crypto/asymmetric_keys/verify_pefile.c

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