hooks.c

  1// SPDX-License-Identifier: GPL-2.0-only
  2/*
  3 * fs/crypto/hooks.c
  4 *
  5 * Encryption hooks for higher-level filesystem operations.
  6 */
  7
  8#include <linux/export.h>
  9
 10#include "fscrypt_private.h"
 11
 12/**
 13 * fscrypt_file_open() - prepare to open a possibly-encrypted regular file
 14 * @inode: the inode being opened
 15 * @filp: the struct file being set up
 16 *
 17 * Currently, an encrypted regular file can only be opened if its encryption key
 18 * is available; access to the raw encrypted contents is not supported.
 19 * Therefore, we first set up the inode's encryption key (if not already done)
 20 * and return an error if it's unavailable.
 21 *
 22 * We also verify that if the parent directory (from the path via which the file
 23 * is being opened) is encrypted, then the inode being opened uses the same
 24 * encryption policy.  This is needed as part of the enforcement that all files
 25 * in an encrypted directory tree use the same encryption policy, as a
 26 * protection against certain types of offline attacks.  Note that this check is
 27 * needed even when opening an *unencrypted* file, since it's forbidden to have
 28 * an unencrypted file in an encrypted directory.
 29 *
 30 * Return: 0 on success, -ENOKEY if the key is missing, or another -errno code
 31 */
 32int fscrypt_file_open(struct inode *inode, struct file *filp)
 33{
 34	int err;
 35	struct dentry *dentry, *dentry_parent;
 36	struct inode *inode_parent;
 37
 38	err = fscrypt_require_key(inode);
 39	if (err)
 40		return err;
 41
 42	dentry = file_dentry(filp);
 43
 44	/*
 45	 * Getting a reference to the parent dentry is needed for the actual
 46	 * encryption policy comparison, but it's expensive on multi-core
 47	 * systems.  Since this function runs on unencrypted files too, start
 48	 * with a lightweight RCU-mode check for the parent directory being
 49	 * unencrypted (in which case it's fine for the child to be either
 50	 * unencrypted, or encrypted with any policy).  Only continue on to the
 51	 * full policy check if the parent directory is actually encrypted.
 52	 */
 53	rcu_read_lock();
 54	dentry_parent = READ_ONCE(dentry->d_parent);
 55	inode_parent = d_inode_rcu(dentry_parent);
 56	if (inode_parent != NULL && !IS_ENCRYPTED(inode_parent)) {
 57		rcu_read_unlock();
 58		return 0;
 59	}
 60	rcu_read_unlock();
 61
 62	dentry_parent = dget_parent(dentry);
 63	if (!fscrypt_has_permitted_context(d_inode(dentry_parent), inode)) {
 64		fscrypt_warn(inode,
 65			     "Inconsistent encryption context (parent directory: %lu)",
 66			     d_inode(dentry_parent)->i_ino);
 67		err = -EPERM;
 68	}
 69	dput(dentry_parent);
 70	return err;
 71}
 72EXPORT_SYMBOL_GPL(fscrypt_file_open);
 73
 74int __fscrypt_prepare_link(struct inode *inode, struct inode *dir,
 75			   struct dentry *dentry)
 76{
 77	if (fscrypt_is_nokey_name(dentry))
 78		return -ENOKEY;
 79	/*
 80	 * We don't need to separately check that the directory inode's key is
 81	 * available, as it's implied by the dentry not being a no-key name.
 82	 */
 83
 84	if (!fscrypt_has_permitted_context(dir, inode))
 85		return -EXDEV;
 86
 87	return 0;
 88}
 89EXPORT_SYMBOL_GPL(__fscrypt_prepare_link);
 90
 91int __fscrypt_prepare_rename(struct inode *old_dir, struct dentry *old_dentry,
 92			     struct inode *new_dir, struct dentry *new_dentry,
 93			     unsigned int flags)
 94{
 95	if (fscrypt_is_nokey_name(old_dentry) ||
 96	    fscrypt_is_nokey_name(new_dentry))
 97		return -ENOKEY;
 98	/*
 99	 * We don't need to separately check that the directory inodes' keys are
100	 * available, as it's implied by the dentries not being no-key names.
101	 */
102
103	if (old_dir != new_dir) {
104		if (IS_ENCRYPTED(new_dir) &&
105		    !fscrypt_has_permitted_context(new_dir,
106						   d_inode(old_dentry)))
107			return -EXDEV;
108
109		if ((flags & RENAME_EXCHANGE) &&
110		    IS_ENCRYPTED(old_dir) &&
111		    !fscrypt_has_permitted_context(old_dir,
112						   d_inode(new_dentry)))
113			return -EXDEV;
114	}
115	return 0;
116}
117EXPORT_SYMBOL_GPL(__fscrypt_prepare_rename);
118
119int __fscrypt_prepare_lookup(struct inode *dir, struct dentry *dentry,
120			     struct fscrypt_name *fname)
121{
122	int err = fscrypt_setup_filename(dir, &dentry->d_name, 1, fname);
123
124	if (err && err != -ENOENT)
125		return err;
126
127	fscrypt_prepare_dentry(dentry, fname->is_nokey_name);
128
129	return err;
130}
131EXPORT_SYMBOL_GPL(__fscrypt_prepare_lookup);
132
133/**
134 * fscrypt_prepare_lookup_partial() - prepare lookup without filename setup
135 * @dir: the encrypted directory being searched
136 * @dentry: the dentry being looked up in @dir
137 *
138 * This function should be used by the ->lookup and ->atomic_open methods of
139 * filesystems that handle filename encryption and no-key name encoding
140 * themselves and thus can't use fscrypt_prepare_lookup().  Like
141 * fscrypt_prepare_lookup(), this will try to set up the directory's encryption
142 * key and will set DCACHE_NOKEY_NAME on the dentry if the key is unavailable.
143 * However, this function doesn't set up a struct fscrypt_name for the filename.
144 *
145 * Return: 0 on success; -errno on error.  Note that the encryption key being
146 *	   unavailable is not considered an error.  It is also not an error if
147 *	   the encryption policy is unsupported by this kernel; that is treated
148 *	   like the key being unavailable, so that files can still be deleted.
149 */
150int fscrypt_prepare_lookup_partial(struct inode *dir, struct dentry *dentry)
151{
152	int err = fscrypt_get_encryption_info(dir, true);
153	bool is_nokey_name = (!err && !fscrypt_has_encryption_key(dir));
154
155	fscrypt_prepare_dentry(dentry, is_nokey_name);
156
157	return err;
158}
159EXPORT_SYMBOL_GPL(fscrypt_prepare_lookup_partial);
160
161int __fscrypt_prepare_readdir(struct inode *dir)
162{
163	return fscrypt_get_encryption_info(dir, true);
164}
165EXPORT_SYMBOL_GPL(__fscrypt_prepare_readdir);
166
167int __fscrypt_prepare_setattr(struct dentry *dentry, struct iattr *attr)
168{
169	if (attr->ia_valid & ATTR_SIZE)
170		return fscrypt_require_key(d_inode(dentry));
171	return 0;
172}
173EXPORT_SYMBOL_GPL(__fscrypt_prepare_setattr);
174
175/**
176 * fscrypt_prepare_setflags() - prepare to change flags with FS_IOC_SETFLAGS
177 * @inode: the inode on which flags are being changed
178 * @oldflags: the old flags
179 * @flags: the new flags
180 *
181 * The caller should be holding i_rwsem for write.
182 *
183 * Return: 0 on success; -errno if the flags change isn't allowed or if
184 *	   another error occurs.
185 */
186int fscrypt_prepare_setflags(struct inode *inode,
187			     unsigned int oldflags, unsigned int flags)
188{
189	struct fscrypt_inode_info *ci;
190	struct fscrypt_master_key *mk;
191	int err;
192
193	/*
194	 * When the CASEFOLD flag is set on an encrypted directory, we must
195	 * derive the secret key needed for the dirhash.  This is only possible
196	 * if the directory uses a v2 encryption policy.
197	 */
198	if (IS_ENCRYPTED(inode) && (flags & ~oldflags & FS_CASEFOLD_FL)) {
199		err = fscrypt_require_key(inode);
200		if (err)
201			return err;
202		ci = fscrypt_get_inode_info_raw(inode);
203		if (ci->ci_policy.version != FSCRYPT_POLICY_V2)
204			return -EINVAL;
205		mk = ci->ci_master_key;
206		down_read(&mk->mk_sem);
207		if (mk->mk_present)
208			fscrypt_derive_dirhash_key(ci, mk);
209		else
210			err = -ENOKEY;
211		up_read(&mk->mk_sem);
212		return err;
213	}
214	return 0;
215}
216
217/**
218 * fscrypt_prepare_symlink() - prepare to create a possibly-encrypted symlink
219 * @dir: directory in which the symlink is being created
220 * @target: plaintext symlink target
221 * @len: length of @target excluding null terminator
222 * @max_len: space the filesystem has available to store the symlink target
223 * @disk_link: (out) the on-disk symlink target being prepared
224 *
225 * This function computes the size the symlink target will require on-disk,
226 * stores it in @disk_link->len, and validates it against @max_len.  An
227 * encrypted symlink may be longer than the original.
228 *
229 * Additionally, @disk_link->name is set to @target if the symlink will be
230 * unencrypted, but left NULL if the symlink will be encrypted.  For encrypted
231 * symlinks, the filesystem must call fscrypt_encrypt_symlink() to create the
232 * on-disk target later.  (The reason for the two-step process is that some
233 * filesystems need to know the size of the symlink target before creating the
234 * inode, e.g. to determine whether it will be a "fast" or "slow" symlink.)
235 *
236 * Return: 0 on success, -ENAMETOOLONG if the symlink target is too long,
237 * -ENOKEY if the encryption key is missing, or another -errno code if a problem
238 * occurred while setting up the encryption key.
239 */
240int fscrypt_prepare_symlink(struct inode *dir, const char *target,
241			    unsigned int len, unsigned int max_len,
242			    struct fscrypt_str *disk_link)
243{
244	const union fscrypt_policy *policy;
245
246	/*
247	 * To calculate the size of the encrypted symlink target we need to know
248	 * the amount of NUL padding, which is determined by the flags set in
249	 * the encryption policy which will be inherited from the directory.
250	 */
251	policy = fscrypt_policy_to_inherit(dir);
252	if (policy == NULL) {
253		/* Not encrypted */
254		disk_link->name = (unsigned char *)target;
255		disk_link->len = len + 1;
256		if (disk_link->len > max_len)
257			return -ENAMETOOLONG;
258		return 0;
259	}
260	if (IS_ERR(policy))
261		return PTR_ERR(policy);
262
263	/*
264	 * Calculate the size of the encrypted symlink and verify it won't
265	 * exceed max_len.  Note that for historical reasons, encrypted symlink
266	 * targets are prefixed with the ciphertext length, despite this
267	 * actually being redundant with i_size.  This decreases by 2 bytes the
268	 * longest symlink target we can accept.
269	 *
270	 * We could recover 1 byte by not counting a null terminator, but
271	 * counting it (even though it is meaningless for ciphertext) is simpler
272	 * for now since filesystems will assume it is there and subtract it.
273	 */
274	if (!__fscrypt_fname_encrypted_size(policy, len,
275					    max_len - sizeof(struct fscrypt_symlink_data) - 1,
276					    &disk_link->len))
277		return -ENAMETOOLONG;
278	disk_link->len += sizeof(struct fscrypt_symlink_data) + 1;
279
280	disk_link->name = NULL;
281	return 0;
282}
283EXPORT_SYMBOL_GPL(fscrypt_prepare_symlink);
284
285int __fscrypt_encrypt_symlink(struct inode *inode, const char *target,
286			      unsigned int len, struct fscrypt_str *disk_link)
287{
288	int err;
289	struct qstr iname = QSTR_INIT(target, len);
290	struct fscrypt_symlink_data *sd;
291	unsigned int ciphertext_len;
292
293	/*
294	 * fscrypt_prepare_new_inode() should have already set up the new
295	 * symlink inode's encryption key.  We don't wait until now to do it,
296	 * since we may be in a filesystem transaction now.
297	 */
298	if (WARN_ON_ONCE(!fscrypt_has_encryption_key(inode)))
299		return -ENOKEY;
300
301	if (disk_link->name) {
302		/* filesystem-provided buffer */
303		sd = (struct fscrypt_symlink_data *)disk_link->name;
304	} else {
305		sd = kmalloc(disk_link->len, GFP_NOFS);
306		if (!sd)
307			return -ENOMEM;
308	}
309	ciphertext_len = disk_link->len - sizeof(*sd) - 1;
310	sd->len = cpu_to_le16(ciphertext_len);
311
312	err = fscrypt_fname_encrypt(inode, &iname, sd->encrypted_path,
313				    ciphertext_len);
314	if (err)
315		goto err_free_sd;
316
317	/*
318	 * Null-terminating the ciphertext doesn't make sense, but we still
319	 * count the null terminator in the length, so we might as well
320	 * initialize it just in case the filesystem writes it out.
321	 */
322	sd->encrypted_path[ciphertext_len] = '\0';
323
324	/* Cache the plaintext symlink target for later use by get_link() */
325	err = -ENOMEM;
326	inode->i_link = kmemdup(target, len + 1, GFP_NOFS);
327	if (!inode->i_link)
328		goto err_free_sd;
329
330	if (!disk_link->name)
331		disk_link->name = (unsigned char *)sd;
332	return 0;
333
334err_free_sd:
335	if (!disk_link->name)
336		kfree(sd);
337	return err;
338}
339EXPORT_SYMBOL_GPL(__fscrypt_encrypt_symlink);
340
341/**
342 * fscrypt_get_symlink() - get the target of an encrypted symlink
343 * @inode: the symlink inode
344 * @caddr: the on-disk contents of the symlink
345 * @max_size: size of @caddr buffer
346 * @done: if successful, will be set up to free the returned target if needed
347 *
348 * If the symlink's encryption key is available, we decrypt its target.
349 * Otherwise, we encode its target for presentation.
350 *
351 * This may sleep, so the filesystem must have dropped out of RCU mode already.
352 *
353 * Return: the presentable symlink target or an ERR_PTR()
354 */
355const char *fscrypt_get_symlink(struct inode *inode, const void *caddr,
356				unsigned int max_size,
357				struct delayed_call *done)
358{
359	const struct fscrypt_symlink_data *sd;
360	struct fscrypt_str cstr, pstr;
361	bool has_key;
362	int err;
363
364	/* This is for encrypted symlinks only */
365	if (WARN_ON_ONCE(!IS_ENCRYPTED(inode)))
366		return ERR_PTR(-EINVAL);
367
368	/* If the decrypted target is already cached, just return it. */
369	pstr.name = READ_ONCE(inode->i_link);
370	if (pstr.name)
371		return pstr.name;
372
373	/*
374	 * Try to set up the symlink's encryption key, but we can continue
375	 * regardless of whether the key is available or not.
376	 */
377	err = fscrypt_get_encryption_info(inode, false);
378	if (err)
379		return ERR_PTR(err);
380	has_key = fscrypt_has_encryption_key(inode);
381
382	/*
383	 * For historical reasons, encrypted symlink targets are prefixed with
384	 * the ciphertext length, even though this is redundant with i_size.
385	 */
386
387	if (max_size < sizeof(*sd) + 1)
388		return ERR_PTR(-EUCLEAN);
389	sd = caddr;
390	cstr.name = (unsigned char *)sd->encrypted_path;
391	cstr.len = le16_to_cpu(sd->len);
392
393	if (cstr.len == 0)
394		return ERR_PTR(-EUCLEAN);
395
396	if (cstr.len + sizeof(*sd) > max_size)
397		return ERR_PTR(-EUCLEAN);
398
399	err = fscrypt_fname_alloc_buffer(cstr.len, &pstr);
400	if (err)
401		return ERR_PTR(err);
402
403	err = fscrypt_fname_disk_to_usr(inode, 0, 0, &cstr, &pstr);
404	if (err)
405		goto err_kfree;
406
407	err = -EUCLEAN;
408	if (pstr.name[0] == '\0')
409		goto err_kfree;
410
411	pstr.name[pstr.len] = '\0';
412
413	/*
414	 * Cache decrypted symlink targets in i_link for later use.  Don't cache
415	 * symlink targets encoded without the key, since those become outdated
416	 * once the key is added.  This pairs with the READ_ONCE() above and in
417	 * the VFS path lookup code.
418	 */
419	if (!has_key ||
420	    cmpxchg_release(&inode->i_link, NULL, pstr.name) != NULL)
421		set_delayed_call(done, kfree_link, pstr.name);
422
423	return pstr.name;
424
425err_kfree:
426	kfree(pstr.name);
427	return ERR_PTR(err);
428}
429EXPORT_SYMBOL_GPL(fscrypt_get_symlink);
430
431/**
432 * fscrypt_symlink_getattr() - set the correct st_size for encrypted symlinks
433 * @path: the path for the encrypted symlink being queried
434 * @stat: the struct being filled with the symlink's attributes
435 *
436 * Override st_size of encrypted symlinks to be the length of the decrypted
437 * symlink target (or the no-key encoded symlink target, if the key is
438 * unavailable) rather than the length of the encrypted symlink target.  This is
439 * necessary for st_size to match the symlink target that userspace actually
440 * sees.  POSIX requires this, and some userspace programs depend on it.
441 *
442 * This requires reading the symlink target from disk if needed, setting up the
443 * inode's encryption key if possible, and then decrypting or encoding the
444 * symlink target.  This makes lstat() more heavyweight than is normally the
445 * case.  However, decrypted symlink targets will be cached in ->i_link, so
446 * usually the symlink won't have to be read and decrypted again later if/when
447 * it is actually followed, readlink() is called, or lstat() is called again.
448 *
449 * Return: 0 on success, -errno on failure
450 */
451int fscrypt_symlink_getattr(const struct path *path, struct kstat *stat)
452{
453	struct dentry *dentry = path->dentry;
454	struct inode *inode = d_inode(dentry);
455	const char *link;
456	DEFINE_DELAYED_CALL(done);
457
458	/*
459	 * To get the symlink target that userspace will see (whether it's the
460	 * decrypted target or the no-key encoded target), we can just get it in
461	 * the same way the VFS does during path resolution and readlink().
462	 */
463	link = READ_ONCE(inode->i_link);
464	if (!link) {
465		link = inode->i_op->get_link(dentry, inode, &done);
466		if (IS_ERR(link))
467			return PTR_ERR(link);
468	}
469	stat->size = strlen(link);
470	do_delayed_call(&done);
471	return 0;
472}
473EXPORT_SYMBOL_GPL(fscrypt_symlink_getattr);