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 <linux/key.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  */
  32 int fscrypt_file_open(struct inode *inode, struct file *filp)
  33 {
  34         int err;
  35         struct dentry *dir;
  36
  37         err = fscrypt_require_key(inode);
  38         if (err)
  39                 return err;
  40
  41         dir = dget_parent(file_dentry(filp));
  42         if (IS_ENCRYPTED(d_inode(dir)) &&
  43             !fscrypt_has_permitted_context(d_inode(dir), inode)) {
  44                 fscrypt_warn(inode,
  45                              "Inconsistent encryption context (parent directory: %lu)",
  46                              d_inode(dir)->i_ino);
  47                 err = -EPERM;
  48         }
  49         dput(dir);
  50         return err;
  51 }
  52 EXPORT_SYMBOL_GPL(fscrypt_file_open);
  53
  54 int __fscrypt_prepare_link(struct inode *inode, struct inode *dir,
  55                            struct dentry *dentry)
  56 {
  57         int err;
  58
  59         err = fscrypt_require_key(dir);
  60         if (err)
  61                 return err;
  62
  63         /* ... in case we looked up no-key name before key was added */
  64         if (fscrypt_is_nokey_name(dentry))
  65                 return -ENOKEY;
  66
  67         if (!fscrypt_has_permitted_context(dir, inode))
  68                 return -EXDEV;
  69
  70         return 0;
  71 }
  72 EXPORT_SYMBOL_GPL(__fscrypt_prepare_link);
  73
  74 int __fscrypt_prepare_rename(struct inode *old_dir, struct dentry *old_dentry,
  75                              struct inode *new_dir, struct dentry *new_dentry,
  76                              unsigned int flags)
  77 {
  78         int err;
  79
  80         err = fscrypt_require_key(old_dir);
  81         if (err)
  82                 return err;
  83
  84         err = fscrypt_require_key(new_dir);
  85         if (err)
  86                 return err;
  87
  88         /* ... in case we looked up no-key name(s) before key was added */
  89         if (fscrypt_is_nokey_name(old_dentry) ||
  90             fscrypt_is_nokey_name(new_dentry))
  91                 return -ENOKEY;
  92
  93         if (old_dir != new_dir) {
  94                 if (IS_ENCRYPTED(new_dir) &&
  95                     !fscrypt_has_permitted_context(new_dir,
  96                                                    d_inode(old_dentry)))
  97                         return -EXDEV;
  98
  99                 if ((flags & RENAME_EXCHANGE) &&
 100                     IS_ENCRYPTED(old_dir) &&
 101                     !fscrypt_has_permitted_context(old_dir,
 102                                                    d_inode(new_dentry)))
 103                         return -EXDEV;
 104         }
 105         return 0;
 106 }
 107 EXPORT_SYMBOL_GPL(__fscrypt_prepare_rename);
 108
 109 int __fscrypt_prepare_lookup(struct inode *dir, struct dentry *dentry,
 110                              struct fscrypt_name *fname)
 111 {
 112         int err = fscrypt_setup_filename(dir, &dentry->d_name, 1, fname);
 113
 114         if (err && err != -ENOENT)
 115                 return err;
 116
 117         if (fname->is_nokey_name) {
 118                 spin_lock(&dentry->d_lock);
 119                 dentry->d_flags |= DCACHE_NOKEY_NAME;
 120                 spin_unlock(&dentry->d_lock);
 121                 d_set_d_op(dentry, &fscrypt_d_ops);
 122         }
 123         return err;
 124 }
 125 EXPORT_SYMBOL_GPL(__fscrypt_prepare_lookup);
 126
 127 /**
 128  * fscrypt_prepare_setflags() - prepare to change flags with FS_IOC_SETFLAGS
 129  * @inode: the inode on which flags are being changed
 130  * @oldflags: the old flags
 131  * @flags: the new flags
 132  *
 133  * The caller should be holding i_rwsem for write.
 134  *
 135  * Return: 0 on success; -errno if the flags change isn't allowed or if
 136  *         another error occurs.
 137  */
 138 int fscrypt_prepare_setflags(struct inode *inode,
 139                              unsigned int oldflags, unsigned int flags)
 140 {
 141         struct fscrypt_info *ci;
 142         struct fscrypt_master_key *mk;
 143         int err;
 144
 145         /*
 146          * When the CASEFOLD flag is set on an encrypted directory, we must
 147          * derive the secret key needed for the dirhash.  This is only possible
 148          * if the directory uses a v2 encryption policy.
 149          */
 150         if (IS_ENCRYPTED(inode) && (flags & ~oldflags & FS_CASEFOLD_FL)) {
 151                 err = fscrypt_require_key(inode);
 152                 if (err)
 153                         return err;
 154                 ci = inode->i_crypt_info;
 155                 if (ci->ci_policy.version != FSCRYPT_POLICY_V2)
 156                         return -EINVAL;
 157                 mk = ci->ci_master_key->payload.data[0];
 158                 down_read(&mk->mk_secret_sem);
 159                 if (is_master_key_secret_present(&mk->mk_secret))
 160                         err = fscrypt_derive_dirhash_key(ci, mk);
 161                 else
 162                         err = -ENOKEY;
 163                 up_read(&mk->mk_secret_sem);
 164                 return err;
 165         }
 166         return 0;
 167 }
 168
 169 /**
 170  * fscrypt_prepare_symlink() - prepare to create a possibly-encrypted symlink
 171  * @dir: directory in which the symlink is being created
 172  * @target: plaintext symlink target
 173  * @len: length of @target excluding null terminator
 174  * @max_len: space the filesystem has available to store the symlink target
 175  * @disk_link: (out) the on-disk symlink target being prepared
 176  *
 177  * This function computes the size the symlink target will require on-disk,
 178  * stores it in @disk_link->len, and validates it against @max_len.  An
 179  * encrypted symlink may be longer than the original.
 180  *
 181  * Additionally, @disk_link->name is set to @target if the symlink will be
 182  * unencrypted, but left NULL if the symlink will be encrypted.  For encrypted
 183  * symlinks, the filesystem must call fscrypt_encrypt_symlink() to create the
 184  * on-disk target later.  (The reason for the two-step process is that some
 185  * filesystems need to know the size of the symlink target before creating the
 186  * inode, e.g. to determine whether it will be a "fast" or "slow" symlink.)
 187  *
 188  * Return: 0 on success, -ENAMETOOLONG if the symlink target is too long,
 189  * -ENOKEY if the encryption key is missing, or another -errno code if a problem
 190  * occurred while setting up the encryption key.
 191  */
 192 int fscrypt_prepare_symlink(struct inode *dir, const char *target,
 193                             unsigned int len, unsigned int max_len,
 194                             struct fscrypt_str *disk_link)
 195 {
 196         const union fscrypt_policy *policy;
 197
 198         /*
 199          * To calculate the size of the encrypted symlink target we need to know
 200          * the amount of NUL padding, which is determined by the flags set in
 201          * the encryption policy which will be inherited from the directory.
 202          */
 203         policy = fscrypt_policy_to_inherit(dir);
 204         if (policy == NULL) {
 205                 /* Not encrypted */
 206                 disk_link->name = (unsigned char *)target;
 207                 disk_link->len = len + 1;
 208                 if (disk_link->len > max_len)
 209                         return -ENAMETOOLONG;
 210                 return 0;
 211         }
 212         if (IS_ERR(policy))
 213                 return PTR_ERR(policy);
 214
 215         /*
 216          * Calculate the size of the encrypted symlink and verify it won't
 217          * exceed max_len.  Note that for historical reasons, encrypted symlink
 218          * targets are prefixed with the ciphertext length, despite this
 219          * actually being redundant with i_size.  This decreases by 2 bytes the
 220          * longest symlink target we can accept.
 221          *
 222          * We could recover 1 byte by not counting a null terminator, but
 223          * counting it (even though it is meaningless for ciphertext) is simpler
 224          * for now since filesystems will assume it is there and subtract it.
 225          */
 226         if (!fscrypt_fname_encrypted_size(policy, len,
 227                                           max_len - sizeof(struct fscrypt_symlink_data),
 228                                           &disk_link->len))
 229                 return -ENAMETOOLONG;
 230         disk_link->len += sizeof(struct fscrypt_symlink_data);
 231
 232         disk_link->name = NULL;
 233         return 0;
 234 }
 235 EXPORT_SYMBOL_GPL(fscrypt_prepare_symlink);
 236
 237 int __fscrypt_encrypt_symlink(struct inode *inode, const char *target,
 238                               unsigned int len, struct fscrypt_str *disk_link)
 239 {
 240         int err;
 241         struct qstr iname = QSTR_INIT(target, len);
 242         struct fscrypt_symlink_data *sd;
 243         unsigned int ciphertext_len;
 244
 245         /*
 246          * fscrypt_prepare_new_inode() should have already set up the new
 247          * symlink inode's encryption key.  We don't wait until now to do it,
 248          * since we may be in a filesystem transaction now.
 249          */
 250         if (WARN_ON_ONCE(!fscrypt_has_encryption_key(inode)))
 251                 return -ENOKEY;
 252
 253         if (disk_link->name) {
 254                 /* filesystem-provided buffer */
 255                 sd = (struct fscrypt_symlink_data *)disk_link->name;
 256         } else {
 257                 sd = kmalloc(disk_link->len, GFP_NOFS);
 258                 if (!sd)
 259                         return -ENOMEM;
 260         }
 261         ciphertext_len = disk_link->len - sizeof(*sd);
 262         sd->len = cpu_to_le16(ciphertext_len);
 263
 264         err = fscrypt_fname_encrypt(inode, &iname, sd->encrypted_path,
 265                                     ciphertext_len);
 266         if (err)
 267                 goto err_free_sd;
 268
 269         /*
 270          * Null-terminating the ciphertext doesn't make sense, but we still
 271          * count the null terminator in the length, so we might as well
 272          * initialize it just in case the filesystem writes it out.
 273          */
 274         sd->encrypted_path[ciphertext_len] = '\0';
 275
 276         /* Cache the plaintext symlink target for later use by get_link() */
 277         err = -ENOMEM;
 278         inode->i_link = kmemdup(target, len + 1, GFP_NOFS);
 279         if (!inode->i_link)
 280                 goto err_free_sd;
 281
 282         if (!disk_link->name)
 283                 disk_link->name = (unsigned char *)sd;
 284         return 0;
 285
 286 err_free_sd:
 287         if (!disk_link->name)
 288                 kfree(sd);
 289         return err;
 290 }
 291 EXPORT_SYMBOL_GPL(__fscrypt_encrypt_symlink);
 292
 293 /**
 294  * fscrypt_get_symlink() - get the target of an encrypted symlink
 295  * @inode: the symlink inode
 296  * @caddr: the on-disk contents of the symlink
 297  * @max_size: size of @caddr buffer
 298  * @done: if successful, will be set up to free the returned target if needed
 299  *
 300  * If the symlink's encryption key is available, we decrypt its target.
 301  * Otherwise, we encode its target for presentation.
 302  *
 303  * This may sleep, so the filesystem must have dropped out of RCU mode already.
 304  *
 305  * Return: the presentable symlink target or an ERR_PTR()
 306  */
 307 const char *fscrypt_get_symlink(struct inode *inode, const void *caddr,
 308                                 unsigned int max_size,
 309                                 struct delayed_call *done)
 310 {
 311         const struct fscrypt_symlink_data *sd;
 312         struct fscrypt_str cstr, pstr;
 313         bool has_key;
 314         int err;
 315
 316         /* This is for encrypted symlinks only */
 317         if (WARN_ON(!IS_ENCRYPTED(inode)))
 318                 return ERR_PTR(-EINVAL);
 319
 320         /* If the decrypted target is already cached, just return it. */
 321         pstr.name = READ_ONCE(inode->i_link);
 322         if (pstr.name)
 323                 return pstr.name;
 324
 325         /*
 326          * Try to set up the symlink's encryption key, but we can continue
 327          * regardless of whether the key is available or not.
 328          */
 329         err = fscrypt_get_encryption_info(inode);
 330         if (err)
 331                 return ERR_PTR(err);
 332         has_key = fscrypt_has_encryption_key(inode);
 333
 334         /*
 335          * For historical reasons, encrypted symlink targets are prefixed with
 336          * the ciphertext length, even though this is redundant with i_size.
 337          */
 338
 339         if (max_size < sizeof(*sd))
 340                 return ERR_PTR(-EUCLEAN);
 341         sd = caddr;
 342         cstr.name = (unsigned char *)sd->encrypted_path;
 343         cstr.len = le16_to_cpu(sd->len);
 344
 345         if (cstr.len == 0)
 346                 return ERR_PTR(-EUCLEAN);
 347
 348         if (cstr.len + sizeof(*sd) - 1 > max_size)
 349                 return ERR_PTR(-EUCLEAN);
 350
 351         err = fscrypt_fname_alloc_buffer(cstr.len, &pstr);
 352         if (err)
 353                 return ERR_PTR(err);
 354
 355         err = fscrypt_fname_disk_to_usr(inode, 0, 0, &cstr, &pstr);
 356         if (err)
 357                 goto err_kfree;
 358
 359         err = -EUCLEAN;
 360         if (pstr.name[0] == '\0')
 361                 goto err_kfree;
 362
 363         pstr.name[pstr.len] = '\0';
 364
 365         /*
 366          * Cache decrypted symlink targets in i_link for later use.  Don't cache
 367          * symlink targets encoded without the key, since those become outdated
 368          * once the key is added.  This pairs with the READ_ONCE() above and in
 369          * the VFS path lookup code.
 370          */
 371         if (!has_key ||
 372             cmpxchg_release(&inode->i_link, NULL, pstr.name) != NULL)
 373                 set_delayed_call(done, kfree_link, pstr.name);
 374
 375         return pstr.name;
 376
 377 err_kfree:
 378         kfree(pstr.name);
 379         return ERR_PTR(err);
 380 }
 381 EXPORT_SYMBOL_GPL(fscrypt_get_symlink);