security/keys/big_key.c

   1 /* Large capacity key type
   2  *
   3  * Copyright (C) 2017 Jason A. Donenfeld <Jason@zx2c4.com>. All Rights Reserved.
   4  * Copyright (C) 2013 Red Hat, Inc. All Rights Reserved.
   5  * Written by David Howells (dhowells@redhat.com)
   6  *
   7  * This program is free software; you can redistribute it and/or
   8  * modify it under the terms of the GNU General Public Licence
   9  * as published by the Free Software Foundation; either version
  10  * 2 of the Licence, or (at your option) any later version.
  11  */
  12
  13 #define pr_fmt(fmt) "big_key: "fmt
  14 #include <linux/init.h>
  15 #include <linux/seq_file.h>
  16 #include <linux/file.h>
  17 #include <linux/shmem_fs.h>
  18 #include <linux/err.h>
  19 #include <linux/scatterlist.h>
  20 #include <linux/random.h>
  21 #include <keys/user-type.h>
  22 #include <keys/big_key-type.h>
  23 #include <crypto/aead.h>
  24
  25 struct big_key_buf {
  26         unsigned int            nr_pages;
  27         void                    *virt;
  28         struct scatterlist      *sg;
  29         struct page             *pages[];
  30 };
  31
  32 /*
  33  * Layout of key payload words.
  34  */
  35 enum {
  36         big_key_data,
  37         big_key_path,
  38         big_key_path_2nd_part,
  39         big_key_len,
  40 };
  41
  42 /*
  43  * Crypto operation with big_key data
  44  */
  45 enum big_key_op {
  46         BIG_KEY_ENC,
  47         BIG_KEY_DEC,
  48 };
  49
  50 /*
  51  * If the data is under this limit, there's no point creating a shm file to
  52  * hold it as the permanently resident metadata for the shmem fs will be at
  53  * least as large as the data.
  54  */
  55 #define BIG_KEY_FILE_THRESHOLD (sizeof(struct inode) + sizeof(struct dentry))
  56
  57 /*
  58  * Key size for big_key data encryption
  59  */
  60 #define ENC_KEY_SIZE 32
  61
  62 /*
  63  * Authentication tag length
  64  */
  65 #define ENC_AUTHTAG_SIZE 16
  66
  67 /*
  68  * big_key defined keys take an arbitrary string as the description and an
  69  * arbitrary blob of data as the payload
  70  */
  71 struct key_type key_type_big_key = {
  72         .name                   = "big_key",
  73         .preparse               = big_key_preparse,
  74         .free_preparse          = big_key_free_preparse,
  75         .instantiate            = generic_key_instantiate,
  76         .revoke                 = big_key_revoke,
  77         .destroy                = big_key_destroy,
  78         .describe               = big_key_describe,
  79         .read                   = big_key_read,
  80         /* no ->update(); don't add it without changing big_key_crypt() nonce */
  81 };
  82
  83 /*
  84  * Crypto names for big_key data authenticated encryption
  85  */
  86 static const char big_key_alg_name[] = "gcm(aes)";
  87
  88 /*
  89  * Crypto algorithms for big_key data authenticated encryption
  90  */
  91 static struct crypto_aead *big_key_aead;
  92
  93 /*
  94  * Since changing the key affects the entire object, we need a mutex.
  95  */
  96 static DEFINE_MUTEX(big_key_aead_lock);
  97
  98 /*
  99  * Encrypt/decrypt big_key data
 100  */
 101 static int big_key_crypt(enum big_key_op op, struct big_key_buf *buf, size_t datalen, u8 *key)
 102 {
 103         int ret;
 104         struct aead_request *aead_req;
 105         /* We always use a zero nonce. The reason we can get away with this is
 106          * because we're using a different randomly generated key for every
 107          * different encryption. Notably, too, key_type_big_key doesn't define
 108          * an .update function, so there's no chance we'll wind up reusing the
 109          * key to encrypt updated data. Simply put: one key, one encryption.
 110          */
 111         u8 zero_nonce[crypto_aead_ivsize(big_key_aead)];
 112
 113         aead_req = aead_request_alloc(big_key_aead, GFP_KERNEL);
 114         if (!aead_req)
 115                 return -ENOMEM;
 116
 117         memset(zero_nonce, 0, sizeof(zero_nonce));
 118         aead_request_set_crypt(aead_req, buf->sg, buf->sg, datalen, zero_nonce);
 119         aead_request_set_callback(aead_req, CRYPTO_TFM_REQ_MAY_SLEEP, NULL, NULL);
 120         aead_request_set_ad(aead_req, 0);
 121
 122         mutex_lock(&big_key_aead_lock);
 123         if (crypto_aead_setkey(big_key_aead, key, ENC_KEY_SIZE)) {
 124                 ret = -EAGAIN;
 125                 goto error;
 126         }
 127         if (op == BIG_KEY_ENC)
 128                 ret = crypto_aead_encrypt(aead_req);
 129         else
 130                 ret = crypto_aead_decrypt(aead_req);
 131 error:
 132         mutex_unlock(&big_key_aead_lock);
 133         aead_request_free(aead_req);
 134         return ret;
 135 }
 136
 137 /*
 138  * Free up the buffer.
 139  */
 140 static void big_key_free_buffer(struct big_key_buf *buf)
 141 {
 142         unsigned int i;
 143
 144         if (buf->virt) {
 145                 memset(buf->virt, 0, buf->nr_pages * PAGE_SIZE);
 146                 vunmap(buf->virt);
 147         }
 148
 149         for (i = 0; i < buf->nr_pages; i++)
 150                 if (buf->pages[i])
 151                         __free_page(buf->pages[i]);
 152
 153         kfree(buf);
 154 }
 155
 156 /*
 157  * Allocate a buffer consisting of a set of pages with a virtual mapping
 158  * applied over them.
 159  */
 160 static void *big_key_alloc_buffer(size_t len)
 161 {
 162         struct big_key_buf *buf;
 163         unsigned int npg = (len + PAGE_SIZE - 1) >> PAGE_SHIFT;
 164         unsigned int i, l;
 165
 166         buf = kzalloc(sizeof(struct big_key_buf) +
 167                       sizeof(struct page) * npg +
 168                       sizeof(struct scatterlist) * npg,
 169                       GFP_KERNEL);
 170         if (!buf)
 171                 return NULL;
 172
 173         buf->nr_pages = npg;
 174         buf->sg = (void *)(buf->pages + npg);
 175         sg_init_table(buf->sg, npg);
 176
 177         for (i = 0; i < buf->nr_pages; i++) {
 178                 buf->pages[i] = alloc_page(GFP_KERNEL);
 179                 if (!buf->pages[i])
 180                         goto nomem;
 181
 182                 l = min_t(size_t, len, PAGE_SIZE);
 183                 sg_set_page(&buf->sg[i], buf->pages[i], l, 0);
 184                 len -= l;
 185         }
 186
 187         buf->virt = vmap(buf->pages, buf->nr_pages, VM_MAP, PAGE_KERNEL);
 188         if (!buf->virt)
 189                 goto nomem;
 190
 191         return buf;
 192
 193 nomem:
 194         big_key_free_buffer(buf);
 195         return NULL;
 196 }
 197
 198 /*
 199  * Preparse a big key
 200  */
 201 int big_key_preparse(struct key_preparsed_payload *prep)
 202 {
 203         struct big_key_buf *buf;
 204         struct path *path = (struct path *)&prep->payload.data[big_key_path];
 205         struct file *file;
 206         u8 *enckey;
 207         ssize_t written;
 208         size_t datalen = prep->datalen, enclen = datalen + ENC_AUTHTAG_SIZE;
 209         int ret;
 210
 211         if (datalen <= 0 || datalen > 1024 * 1024 || !prep->data)
 212                 return -EINVAL;
 213
 214         /* Set an arbitrary quota */
 215         prep->quotalen = 16;
 216
 217         prep->payload.data[big_key_len] = (void *)(unsigned long)datalen;
 218
 219         if (datalen > BIG_KEY_FILE_THRESHOLD) {
 220                 /* Create a shmem file to store the data in.  This will permit the data
 221                  * to be swapped out if needed.
 222                  *
 223                  * File content is stored encrypted with randomly generated key.
 224                  */
 225                 loff_t pos = 0;
 226
 227                 buf = big_key_alloc_buffer(enclen);
 228                 if (!buf)
 229                         return -ENOMEM;
 230                 memcpy(buf->virt, prep->data, datalen);
 231
 232                 /* generate random key */
 233                 enckey = kmalloc(ENC_KEY_SIZE, GFP_KERNEL);
 234                 if (!enckey) {
 235                         ret = -ENOMEM;
 236                         goto error;
 237                 }
 238                 ret = get_random_bytes_wait(enckey, ENC_KEY_SIZE);
 239                 if (unlikely(ret))
 240                         goto err_enckey;
 241
 242                 /* encrypt aligned data */
 243                 ret = big_key_crypt(BIG_KEY_ENC, buf, datalen, enckey);
 244                 if (ret)
 245                         goto err_enckey;
 246
 247                 /* save aligned data to file */
 248                 file = shmem_kernel_file_setup("", enclen, 0);
 249                 if (IS_ERR(file)) {
 250                         ret = PTR_ERR(file);
 251                         goto err_enckey;
 252                 }
 253
 254                 written = kernel_write(file, buf->virt, enclen, &pos);
 255                 if (written != enclen) {
 256                         ret = written;
 257                         if (written >= 0)
 258                                 ret = -ENOMEM;
 259                         goto err_fput;
 260                 }
 261
 262                 /* Pin the mount and dentry to the key so that we can open it again
 263                  * later
 264                  */
 265                 prep->payload.data[big_key_data] = enckey;
 266                 *path = file->f_path;
 267                 path_get(path);
 268                 fput(file);
 269                 big_key_free_buffer(buf);
 270         } else {
 271                 /* Just store the data in a buffer */
 272                 void *data = kmalloc(datalen, GFP_KERNEL);
 273
 274                 if (!data)
 275                         return -ENOMEM;
 276
 277                 prep->payload.data[big_key_data] = data;
 278                 memcpy(data, prep->data, prep->datalen);
 279         }
 280         return 0;
 281
 282 err_fput:
 283         fput(file);
 284 err_enckey:
 285         kzfree(enckey);
 286 error:
 287         big_key_free_buffer(buf);
 288         return ret;
 289 }
 290
 291 /*
 292  * Clear preparsement.
 293  */
 294 void big_key_free_preparse(struct key_preparsed_payload *prep)
 295 {
 296         if (prep->datalen > BIG_KEY_FILE_THRESHOLD) {
 297                 struct path *path = (struct path *)&prep->payload.data[big_key_path];
 298
 299                 path_put(path);
 300         }
 301         kzfree(prep->payload.data[big_key_data]);
 302 }
 303
 304 /*
 305  * dispose of the links from a revoked keyring
 306  * - called with the key sem write-locked
 307  */
 308 void big_key_revoke(struct key *key)
 309 {
 310         struct path *path = (struct path *)&key->payload.data[big_key_path];
 311
 312         /* clear the quota */
 313         key_payload_reserve(key, 0);
 314         if (key_is_positive(key) &&
 315             (size_t)key->payload.data[big_key_len] > BIG_KEY_FILE_THRESHOLD)
 316                 vfs_truncate(path, 0);
 317 }
 318
 319 /*
 320  * dispose of the data dangling from the corpse of a big_key key
 321  */
 322 void big_key_destroy(struct key *key)
 323 {
 324         size_t datalen = (size_t)key->payload.data[big_key_len];
 325
 326         if (datalen > BIG_KEY_FILE_THRESHOLD) {
 327                 struct path *path = (struct path *)&key->payload.data[big_key_path];
 328
 329                 path_put(path);
 330                 path->mnt = NULL;
 331                 path->dentry = NULL;
 332         }
 333         kzfree(key->payload.data[big_key_data]);
 334         key->payload.data[big_key_data] = NULL;
 335 }
 336
 337 /*
 338  * describe the big_key key
 339  */
 340 void big_key_describe(const struct key *key, struct seq_file *m)
 341 {
 342         size_t datalen = (size_t)key->payload.data[big_key_len];
 343
 344         seq_puts(m, key->description);
 345
 346         if (key_is_positive(key))
 347                 seq_printf(m, ": %zu [%s]",
 348                            datalen,
 349                            datalen > BIG_KEY_FILE_THRESHOLD ? "file" : "buff");
 350 }
 351
 352 /*
 353  * read the key data
 354  * - the key's semaphore is read-locked
 355  */
 356 long big_key_read(const struct key *key, char __user *buffer, size_t buflen)
 357 {
 358         size_t datalen = (size_t)key->payload.data[big_key_len];
 359         long ret;
 360
 361         if (!buffer || buflen < datalen)
 362                 return datalen;
 363
 364         if (datalen > BIG_KEY_FILE_THRESHOLD) {
 365                 struct big_key_buf *buf;
 366                 struct path *path = (struct path *)&key->payload.data[big_key_path];
 367                 struct file *file;
 368                 u8 *enckey = (u8 *)key->payload.data[big_key_data];
 369                 size_t enclen = datalen + ENC_AUTHTAG_SIZE;
 370                 loff_t pos = 0;
 371
 372                 buf = big_key_alloc_buffer(enclen);
 373                 if (!buf)
 374                         return -ENOMEM;
 375
 376                 file = dentry_open(path, O_RDONLY, current_cred());
 377                 if (IS_ERR(file)) {
 378                         ret = PTR_ERR(file);
 379                         goto error;
 380                 }
 381
 382                 /* read file to kernel and decrypt */
 383                 ret = kernel_read(file, buf->virt, enclen, &pos);
 384                 if (ret >= 0 && ret != enclen) {
 385                         ret = -EIO;
 386                         goto err_fput;
 387                 }
 388
 389                 ret = big_key_crypt(BIG_KEY_DEC, buf, enclen, enckey);
 390                 if (ret)
 391                         goto err_fput;
 392
 393                 ret = datalen;
 394
 395                 /* copy decrypted data to user */
 396                 if (copy_to_user(buffer, buf->virt, datalen) != 0)
 397                         ret = -EFAULT;
 398
 399 err_fput:
 400                 fput(file);
 401 error:
 402                 big_key_free_buffer(buf);
 403         } else {
 404                 ret = datalen;
 405                 if (copy_to_user(buffer, key->payload.data[big_key_data],
 406                                  datalen) != 0)
 407                         ret = -EFAULT;
 408         }
 409
 410         return ret;
 411 }
 412
 413 /*
 414  * Register key type
 415  */
 416 static int __init big_key_init(void)
 417 {
 418         int ret;
 419
 420         /* init block cipher */
 421         big_key_aead = crypto_alloc_aead(big_key_alg_name, 0, CRYPTO_ALG_ASYNC);
 422         if (IS_ERR(big_key_aead)) {
 423                 ret = PTR_ERR(big_key_aead);
 424                 pr_err("Can't alloc crypto: %d\n", ret);
 425                 return ret;
 426         }
 427         ret = crypto_aead_setauthsize(big_key_aead, ENC_AUTHTAG_SIZE);
 428         if (ret < 0) {
 429                 pr_err("Can't set crypto auth tag len: %d\n", ret);
 430                 goto free_aead;
 431         }
 432
 433         ret = register_key_type(&key_type_big_key);
 434         if (ret < 0) {
 435                 pr_err("Can't register type: %d\n", ret);
 436                 goto free_aead;
 437         }
 438
 439         return 0;
 440
 441 free_aead:
 442         crypto_free_aead(big_key_aead);
 443         return ret;
 444 }
 445
 446 late_initcall(big_key_init);