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 *dir;
  34
  35         err = fscrypt_require_key(inode);
  36         if (err)
  37                 return err;
  38
  39         dir = dget_parent(file_dentry(filp));
  40         if (IS_ENCRYPTED(d_inode(dir)) &&
  41             !fscrypt_has_permitted_context(d_inode(dir), inode)) {
  42                 fscrypt_warn(inode,
  43                              "Inconsistent encryption context (parent directory: %lu)",
  44                              d_inode(dir)->i_ino);
  45                 err = -EPERM;
  46         }
  47         dput(dir);
  48         return err;
  49 }
  50 EXPORT_SYMBOL_GPL(fscrypt_file_open);
  51
  52 int __fscrypt_prepare_link(struct inode *inode, struct inode *dir,
  53                            struct dentry *dentry)
  54 {
  55         int err;
  56
  57         err = fscrypt_require_key(dir);
  58         if (err)
  59                 return err;
  60
  61         /* ... in case we looked up ciphertext name before key was added */
  62         if (dentry->d_flags & DCACHE_ENCRYPTED_NAME)
  63                 return -ENOKEY;
  64
  65         if (!fscrypt_has_permitted_context(dir, inode))
  66                 return -EXDEV;
  67
  68         return 0;
  69 }
  70 EXPORT_SYMBOL_GPL(__fscrypt_prepare_link);
  71
  72 int __fscrypt_prepare_rename(struct inode *old_dir, struct dentry *old_dentry,
  73                              struct inode *new_dir, struct dentry *new_dentry,
  74                              unsigned int flags)
  75 {
  76         int err;
  77
  78         err = fscrypt_require_key(old_dir);
  79         if (err)
  80                 return err;
  81
  82         err = fscrypt_require_key(new_dir);
  83         if (err)
  84                 return err;
  85
  86         /* ... in case we looked up ciphertext name(s) before key was added */
  87         if ((old_dentry->d_flags | new_dentry->d_flags) &
  88             DCACHE_ENCRYPTED_NAME)
  89                 return -ENOKEY;
  90
  91         if (old_dir != new_dir) {
  92                 if (IS_ENCRYPTED(new_dir) &&
  93                     !fscrypt_has_permitted_context(new_dir,
  94                                                    d_inode(old_dentry)))
  95                         return -EXDEV;
  96
  97                 if ((flags & RENAME_EXCHANGE) &&
  98                     IS_ENCRYPTED(old_dir) &&
  99                     !fscrypt_has_permitted_context(old_dir,
 100                                                    d_inode(new_dentry)))
 101                         return -EXDEV;
 102         }
 103         return 0;
 104 }
 105 EXPORT_SYMBOL_GPL(__fscrypt_prepare_rename);
 106
 107 int __fscrypt_prepare_lookup(struct inode *dir, struct dentry *dentry,
 108                              struct fscrypt_name *fname)
 109 {
 110         int err = fscrypt_setup_filename(dir, &dentry->d_name, 1, fname);
 111
 112         if (err && err != -ENOENT)
 113                 return err;
 114
 115         if (fname->is_ciphertext_name) {
 116                 spin_lock(&dentry->d_lock);
 117                 dentry->d_flags |= DCACHE_ENCRYPTED_NAME;
 118                 spin_unlock(&dentry->d_lock);
 119                 d_set_d_op(dentry, &fscrypt_d_ops);
 120         }
 121         return err;
 122 }
 123 EXPORT_SYMBOL_GPL(__fscrypt_prepare_lookup);
 124
 125 int __fscrypt_prepare_symlink(struct inode *dir, unsigned int len,
 126                               unsigned int max_len,
 127                               struct fscrypt_str *disk_link)
 128 {
 129         int err;
 130
 131         /*
 132          * To calculate the size of the encrypted symlink target we need to know
 133          * the amount of NUL padding, which is determined by the flags set in
 134          * the encryption policy which will be inherited from the directory.
 135          * The easiest way to get access to this is to just load the directory's
 136          * fscrypt_info, since we'll need it to create the dir_entry anyway.
 137          *
 138          * Note: in test_dummy_encryption mode, @dir may be unencrypted.
 139          */
 140         err = fscrypt_get_encryption_info(dir);
 141         if (err)
 142                 return err;
 143         if (!fscrypt_has_encryption_key(dir))
 144                 return -ENOKEY;
 145
 146         /*
 147          * Calculate the size of the encrypted symlink and verify it won't
 148          * exceed max_len.  Note that for historical reasons, encrypted symlink
 149          * targets are prefixed with the ciphertext length, despite this
 150          * actually being redundant with i_size.  This decreases by 2 bytes the
 151          * longest symlink target we can accept.
 152          *
 153          * We could recover 1 byte by not counting a null terminator, but
 154          * counting it (even though it is meaningless for ciphertext) is simpler
 155          * for now since filesystems will assume it is there and subtract it.
 156          */
 157         if (!fscrypt_fname_encrypted_size(dir, len,
 158                                           max_len - sizeof(struct fscrypt_symlink_data),
 159                                           &disk_link->len))
 160                 return -ENAMETOOLONG;
 161         disk_link->len += sizeof(struct fscrypt_symlink_data);
 162
 163         disk_link->name = NULL;
 164         return 0;
 165 }
 166 EXPORT_SYMBOL_GPL(__fscrypt_prepare_symlink);
 167
 168 int __fscrypt_encrypt_symlink(struct inode *inode, const char *target,
 169                               unsigned int len, struct fscrypt_str *disk_link)
 170 {
 171         int err;
 172         struct qstr iname = QSTR_INIT(target, len);
 173         struct fscrypt_symlink_data *sd;
 174         unsigned int ciphertext_len;
 175
 176         err = fscrypt_require_key(inode);
 177         if (err)
 178                 return err;
 179
 180         if (disk_link->name) {
 181                 /* filesystem-provided buffer */
 182                 sd = (struct fscrypt_symlink_data *)disk_link->name;
 183         } else {
 184                 sd = kmalloc(disk_link->len, GFP_NOFS);
 185                 if (!sd)
 186                         return -ENOMEM;
 187         }
 188         ciphertext_len = disk_link->len - sizeof(*sd);
 189         sd->len = cpu_to_le16(ciphertext_len);
 190
 191         err = fname_encrypt(inode, &iname, sd->encrypted_path, ciphertext_len);
 192         if (err)
 193                 goto err_free_sd;
 194
 195         /*
 196          * Null-terminating the ciphertext doesn't make sense, but we still
 197          * count the null terminator in the length, so we might as well
 198          * initialize it just in case the filesystem writes it out.
 199          */
 200         sd->encrypted_path[ciphertext_len] = '\0';
 201
 202         /* Cache the plaintext symlink target for later use by get_link() */
 203         err = -ENOMEM;
 204         inode->i_link = kmemdup(target, len + 1, GFP_NOFS);
 205         if (!inode->i_link)
 206                 goto err_free_sd;
 207
 208         if (!disk_link->name)
 209                 disk_link->name = (unsigned char *)sd;
 210         return 0;
 211
 212 err_free_sd:
 213         if (!disk_link->name)
 214                 kfree(sd);
 215         return err;
 216 }
 217 EXPORT_SYMBOL_GPL(__fscrypt_encrypt_symlink);
 218
 219 /**
 220  * fscrypt_get_symlink - get the target of an encrypted symlink
 221  * @inode: the symlink inode
 222  * @caddr: the on-disk contents of the symlink
 223  * @max_size: size of @caddr buffer
 224  * @done: if successful, will be set up to free the returned target if needed
 225  *
 226  * If the symlink's encryption key is available, we decrypt its target.
 227  * Otherwise, we encode its target for presentation.
 228  *
 229  * This may sleep, so the filesystem must have dropped out of RCU mode already.
 230  *
 231  * Return: the presentable symlink target or an ERR_PTR()
 232  */
 233 const char *fscrypt_get_symlink(struct inode *inode, const void *caddr,
 234                                 unsigned int max_size,
 235                                 struct delayed_call *done)
 236 {
 237         const struct fscrypt_symlink_data *sd;
 238         struct fscrypt_str cstr, pstr;
 239         bool has_key;
 240         int err;
 241
 242         /* This is for encrypted symlinks only */
 243         if (WARN_ON(!IS_ENCRYPTED(inode)))
 244                 return ERR_PTR(-EINVAL);
 245
 246         /* If the decrypted target is already cached, just return it. */
 247         pstr.name = READ_ONCE(inode->i_link);
 248         if (pstr.name)
 249                 return pstr.name;
 250
 251         /*
 252          * Try to set up the symlink's encryption key, but we can continue
 253          * regardless of whether the key is available or not.
 254          */
 255         err = fscrypt_get_encryption_info(inode);
 256         if (err)
 257                 return ERR_PTR(err);
 258         has_key = fscrypt_has_encryption_key(inode);
 259
 260         /*
 261          * For historical reasons, encrypted symlink targets are prefixed with
 262          * the ciphertext length, even though this is redundant with i_size.
 263          */
 264
 265         if (max_size < sizeof(*sd))
 266                 return ERR_PTR(-EUCLEAN);
 267         sd = caddr;
 268         cstr.name = (unsigned char *)sd->encrypted_path;
 269         cstr.len = le16_to_cpu(sd->len);
 270
 271         if (cstr.len == 0)
 272                 return ERR_PTR(-EUCLEAN);
 273
 274         if (cstr.len + sizeof(*sd) - 1 > max_size)
 275                 return ERR_PTR(-EUCLEAN);
 276
 277         err = fscrypt_fname_alloc_buffer(inode, cstr.len, &pstr);
 278         if (err)
 279                 return ERR_PTR(err);
 280
 281         err = fscrypt_fname_disk_to_usr(inode, 0, 0, &cstr, &pstr);
 282         if (err)
 283                 goto err_kfree;
 284
 285         err = -EUCLEAN;
 286         if (pstr.name[0] == '\0')
 287                 goto err_kfree;
 288
 289         pstr.name[pstr.len] = '\0';
 290
 291         /*
 292          * Cache decrypted symlink targets in i_link for later use.  Don't cache
 293          * symlink targets encoded without the key, since those become outdated
 294          * once the key is added.  This pairs with the READ_ONCE() above and in
 295          * the VFS path lookup code.
 296          */
 297         if (!has_key ||
 298             cmpxchg_release(&inode->i_link, NULL, pstr.name) != NULL)
 299                 set_delayed_call(done, kfree_link, pstr.name);
 300
 301         return pstr.name;
 302
 303 err_kfree:
 304         kfree(pstr.name);
 305         return ERR_PTR(err);
 306 }
 307 EXPORT_SYMBOL_GPL(fscrypt_get_symlink);