fname.c

  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(str->name, 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, 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, &sg, &sg, olen, &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, 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_sg, &dst_sg, iname->len, &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(oname->name, 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(&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(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(&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(&iname->name[sizeof(nokey_name.bytes)],
309		       iname->len - sizeof(nokey_name.bytes),
310		       nokey_name.sha256);
311		size = FSCRYPT_NOKEY_NAME_MAX;
312	}
313	oname->len = base64_encode((const u8 *)&nokey_name, size,
314				   oname->name, false, 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(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(dir, lookup);
357	if (ret)
358		return ret;
359
360	if (fscrypt_has_encryption_key(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(iname->name, iname->len,
394			    fname->crypto_buf.name, false, 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(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(de_name, fname->disk_name.name, de_name_len);
445	}
446	if (de_name_len <= sizeof(nokey_name->bytes))
447		return false;
448	if (memcmp(de_name, nokey_name->bytes, sizeof(nokey_name->bytes)))
449		return false;
450	sha256(&de_name[sizeof(nokey_name->bytes)],
451	       de_name_len - sizeof(nokey_name->bytes), digest);
452	return !memcmp(digest, nokey_name->sha256, 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(dir);
470
471	WARN_ON_ONCE(!ci->ci_dirhash_key_initialized);
472
473	return siphash(name->name, name->len, &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(dir, true);
509	if (err < 0)
510		return err;
511
512	return !fscrypt_has_encryption_key(dir);
513}
514EXPORT_SYMBOL_GPL(fscrypt_d_revalidate);