypto_str: the buffer to free
312 * Free a buffer that was allocated by fscrypt_fname_alloc_buffer().
314 void fscrypt_fname_free_buffer(struct fscrypt_str *crypto_str)
318 kfree(crypto_str->name);
319 crypto_str->name = NULL;
321 EXPORT_SYMBOL(fscrypt_fname_free_buffer);
324 * fscrypt_fname_disk_to_usr() - convert an encrypted filename to
325 * user-presentable form
326 * @inode: inode of the parent directory (for regular filenames)
327 * or of the symlink (for symlink targets)
328 * @hash: first part of the name's dirhash, if applicable. This only needs to
329 * be provided if the filename is located in an indexed directory whose
330 * encryption key may be unavailable. Not needed for symlink targets.
331 * @minor_hash: second part of the name's dirhash, if applicable
332 * @iname: encrypted filename to convert. May also be "." or "..", which
333 * aren't actually encrypted.
334 * @oname: output buffer for the user-presentable filename. The caller must
335 * have allocated enough space for this, e.g. using
336 * fscrypt_fname_alloc_buffer().
338 * If the key is available, we'll decrypt the disk name. Otherwise, we'll
339 * encode it for presentation in fscrypt_nokey_name format.
340 * See struct fscrypt_nokey_name for details.
342 * Return: 0 on success, -errno on failure
344 int fscrypt_fname_disk_to_usr(const struct inode *inode,
345 u32 hash, u32 minor_hash,
346 const struct fscrypt_str *iname,
347 struct fscrypt_str *oname)
349 const struct qstr qname = FSTR_TO_QSTR(iname);
350 struct fscrypt_nokey_name nokey_name;
351 u32 size; /* size of the unencoded no-key name */
353 if (fscrypt_is_dot_dotdot(&qname)) {
354 oname->name[0] = '.';
355 oname->name[iname->len - 1] = '.';
356 oname->len = iname->len;
360 if (iname->len < FSCRYPT_FNAME_MIN_MSG_LEN)
363 if (fscrypt_has_encryption_key(inode))
364 return fname_decrypt(inode, iname, oname);
367 * Sanity check that struct fscrypt_nokey_name doesn't have padding
368 * between fields and that its encoded size never exceeds NAME_MAX.
370 BUILD_BUG_ON(offsetofend(struct fscrypt_nokey_name, dirhash) !=
371 offsetof(struct fscrypt_nokey_name, bytes));
372 BUILD_BUG_ON(offsetofend(struct fscrypt_nokey_name, bytes) !=
373 offsetof(struct fscrypt_nokey_name, sha256));
374 BUILD_BUG_ON(FSCRYPT_NOKEY_NAME_MAX_ENCODED > NAME_MAX);
376 nokey_name.dirhash[0] = hash;
377 nokey_name.dirhash[1] = minor_hash;
379 if (iname->len <= sizeof(nokey_name.bytes)) {
380 memcpy(nokey_name.bytes, iname->name, iname->len);
381 size = offsetof(struct fscrypt_nokey_name, bytes[iname->len]);
383 memcpy(nokey_name.bytes, iname->name, sizeof(nokey_name.bytes));
384 /* Compute strong hash of remaining part of name. */
385 sha256(&iname->name[sizeof(nokey_name.bytes)],
386 iname->len - sizeof(nokey_name.bytes),
388 size = FSCRYPT_NOKEY_NAME_MAX;
390 oname->len = fscrypt_base64url_encode((const u8 *)&nokey_name, size,
394 EXPORT_SYMBOL(fscrypt_fname_disk_to_usr);
397 * fscrypt_setup_filename() - prepare to search a possibly encrypted directory
398 * @dir: the directory that will be searched
399 * @iname: the user-provided filename being searched for
400 * @lookup: 1 if we're allowed to proceed without the key because it's
401 * ->lookup() or we're finding the dir_entry for deletion; 0 if we cannot
402 * proceed without the key because we're going to create the dir_entry.
403 * @fname: the filename information to be filled in
405 * Given a user-provided filename @iname, this function sets @fname->disk_name
406 * to the name that would be stored in the on-disk directory entry, if possible.
407 * If the directory is unencrypted this is simply @iname. Else, if we have the
408 * directory's encryption key, then @iname is the plaintext, so we encrypt it to
411 * Else, for keyless @lookup operations, @iname should be a no-key name, so we
412 * decode it to get the struct fscrypt_nokey_name. Non-@lookup operations will
413 * be impossible in this case, so we fail them with ENOKEY.
415 * If successful, fscrypt_free_filename() must be called later to clean up.
417 * Return: 0 on success, -errno on failure
419 int fscrypt_setup_filename(struct inode *dir, const struct qstr *iname,
420 int lookup, struct fscrypt_name *fname)
422 struct fscrypt_nokey_name *nokey_name;
425 memset(fname, 0, sizeof(struct fscrypt_name));
426 fname->usr_fname = iname;
428 if (!IS_ENCRYPTED(dir) || fscrypt_is_dot_dotdot(iname)) {
429 fname->disk_name.name = (unsigned char *)iname->name;
430 fname->disk_name.len = iname->len;
433 ret = fscrypt_get_encryption_info(dir, lookup);
437 if (fscrypt_has_encryption_key(dir)) {
438 if (!fscrypt_fname_encrypted_size(&dir->i_crypt_info->ci_policy,
439 iname->len, NAME_MAX,
440 &fname->crypto_buf.len))
441 return -ENAMETOOLONG;
442 fname->crypto_buf.name = kmalloc(fname->crypto_buf.len,
444 if (!fname->crypto_buf.name)
447 ret = fscrypt_fname_encrypt(dir, iname, fname->crypto_buf.name,
448 fname->crypto_buf.len);
451 fname->disk_name.name = fname->crypto_buf.name;
452 fname->disk_name.len = fname->crypto_buf.len;
457 fname->is_nokey_name = true;
460 * We don't have the key and we are doing a lookup; decode the
464 if (iname->len > FSCRYPT_NOKEY_NAME_MAX_ENCODED)
467 fname->crypto_buf.name = kmalloc(FSCRYPT_NOKEY_NAME_MAX, GFP_KERNEL);
468 if (fname->crypto_buf.name == NULL)
471 ret = fscrypt_base64url_decode(iname->name, iname->len,
472 fname->crypto_buf.name);
473 if (ret < (int)offsetof(struct fscrypt_nokey_name, bytes[1]) ||
474 (ret > offsetof(struct fscrypt_nokey_name, sha256) &&
475 ret != FSCRYPT_NOKEY_NAME_MAX)) {
479 fname->crypto_buf.len = ret;
481 nokey_name = (void *)fname->crypto_buf.name;
482 fname->hash = nokey_name->dirhash[0];
483 fname->minor_hash = nokey_name->dirhash[1];
484 if (ret != FSCRYPT_NOKEY_NAME_MAX) {
485 /* The full ciphertext filename is available. */
486 fname->disk_name.name = nokey_name->bytes;
487 fname->disk_name.len =
488 ret - offsetof(struct fscrypt_nokey_name, bytes);
493 kfree(fname->crypto_buf.name);
496 EXPORT_SYMBOL(fscrypt_setup_filename);
499 * fscrypt_match_name() - test whether the given name matches a directory entry
500 * @fname: the name being searched for
501 * @de_name: the name from the directory entry
502 * @de_name_len: the length of @de_name in bytes
504 * Normally @fname->disk_name will be set, and in that case we simply compare
505 * that to the name stored in the directory entry. The only exception is that
506 * if we don't have the key for an encrypted directory and the name we're
507 * looking for is very long, then we won't have the full disk_name and instead
508 * we'll need to match against a fscrypt_nokey_name that includes a strong hash.
510 * Return: %true if the name matches, otherwise %false.
512 bool fscrypt_match_name(const struct fscrypt_name *fname,
513 const u8 *de_name, u32 de_name_len)
515 const struct fscrypt_nokey_name *nokey_name =
516 (const void *)fname->crypto_buf.name;
517 u8 digest[SHA256_DIGEST_SIZE];
519 if (likely(fname->disk_name.name)) {
520 if (de_name_len != fname->disk_name.len)
522 return !memcmp(de_name, fname->disk_name.name, de_name_len);
524 if (de_name_len <= sizeof(nokey_name->bytes))
526 if (memcmp(de_name, nokey_name->bytes, sizeof(nokey_name->bytes)))
528 sha256(&de_name[sizeof(nokey_name->bytes)],
529 de_name_len - sizeof(nokey_name->bytes), digest);
530 return !memcmp(digest, nokey_name->sha256, sizeof(digest));
532 EXPORT_SYMBOL_GPL(fscrypt_match_name);
535 * fscrypt_fname_siphash() - calculate the SipHash of a filename
536 * @dir: the parent directory
537 * @name: the filename to calculate the SipHash of
539 * Given a plaintext filename @name and a directory @dir which uses SipHash as
540 * its dirhash method and has had its fscrypt key set up, this function
541 * calculates the SipHash of that name using the directory's secret dirhash key.
543 * Return: the SipHash of @name using the hash key of @dir
545 u64 fscrypt_fname_siphash(const struct inode *dir, const struct qstr *name)
547 const struct fscrypt_info *ci = dir->i_crypt_info;
549 WARN_ON(!ci->ci_dirhash_key_initialized);
551 return siphash(name->name, name->len, &ci->ci_dirhash_key);
553 EXPORT_SYMBOL_GPL(fscrypt_fname_siphash);
556 * Validate dentries in encrypted directories to make sure we aren't potentially
557 * caching stale dentries after a key has been added.
559 int fscrypt_d_revalidate(struct dentry *dentry, unsigned int flags)
566 * Plaintext names are always valid, since fscrypt doesn't support
567 * reverting to no-key names without evicting the directory's inode
568 * -- which implies eviction of the dentries in the directory.
570 if (!(dentry->d_flags & DCACHE_NOKEY_NAME))
574 * No-key name; valid if the directory's key is still unavailable.
576 * Although fscrypt forbids rename() on no-key names, we still must use
577 * dget_parent() here rather than use ->d_parent directly. That's
578 * because a corrupted fs image may contain directory hard links, which
579 * the VFS handles by moving the directory's dentry tree in the dcache
580 * each time ->lookup() finds the directory and it already has a dentry
581 * elsewhere. Thus ->d_parent can be changing, and we must safely grab
582 * a reference to some ->d_parent to prevent it from being freed.
585 if (flags & LOOKUP_RCU)
588 dir = dget_parent(dentry);
590 * Pass allow_unsupported=true, so that files with an unsupported
591 * encryption policy can be deleted.
593 err = fscrypt_get_encryption_info(d_inode(dir), true);
594 valid = !fscrypt_has_encryption_key(d_inode(dir));
602 EXPORT_SYMBOL_GPL(fscrypt_d_revalidate);