crypto/asymmetric_keys/verify_pefile.c

   1 /* Parse a signed PE binary
   2  *
   3  * Copyright (C) 2014 Red Hat, Inc. All Rights Reserved.
   4  * Written by David Howells (dhowells@redhat.com)
   5  *
   6  * This program is free software; you can redistribute it and/or
   7  * modify it under the terms of the GNU General Public Licence
   8  * as published by the Free Software Foundation; either version
   9  * 2 of the Licence, or (at your option) any later version.
  10  */
  11
  12 #define pr_fmt(fmt) "PEFILE: "fmt
  13 #include <linux/module.h>
  14 #include <linux/kernel.h>
  15 #include <linux/slab.h>
  16 #include <linux/err.h>
  17 #include <linux/pe.h>
  18 #include <linux/asn1.h>
  19 #include <linux/verification.h>
  20 #include <crypto/hash.h>
  21 #include "verify_pefile.h"
  22
  23 /*
  24  * Parse a PE binary.
  25  */
  26 static int pefile_parse_binary(const void *pebuf, unsigned int pelen,
  27                                struct pefile_context *ctx)
  28 {
  29         const struct mz_hdr *mz = pebuf;
  30         const struct pe_hdr *pe;
  31         const struct pe32_opt_hdr *pe32;
  32         const struct pe32plus_opt_hdr *pe64;
  33         const struct data_directory *ddir;
  34         const struct data_dirent *dde;
  35         const struct section_header *secs, *sec;
  36         size_t cursor, datalen = pelen;
  37
  38         kenter("");
  39
  40 #define chkaddr(base, x, s)                                             \
  41         do {                                                            \
  42                 if ((x) < base || (s) >= datalen || (x) > datalen - (s)) \
  43                         return -ELIBBAD;                                \
  44         } while (0)
  45
  46         chkaddr(0, 0, sizeof(*mz));
  47         if (mz->magic != MZ_MAGIC)
  48                 return -ELIBBAD;
  49         cursor = sizeof(*mz);
  50
  51         chkaddr(cursor, mz->peaddr, sizeof(*pe));
  52         pe = pebuf + mz->peaddr;
  53         if (pe->magic != PE_MAGIC)
  54                 return -ELIBBAD;
  55         cursor = mz->peaddr + sizeof(*pe);
  56
  57         chkaddr(0, cursor, sizeof(pe32->magic));
  58         pe32 = pebuf + cursor;
  59         pe64 = pebuf + cursor;
  60
  61         switch (pe32->magic) {
  62         case PE_OPT_MAGIC_PE32:
  63                 chkaddr(0, cursor, sizeof(*pe32));
  64                 ctx->image_checksum_offset =
  65                         (unsigned long)&pe32->csum - (unsigned long)pebuf;
  66                 ctx->header_size = pe32->header_size;
  67                 cursor += sizeof(*pe32);
  68                 ctx->n_data_dirents = pe32->data_dirs;
  69                 break;
  70
  71         case PE_OPT_MAGIC_PE32PLUS:
  72                 chkaddr(0, cursor, sizeof(*pe64));
  73                 ctx->image_checksum_offset =
  74                         (unsigned long)&pe64->csum - (unsigned long)pebuf;
  75                 ctx->header_size = pe64->header_size;
  76                 cursor += sizeof(*pe64);
  77                 ctx->n_data_dirents = pe64->data_dirs;
  78                 break;
  79
  80         default:
  81                 pr_debug("Unknown PEOPT magic = %04hx\n", pe32->magic);
  82                 return -ELIBBAD;
  83         }
  84
  85         pr_debug("checksum @ %x\n", ctx->image_checksum_offset);
  86         pr_debug("header size = %x\n", ctx->header_size);
  87
  88         if (cursor >= ctx->header_size || ctx->header_size >= datalen)
  89                 return -ELIBBAD;
  90
  91         if (ctx->n_data_dirents > (ctx->header_size - cursor) / sizeof(*dde))
  92                 return -ELIBBAD;
  93
  94         ddir = pebuf + cursor;
  95         cursor += sizeof(*dde) * ctx->n_data_dirents;
  96
  97         ctx->cert_dirent_offset =
  98                 (unsigned long)&ddir->certs - (unsigned long)pebuf;
  99         ctx->certs_size = ddir->certs.size;
 100
 101         if (!ddir->certs.virtual_address || !ddir->certs.size) {
 102                 pr_debug("Unsigned PE binary\n");
 103                 return -EKEYREJECTED;
 104         }
 105
 106         chkaddr(ctx->header_size, ddir->certs.virtual_address,
 107                 ddir->certs.size);
 108         ctx->sig_offset = ddir->certs.virtual_address;
 109         ctx->sig_len = ddir->certs.size;
 110         pr_debug("cert = %x @%x [%*ph]\n",
 111                  ctx->sig_len, ctx->sig_offset,
 112                  ctx->sig_len, pebuf + ctx->sig_offset);
 113
 114         ctx->n_sections = pe->sections;
 115         if (ctx->n_sections > (ctx->header_size - cursor) / sizeof(*sec))
 116                 return -ELIBBAD;
 117         ctx->secs = secs = pebuf + cursor;
 118
 119         return 0;
 120 }
 121
 122 /*
 123  * Check and strip the PE wrapper from around the signature and check that the
 124  * remnant looks something like PKCS#7.
 125  */
 126 static int pefile_strip_sig_wrapper(const void *pebuf,
 127                                     struct pefile_context *ctx)
 128 {
 129         struct win_certificate wrapper;
 130         const u8 *pkcs7;
 131         unsigned len;
 132
 133         if (ctx->sig_len < sizeof(wrapper)) {
 134                 pr_debug("Signature wrapper too short\n");
 135                 return -ELIBBAD;
 136         }
 137
 138         memcpy(&wrapper, pebuf + ctx->sig_offset, sizeof(wrapper));
 139         pr_debug("sig wrapper = { %x, %x, %x }\n",
 140                  wrapper.length, wrapper.revision, wrapper.cert_type);
 141
 142         /* Both pesign and sbsign round up the length of certificate table
 143          * (in optional header data directories) to 8 byte alignment.
 144          */
 145         if (round_up(wrapper.length, 8) != ctx->sig_len) {
 146                 pr_debug("Signature wrapper len wrong\n");
 147                 return -ELIBBAD;
 148         }
 149         if (wrapper.revision != WIN_CERT_REVISION_2_0) {
 150                 pr_debug("Signature is not revision 2.0\n");
 151                 return -ENOTSUPP;
 152         }
 153         if (wrapper.cert_type != WIN_CERT_TYPE_PKCS_SIGNED_DATA) {
 154                 pr_debug("Signature certificate type is not PKCS\n");
 155                 return -ENOTSUPP;
 156         }
 157
 158         /* It looks like the pkcs signature length in wrapper->length and the
 159          * size obtained from the data dir entries, which lists the total size
 160          * of certificate table, are both aligned to an octaword boundary, so
 161          * we may have to deal with some padding.
 162          */
 163         ctx->sig_len = wrapper.length;
 164         ctx->sig_offset += sizeof(wrapper);
 165         ctx->sig_len -= sizeof(wrapper);
 166         if (ctx->sig_len < 4) {
 167                 pr_debug("Signature data missing\n");
 168                 return -EKEYREJECTED;
 169         }
 170
 171         /* What's left should be a PKCS#7 cert */
 172         pkcs7 = pebuf + ctx->sig_offset;
 173         if (pkcs7[0] != (ASN1_CONS_BIT | ASN1_SEQ))
 174                 goto not_pkcs7;
 175
 176         switch (pkcs7[1]) {
 177         case 0 ... 0x7f:
 178                 len = pkcs7[1] + 2;
 179                 goto check_len;
 180         case ASN1_INDEFINITE_LENGTH:
 181                 return 0;
 182         case 0x81:
 183                 len = pkcs7[2] + 3;
 184                 goto check_len;
 185         case 0x82:
 186                 len = ((pkcs7[2] << 8) | pkcs7[3]) + 4;
 187                 goto check_len;
 188         case 0x83 ... 0xff:
 189                 return -EMSGSIZE;
 190         default:
 191                 goto not_pkcs7;
 192         }
 193
 194 check_len:
 195         if (len <= ctx->sig_len) {
 196                 /* There may be padding */
 197                 ctx->sig_len = len;
 198                 return 0;
 199         }
 200 not_pkcs7:
 201         pr_debug("Signature data not PKCS#7\n");
 202         return -ELIBBAD;
 203 }
 204
 205 /*
 206  * Compare two sections for canonicalisation.
 207  */
 208 static int pefile_compare_shdrs(const void *a, const void *b)
 209 {
 210         const struct section_header *shdra = a;
 211         const struct section_header *shdrb = b;
 212         int rc;
 213
 214         if (shdra->data_addr > shdrb->data_addr)
 215                 return 1;
 216         if (shdrb->data_addr > shdra->data_addr)
 217                 return -1;
 218
 219         if (shdra->virtual_address > shdrb->virtual_address)
 220                 return 1;
 221         if (shdrb->virtual_address > shdra->virtual_address)
 222                 return -1;
 223
 224         rc = strcmp(shdra->name, shdrb->name);
 225         if (rc != 0)
 226                 return rc;
 227
 228         if (shdra->virtual_size > shdrb->virtual_size)
 229                 return 1;
 230         if (shdrb->virtual_size > shdra->virtual_size)
 231                 return -1;
 232
 233         if (shdra->raw_data_size > shdrb->raw_data_size)
 234                 return 1;
 235         if (shdrb->raw_data_size > shdra->raw_data_size)
 236                 return -1;
 237
 238         return 0;
 239 }
 240
 241 /*
 242  * Load the contents of the PE binary into the digest, leaving out the image
 243  * checksum and the certificate data block.
 244  */
 245 static int pefile_digest_pe_contents(const void *pebuf, unsigned int pelen,
 246                                      struct pefile_context *ctx,
 247                                      struct shash_desc *desc)
 248 {
 249         unsigned *canon, tmp, loop, i, hashed_bytes;
 250         int ret;
 251
 252         /* Digest the header and data directory, but leave out the image
 253          * checksum and the data dirent for the signature.
 254          */
 255         ret = crypto_shash_update(desc, pebuf, ctx->image_checksum_offset);
 256         if (ret < 0)
 257                 return ret;
 258
 259         tmp = ctx->image_checksum_offset + sizeof(uint32_t);
 260         ret = crypto_shash_update(desc, pebuf + tmp,
 261                                   ctx->cert_dirent_offset - tmp);
 262         if (ret < 0)
 263                 return ret;
 264
 265         tmp = ctx->cert_dirent_offset + sizeof(struct data_dirent);
 266         ret = crypto_shash_update(desc, pebuf + tmp, ctx->header_size - tmp);
 267         if (ret < 0)
 268                 return ret;
 269
 270         canon = kcalloc(ctx->n_sections, sizeof(unsigned), GFP_KERNEL);
 271         if (!canon)
 272                 return -ENOMEM;
 273
 274         /* We have to canonicalise the section table, so we perform an
 275          * insertion sort.
 276          */
 277         canon[0] = 0;
 278         for (loop = 1; loop < ctx->n_sections; loop++) {
 279                 for (i = 0; i < loop; i++) {
 280                         if (pefile_compare_shdrs(&ctx->secs[canon[i]],
 281                                                  &ctx->secs[loop]) > 0) {
 282                                 memmove(&canon[i + 1], &canon[i],
 283                                         (loop - i) * sizeof(canon[0]));
 284                                 break;
 285                         }
 286                 }
 287                 canon[i] = loop;
 288         }
 289
 290         hashed_bytes = ctx->header_size;
 291         for (loop = 0; loop < ctx->n_sections; loop++) {
 292                 i = canon[loop];
 293                 if (ctx->secs[i].raw_data_size == 0)
 294                         continue;
 295                 ret = crypto_shash_update(desc,
 296                                           pebuf + ctx->secs[i].data_addr,
 297                                           ctx->secs[i].raw_data_size);
 298                 if (ret < 0) {
 299                         kfree(canon);
 300                         return ret;
 301                 }
 302                 hashed_bytes += ctx->secs[i].raw_data_size;
 303         }
 304         kfree(canon);
 305
 306         if (pelen > hashed_bytes) {
 307                 tmp = hashed_bytes + ctx->certs_size;
 308                 ret = crypto_shash_update(desc,
 309                                           pebuf + hashed_bytes,
 310                                           pelen - tmp);
 311                 if (ret < 0)
 312                         return ret;
 313         }
 314
 315         return 0;
 316 }
 317
 318 /*
 319  * Digest the contents of the PE binary, leaving out the image checksum and the
 320  * certificate data block.
 321  */
 322 static int pefile_digest_pe(const void *pebuf, unsigned int pelen,
 323                             struct pefile_context *ctx)
 324 {
 325         struct crypto_shash *tfm;
 326         struct shash_desc *desc;
 327         size_t digest_size, desc_size;
 328         void *digest;
 329         int ret;
 330
 331         kenter(",%s", ctx->digest_algo);
 332
 333         /* Allocate the hashing algorithm we're going to need and find out how
 334          * big the hash operational data will be.
 335          */
 336         tfm = crypto_alloc_shash(ctx->digest_algo, 0, 0);
 337         if (IS_ERR(tfm))
 338                 return (PTR_ERR(tfm) == -ENOENT) ? -ENOPKG : PTR_ERR(tfm);
 339
 340         desc_size = crypto_shash_descsize(tfm) + sizeof(*desc);
 341         digest_size = crypto_shash_digestsize(tfm);
 342
 343         if (digest_size != ctx->digest_len) {
 344                 pr_debug("Digest size mismatch (%zx != %x)\n",
 345                          digest_size, ctx->digest_len);
 346                 ret = -EBADMSG;
 347                 goto error_no_desc;
 348         }
 349         pr_debug("Digest: desc=%zu size=%zu\n", desc_size, digest_size);
 350
 351         ret = -ENOMEM;
 352         desc = kzalloc(desc_size + digest_size, GFP_KERNEL);
 353         if (!desc)
 354                 goto error_no_desc;
 355
 356         desc->tfm   = tfm;
 357         desc->flags = CRYPTO_TFM_REQ_MAY_SLEEP;
 358         ret = crypto_shash_init(desc);
 359         if (ret < 0)
 360                 goto error;
 361
 362         ret = pefile_digest_pe_contents(pebuf, pelen, ctx, desc);
 363         if (ret < 0)
 364                 goto error;
 365
 366         digest = (void *)desc + desc_size;
 367         ret = crypto_shash_final(desc, digest);
 368         if (ret < 0)
 369                 goto error;
 370
 371         pr_debug("Digest calc = [%*ph]\n", ctx->digest_len, digest);
 372
 373         /* Check that the PE file digest matches that in the MSCODE part of the
 374          * PKCS#7 certificate.
 375          */
 376         if (memcmp(digest, ctx->digest, ctx->digest_len) != 0) {
 377                 pr_debug("Digest mismatch\n");
 378                 ret = -EKEYREJECTED;
 379         } else {
 380                 pr_debug("The digests match!\n");
 381         }
 382
 383 error:
 384         kzfree(desc);
 385 error_no_desc:
 386         crypto_free_shash(tfm);
 387         kleave(" = %d", ret);
 388         return ret;
 389 }
 390
 391 /**
 392  * verify_pefile_signature - Verify the signature on a PE binary image
 393  * @pebuf: Buffer containing the PE binary image
 394  * @pelen: Length of the binary image
 395  * @trust_keys: Signing certificate(s) to use as starting points
 396  * @usage: The use to which the key is being put.
 397  *
 398  * Validate that the certificate chain inside the PKCS#7 message inside the PE
 399  * binary image intersects keys we already know and trust.
 400  *
 401  * Returns, in order of descending priority:
 402  *
 403  *  (*) -ELIBBAD if the image cannot be parsed, or:
 404  *
 405  *  (*) -EKEYREJECTED if a signature failed to match for which we have a valid
 406  *      key, or:
 407  *
 408  *  (*) 0 if at least one signature chain intersects with the keys in the trust
 409  *      keyring, or:
 410  *
 411  *  (*) -ENOPKG if a suitable crypto module couldn't be found for a check on a
 412  *      chain.
 413  *
 414  *  (*) -ENOKEY if we couldn't find a match for any of the signature chains in
 415  *      the message.
 416  *
 417  * May also return -ENOMEM.
 418  */
 419 int verify_pefile_signature(const void *pebuf, unsigned pelen,
 420                             struct key *trusted_keys,
 421                             enum key_being_used_for usage)
 422 {
 423         struct pefile_context ctx;
 424         int ret;
 425
 426         kenter("");
 427
 428         memset(&ctx, 0, sizeof(ctx));
 429         ret = pefile_parse_binary(pebuf, pelen, &ctx);
 430         if (ret < 0)
 431                 return ret;
 432
 433         ret = pefile_strip_sig_wrapper(pebuf, &ctx);
 434         if (ret < 0)
 435                 return ret;
 436
 437         ret = verify_pkcs7_signature(NULL, 0,
 438                                      pebuf + ctx.sig_offset, ctx.sig_len,
 439                                      trusted_keys, usage,
 440                                      mscode_parse, &ctx);
 441         if (ret < 0)
 442                 goto error;
 443
 444         pr_debug("Digest: %u [%*ph]\n",
 445                  ctx.digest_len, ctx.digest_len, ctx.digest);
 446
 447         /* Generate the digest and check against the PKCS7 certificate
 448          * contents.
 449          */
 450         ret = pefile_digest_pe(pebuf, pelen, &ctx);
 451
 452 error:
 453         kzfree(ctx.digest);
 454         return ret;
 455 }