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/ratelimit.h>
   9 #include "fscrypt_private.h"
  10
  11 /**
  12  * fscrypt_file_open - prepare to open a possibly-encrypted regular file
  13  * @inode: the inode being opened
  14  * @filp: the struct file being set up
  15  *
  16  * Currently, an encrypted regular file can only be opened if its encryption key
  17  * is available; access to the raw encrypted contents is not supported.
  18  * Therefore, we first set up the inode's encryption key (if not already done)
  19  * and return an error if it's unavailable.
  20  *
  21  * We also verify that if the parent directory (from the path via which the file
  22  * is being opened) is encrypted, then the inode being opened uses the same
  23  * encryption policy.  This is needed as part of the enforcement that all files
  24  * in an encrypted directory tree use the same encryption policy, as a
  25  * protection against certain types of offline attacks.  Note that this check is
  26  * needed even when opening an *unencrypted* file, since it's forbidden to have
  27  * an unencrypted file in an encrypted directory.
  28  *
  29  * Return: 0 on success, -ENOKEY if the key is missing, or another -errno code
  30  */
  31 int fscrypt_file_open(struct inode *inode, struct file *filp)
  32 {
  33         int err;
  34         struct dentry *dir;
  35
  36         err = fscrypt_require_key(inode);
  37         if (err)
  38                 return err;
  39
  40         dir = dget_parent(file_dentry(filp));
  41         if (IS_ENCRYPTED(d_inode(dir)) &&
  42             !fscrypt_has_permitted_context(d_inode(dir), inode)) {
  43                 fscrypt_warn(inode->i_sb,
  44                              "inconsistent encryption contexts: %lu/%lu",
  45                              d_inode(dir)->i_ino, inode->i_ino);
  46                 err = -EPERM;
  47         }
  48         dput(dir);
  49         return err;
  50 }
  51 EXPORT_SYMBOL_GPL(fscrypt_file_open);
  52
  53 int __fscrypt_prepare_link(struct inode *inode, struct inode *dir,
  54                            struct dentry *dentry)
  55 {
  56         int err;
  57
  58         err = fscrypt_require_key(dir);
  59         if (err)
  60                 return err;
  61
  62         /* ... in case we looked up ciphertext name before key was added */
  63         if (dentry->d_flags & DCACHE_ENCRYPTED_NAME)
  64                 return -ENOKEY;
  65
  66         if (!fscrypt_has_permitted_context(dir, inode))
  67                 return -EXDEV;
  68
  69         return 0;
  70 }
  71 EXPORT_SYMBOL_GPL(__fscrypt_prepare_link);
  72
  73 int __fscrypt_prepare_rename(struct inode *old_dir, struct dentry *old_dentry,
  74                              struct inode *new_dir, struct dentry *new_dentry,
  75                              unsigned int flags)
  76 {
  77         int err;
  78
  79         err = fscrypt_require_key(old_dir);
  80         if (err)
  81                 return err;
  82
  83         err = fscrypt_require_key(new_dir);
  84         if (err)
  85                 return err;
  86
  87         /* ... in case we looked up ciphertext name(s) before key was added */
  88         if ((old_dentry->d_flags | new_dentry->d_flags) &
  89             DCACHE_ENCRYPTED_NAME)
  90                 return -ENOKEY;
  91
  92         if (old_dir != new_dir) {
  93                 if (IS_ENCRYPTED(new_dir) &&
  94                     !fscrypt_has_permitted_context(new_dir,
  95                                                    d_inode(old_dentry)))
  96                         return -EXDEV;
  97
  98                 if ((flags & RENAME_EXCHANGE) &&
  99                     IS_ENCRYPTED(old_dir) &&
 100                     !fscrypt_has_permitted_context(old_dir,
 101                                                    d_inode(new_dentry)))
 102                         return -EXDEV;
 103         }
 104         return 0;
 105 }
 106 EXPORT_SYMBOL_GPL(__fscrypt_prepare_rename);
 107
 108 int __fscrypt_prepare_lookup(struct inode *dir, struct dentry *dentry,
 109                              struct fscrypt_name *fname)
 110 {
 111         int err = fscrypt_setup_filename(dir, &dentry->d_name, 1, fname);
 112
 113         if (err && err != -ENOENT)
 114                 return err;
 115
 116         if (fname->is_ciphertext_name) {
 117                 spin_lock(&dentry->d_lock);
 118                 dentry->d_flags |= DCACHE_ENCRYPTED_NAME;
 119                 spin_unlock(&dentry->d_lock);
 120                 d_set_d_op(dentry, &fscrypt_d_ops);
 121         }
 122         return err;
 123 }
 124 EXPORT_SYMBOL_GPL(__fscrypt_prepare_lookup);
 125
 126 int __fscrypt_prepare_symlink(struct inode *dir, unsigned int len,
 127                               unsigned int max_len,
 128                               struct fscrypt_str *disk_link)
 129 {
 130         int err;
 131
 132         /*
 133          * To calculate the size of the encrypted symlink target we need to know
 134          * the amount of NUL padding, which is determined by the flags set in
 135          * the encryption policy which will be inherited from the directory.
 136          * The easiest way to get access to this is to just load the directory's
 137          * fscrypt_info, since we'll need it to create the dir_entry anyway.
 138          *
 139          * Note: in test_dummy_encryption mode, @dir may be unencrypted.
 140          */
 141         err = fscrypt_get_encryption_info(dir);
 142         if (err)
 143                 return err;
 144         if (!fscrypt_has_encryption_key(dir))
 145                 return -ENOKEY;
 146
 147         /*
 148          * Calculate the size of the encrypted symlink and verify it won't
 149          * exceed max_len.  Note that for historical reasons, encrypted symlink
 150          * targets are prefixed with the ciphertext length, despite this
 151          * actually being redundant with i_size.  This decreases by 2 bytes the
 152          * longest symlink target we can accept.
 153          *
 154          * We could recover 1 byte by not counting a null terminator, but
 155          * counting it (even though it is meaningless for ciphertext) is simpler
 156          * for now since filesystems will assume it is there and subtract it.
 157          */
 158         if (!fscrypt_fname_encrypted_size(dir, len,
 159                                           max_len - sizeof(struct fscrypt_symlink_data),
 160                                           &disk_link->len))
 161                 return -ENAMETOOLONG;
 162         disk_link->len += sizeof(struct fscrypt_symlink_data);
 163
 164         disk_link->name = NULL;
 165         return 0;
 166 }
 167 EXPORT_SYMBOL_GPL(__fscrypt_prepare_symlink);
 168
 169 int __fscrypt_encrypt_symlink(struct inode *inode, const char *target,
 170                               unsigned int len, struct fscrypt_str *disk_link)
 171 {
 172         int err;
 173         struct qstr iname = QSTR_INIT(target, len);
 174         struct fscrypt_symlink_data *sd;
 175         unsigned int ciphertext_len;
 176
 177         err = fscrypt_require_key(inode);
 178         if (err)
 179                 return err;
 180
 181         if (disk_link->name) {
 182                 /* filesystem-provided buffer */
 183                 sd = (struct fscrypt_symlink_data *)disk_link->name;
 184         } else {
 185                 sd = kmalloc(disk_link->len, GFP_NOFS);
 186                 if (!sd)
 187                         return -ENOMEM;
 188         }
 189         ciphertext_len = disk_link->len - sizeof(*sd);
 190         sd->len = cpu_to_le16(ciphertext_len);
 191
 192         err = fname_encrypt(inode, &iname, sd->encrypted_path, ciphertext_len);
 193         if (err)
 194                 goto err_free_sd;
 195
 196         /*
 197          * Null-terminating the ciphertext doesn't make sense, but we still
 198          * count the null terminator in the length, so we might as well
 199          * initialize it just in case the filesystem writes it out.
 200          */
 201         sd->encrypted_path[ciphertext_len] = '\0';
 202
 203         /* Cache the plaintext symlink target for later use by get_link() */
 204         err = -ENOMEM;
 205         inode->i_link = kmemdup(target, len + 1, GFP_NOFS);
 206         if (!inode->i_link)
 207                 goto err_free_sd;
 208
 209         if (!disk_link->name)
 210                 disk_link->name = (unsigned char *)sd;
 211         return 0;
 212
 213 err_free_sd:
 214         if (!disk_link->name)
 215                 kfree(sd);
 216         return err;
 217 }
 218 EXPORT_SYMBOL_GPL(__fscrypt_encrypt_symlink);
 219
 220 /**
 221  * fscrypt_get_symlink - get the target of an encrypted symlink
 222  * @inode: the symlink inode
 223  * @caddr: the on-disk contents of the symlink
 224  * @max_size: size of @caddr buffer
 225  * @done: if successful, will be set up to free the returned target if needed
 226  *
 227  * If the symlink's encryption key is available, we decrypt its target.
 228  * Otherwise, we encode its target for presentation.
 229  *
 230  * This may sleep, so the filesystem must have dropped out of RCU mode already.
 231  *
 232  * Return: the presentable symlink target or an ERR_PTR()
 233  */
 234 const char *fscrypt_get_symlink(struct inode *inode, const void *caddr,
 235                                 unsigned int max_size,
 236                                 struct delayed_call *done)
 237 {
 238         const struct fscrypt_symlink_data *sd;
 239         struct fscrypt_str cstr, pstr;
 240         bool has_key;
 241         int err;
 242
 243         /* This is for encrypted symlinks only */
 244         if (WARN_ON(!IS_ENCRYPTED(inode)))
 245                 return ERR_PTR(-EINVAL);
 246
 247         /* If the decrypted target is already cached, just return it. */
 248         pstr.name = READ_ONCE(inode->i_link);
 249         if (pstr.name)
 250                 return pstr.name;
 251
 252         /*
 253          * Try to set up the symlink's encryption key, but we can continue
 254          * regardless of whether the key is available or not.
 255          */
 256         err = fscrypt_get_encryption_info(inode);
 257         if (err)
 258                 return ERR_PTR(err);
 259         has_key = fscrypt_has_encryption_key(inode);
 260
 261         /*
 262          * For historical reasons, encrypted symlink targets are prefixed with
 263          * the ciphertext length, even though this is redundant with i_size.
 264          */
 265
 266         if (max_size < sizeof(*sd))
 267                 return ERR_PTR(-EUCLEAN);
 268         sd = caddr;
 269         cstr.name = (unsigned char *)sd->encrypted_path;
 270         cstr.len = le16_to_cpu(sd->len);
 271
 272         if (cstr.len == 0)
 273                 return ERR_PTR(-EUCLEAN);
 274
 275         if (cstr.len + sizeof(*sd) - 1 > max_size)
 276                 return ERR_PTR(-EUCLEAN);
 277
 278         err = fscrypt_fname_alloc_buffer(inode, cstr.len, &pstr);
 279         if (err)
 280                 return ERR_PTR(err);
 281
 282         err = fscrypt_fname_disk_to_usr(inode, 0, 0, &cstr, &pstr);
 283         if (err)
 284                 goto err_kfree;
 285
 286         err = -EUCLEAN;
 287         if (pstr.name[0] == '\0')
 288                 goto err_kfree;
 289
 290         pstr.name[pstr.len] = '\0';
 291
 292         /*
 293          * Cache decrypted symlink targets in i_link for later use.  Don't cache
 294          * symlink targets encoded without the key, since those become outdated
 295          * once the key is added.  This pairs with the READ_ONCE() above and in
 296          * the VFS path lookup code.
 297          */
 298         if (!has_key ||
 299             cmpxchg_release(&inode->i_link, NULL, pstr.name) != NULL)
 300                 set_delayed_call(done, kfree_link, pstr.name);
 301
 302         return pstr.name;
 303
 304 err_kfree:
 305         kfree(pstr.name);
 306         return ERR_PTR(err);
 307 }
 308 EXPORT_SYMBOL_GPL(fscrypt_get_symlink);