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