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 ciphertext name before key was added */
  64         if (dentry->d_flags & DCACHE_ENCRYPTED_NAME)
  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 ciphertext name(s) before key was added */
  89         if ((old_dentry->d_flags | new_dentry->d_flags) &
  90             DCACHE_ENCRYPTED_NAME)
  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_ciphertext_name) {
 118                 spin_lock(&dentry->d_lock);
 119                 dentry->d_flags |= DCACHE_ENCRYPTED_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 int __fscrypt_prepare_symlink(struct inode *dir, unsigned int len,
 170                               unsigned int max_len,
 171                               struct fscrypt_str *disk_link)
 172 {
 173         int err;
 174
 175         /*
 176          * To calculate the size of the encrypted symlink target we need to know
 177          * the amount of NUL padding, which is determined by the flags set in
 178          * the encryption policy which will be inherited from the directory.
 179          * The easiest way to get access to this is to just load the directory's
 180          * fscrypt_info, since we'll need it to create the dir_entry anyway.
 181          *
 182          * Note: in test_dummy_encryption mode, @dir may be unencrypted.
 183          */
 184         err = fscrypt_get_encryption_info(dir);
 185         if (err)
 186                 return err;
 187         if (!fscrypt_has_encryption_key(dir))
 188                 return -ENOKEY;
 189
 190         /*
 191          * Calculate the size of the encrypted symlink and verify it won't
 192          * exceed max_len.  Note that for historical reasons, encrypted symlink
 193          * targets are prefixed with the ciphertext length, despite this
 194          * actually being redundant with i_size.  This decreases by 2 bytes the
 195          * longest symlink target we can accept.
 196          *
 197          * We could recover 1 byte by not counting a null terminator, but
 198          * counting it (even though it is meaningless for ciphertext) is simpler
 199          * for now since filesystems will assume it is there and subtract it.
 200          */
 201         if (!fscrypt_fname_encrypted_size(dir, len,
 202                                           max_len - sizeof(struct fscrypt_symlink_data),
 203                                           &disk_link->len))
 204                 return -ENAMETOOLONG;
 205         disk_link->len += sizeof(struct fscrypt_symlink_data);
 206
 207         disk_link->name = NULL;
 208         return 0;
 209 }
 210 EXPORT_SYMBOL_GPL(__fscrypt_prepare_symlink);
 211
 212 int __fscrypt_encrypt_symlink(struct inode *inode, const char *target,
 213                               unsigned int len, struct fscrypt_str *disk_link)
 214 {
 215         int err;
 216         struct qstr iname = QSTR_INIT(target, len);
 217         struct fscrypt_symlink_data *sd;
 218         unsigned int ciphertext_len;
 219
 220         err = fscrypt_require_key(inode);
 221         if (err)
 222                 return err;
 223
 224         if (disk_link->name) {
 225                 /* filesystem-provided buffer */
 226                 sd = (struct fscrypt_symlink_data *)disk_link->name;
 227         } else {
 228                 sd = kmalloc(disk_link->len, GFP_NOFS);
 229                 if (!sd)
 230                         return -ENOMEM;
 231         }
 232         ciphertext_len = disk_link->len - sizeof(*sd);
 233         sd->len = cpu_to_le16(ciphertext_len);
 234
 235         err = fscrypt_fname_encrypt(inode, &iname, sd->encrypted_path,
 236                                     ciphertext_len);
 237         if (err)
 238                 goto err_free_sd;
 239
 240         /*
 241          * Null-terminating the ciphertext doesn't make sense, but we still
 242          * count the null terminator in the length, so we might as well
 243          * initialize it just in case the filesystem writes it out.
 244          */
 245         sd->encrypted_path[ciphertext_len] = '\0';
 246
 247         /* Cache the plaintext symlink target for later use by get_link() */
 248         err = -ENOMEM;
 249         inode->i_link = kmemdup(target, len + 1, GFP_NOFS);
 250         if (!inode->i_link)
 251                 goto err_free_sd;
 252
 253         if (!disk_link->name)
 254                 disk_link->name = (unsigned char *)sd;
 255         return 0;
 256
 257 err_free_sd:
 258         if (!disk_link->name)
 259                 kfree(sd);
 260         return err;
 261 }
 262 EXPORT_SYMBOL_GPL(__fscrypt_encrypt_symlink);
 263
 264 /**
 265  * fscrypt_get_symlink - get the target of an encrypted symlink
 266  * @inode: the symlink inode
 267  * @caddr: the on-disk contents of the symlink
 268  * @max_size: size of @caddr buffer
 269  * @done: if successful, will be set up to free the returned target if needed
 270  *
 271  * If the symlink's encryption key is available, we decrypt its target.
 272  * Otherwise, we encode its target for presentation.
 273  *
 274  * This may sleep, so the filesystem must have dropped out of RCU mode already.
 275  *
 276  * Return: the presentable symlink target or an ERR_PTR()
 277  */
 278 const char *fscrypt_get_symlink(struct inode *inode, const void *caddr,
 279                                 unsigned int max_size,
 280                                 struct delayed_call *done)
 281 {
 282         const struct fscrypt_symlink_data *sd;
 283         struct fscrypt_str cstr, pstr;
 284         bool has_key;
 285         int err;
 286
 287         /* This is for encrypted symlinks only */
 288         if (WARN_ON(!IS_ENCRYPTED(inode)))
 289                 return ERR_PTR(-EINVAL);
 290
 291         /* If the decrypted target is already cached, just return it. */
 292         pstr.name = READ_ONCE(inode->i_link);
 293         if (pstr.name)
 294                 return pstr.name;
 295
 296         /*
 297          * Try to set up the symlink's encryption key, but we can continue
 298          * regardless of whether the key is available or not.
 299          */
 300         err = fscrypt_get_encryption_info(inode);
 301         if (err)
 302                 return ERR_PTR(err);
 303         has_key = fscrypt_has_encryption_key(inode);
 304
 305         /*
 306          * For historical reasons, encrypted symlink targets are prefixed with
 307          * the ciphertext length, even though this is redundant with i_size.
 308          */
 309
 310         if (max_size < sizeof(*sd))
 311                 return ERR_PTR(-EUCLEAN);
 312         sd = caddr;
 313         cstr.name = (unsigned char *)sd->encrypted_path;
 314         cstr.len = le16_to_cpu(sd->len);
 315
 316         if (cstr.len == 0)
 317                 return ERR_PTR(-EUCLEAN);
 318
 319         if (cstr.len + sizeof(*sd) - 1 > max_size)
 320                 return ERR_PTR(-EUCLEAN);
 321
 322         err = fscrypt_fname_alloc_buffer(inode, cstr.len, &pstr);
 323         if (err)
 324                 return ERR_PTR(err);
 325
 326         err = fscrypt_fname_disk_to_usr(inode, 0, 0, &cstr, &pstr);
 327         if (err)
 328                 goto err_kfree;
 329
 330         err = -EUCLEAN;
 331         if (pstr.name[0] == '\0')
 332                 goto err_kfree;
 333
 334         pstr.name[pstr.len] = '\0';
 335
 336         /*
 337          * Cache decrypted symlink targets in i_link for later use.  Don't cache
 338          * symlink targets encoded without the key, since those become outdated
 339          * once the key is added.  This pairs with the READ_ONCE() above and in
 340          * the VFS path lookup code.
 341          */
 342         if (!has_key ||
 343             cmpxchg_release(&inode->i_link, NULL, pstr.name) != NULL)
 344                 set_delayed_call(done, kfree_link, pstr.name);
 345
 346         return pstr.name;
 347
 348 err_kfree:
 349         kfree(pstr.name);
 350         return ERR_PTR(err);
 351 }
 352 EXPORT_SYMBOL_GPL(fscrypt_get_symlink);