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 <crypto/pkcs7.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 
132 	if (ctx->sig_len < sizeof(wrapper)) {
133 		pr_debug("Signature wrapper too short\n");
134 		return -ELIBBAD;
135 	}
136 
137 	memcpy(&wrapper, pebuf + ctx->sig_offset, sizeof(wrapper));
138 	pr_debug("sig wrapper = { %x, %x, %x }\n",
139 		 wrapper.length, wrapper.revision, wrapper.cert_type);
140 
141 	/* Both pesign and sbsign round up the length of certificate table
142 	 * (in optional header data directories) to 8 byte alignment.
143 	 */
144 	if (round_up(wrapper.length, 8) != ctx->sig_len) {
145 		pr_debug("Signature wrapper len wrong\n");
146 		return -ELIBBAD;
147 	}
148 	if (wrapper.revision != WIN_CERT_REVISION_2_0) {
149 		pr_debug("Signature is not revision 2.0\n");
150 		return -ENOTSUPP;
151 	}
152 	if (wrapper.cert_type != WIN_CERT_TYPE_PKCS_SIGNED_DATA) {
153 		pr_debug("Signature certificate type is not PKCS\n");
154 		return -ENOTSUPP;
155 	}
156 
157 	/* Looks like actual pkcs signature length is in wrapper->length.
158 	 * size obtained from data dir entries lists the total size of
159 	 * certificate table which is also aligned to octawrod boundary.
160 	 *
161 	 * So set signature length field appropriately.
162 	 */
163 	ctx->sig_len = wrapper.length;
164 	ctx->sig_offset += sizeof(wrapper);
165 	ctx->sig_len -= sizeof(wrapper);
166 	if (ctx->sig_len == 0) {
167 		pr_debug("Signature data missing\n");
168 		return -EKEYREJECTED;
169 	}
170 
171 	/* What's left should a PKCS#7 cert */
172 	pkcs7 = pebuf + ctx->sig_offset;
173 	if (pkcs7[0] == (ASN1_CONS_BIT | ASN1_SEQ)) {
174 		if (pkcs7[1] == 0x82 &&
175 		    pkcs7[2] == (((ctx->sig_len - 4) >> 8) & 0xff) &&
176 		    pkcs7[3] ==  ((ctx->sig_len - 4)       & 0xff))
177 			return 0;
178 		if (pkcs7[1] == 0x80)
179 			return 0;
180 		if (pkcs7[1] > 0x82)
181 			return -EMSGSIZE;
182 	}
183 
184 	pr_debug("Signature data not PKCS#7\n");
185 	return -ELIBBAD;
186 }
187 
188 /*
189  * Compare two sections for canonicalisation.
190  */
191 static int pefile_compare_shdrs(const void *a, const void *b)
192 {
193 	const struct section_header *shdra = a;
194 	const struct section_header *shdrb = b;
195 	int rc;
196 
197 	if (shdra->data_addr > shdrb->data_addr)
198 		return 1;
199 	if (shdrb->data_addr > shdra->data_addr)
200 		return -1;
201 
202 	if (shdra->virtual_address > shdrb->virtual_address)
203 		return 1;
204 	if (shdrb->virtual_address > shdra->virtual_address)
205 		return -1;
206 
207 	rc = strcmp(shdra->name, shdrb->name);
208 	if (rc != 0)
209 		return rc;
210 
211 	if (shdra->virtual_size > shdrb->virtual_size)
212 		return 1;
213 	if (shdrb->virtual_size > shdra->virtual_size)
214 		return -1;
215 
216 	if (shdra->raw_data_size > shdrb->raw_data_size)
217 		return 1;
218 	if (shdrb->raw_data_size > shdra->raw_data_size)
219 		return -1;
220 
221 	return 0;
222 }
223 
224 /*
225  * Load the contents of the PE binary into the digest, leaving out the image
226  * checksum and the certificate data block.
227  */
228 static int pefile_digest_pe_contents(const void *pebuf, unsigned int pelen,
229 				     struct pefile_context *ctx,
230 				     struct shash_desc *desc)
231 {
232 	unsigned *canon, tmp, loop, i, hashed_bytes;
233 	int ret;
234 
235 	/* Digest the header and data directory, but leave out the image
236 	 * checksum and the data dirent for the signature.
237 	 */
238 	ret = crypto_shash_update(desc, pebuf, ctx->image_checksum_offset);
239 	if (ret < 0)
240 		return ret;
241 
242 	tmp = ctx->image_checksum_offset + sizeof(uint32_t);
243 	ret = crypto_shash_update(desc, pebuf + tmp,
244 				  ctx->cert_dirent_offset - tmp);
245 	if (ret < 0)
246 		return ret;
247 
248 	tmp = ctx->cert_dirent_offset + sizeof(struct data_dirent);
249 	ret = crypto_shash_update(desc, pebuf + tmp, ctx->header_size - tmp);
250 	if (ret < 0)
251 		return ret;
252 
253 	canon = kcalloc(ctx->n_sections, sizeof(unsigned), GFP_KERNEL);
254 	if (!canon)
255 		return -ENOMEM;
256 
257 	/* We have to canonicalise the section table, so we perform an
258 	 * insertion sort.
259 	 */
260 	canon[0] = 0;
261 	for (loop = 1; loop < ctx->n_sections; loop++) {
262 		for (i = 0; i < loop; i++) {
263 			if (pefile_compare_shdrs(&ctx->secs[canon[i]],
264 						 &ctx->secs[loop]) > 0) {
265 				memmove(&canon[i + 1], &canon[i],
266 					(loop - i) * sizeof(canon[0]));
267 				break;
268 			}
269 		}
270 		canon[i] = loop;
271 	}
272 
273 	hashed_bytes = ctx->header_size;
274 	for (loop = 0; loop < ctx->n_sections; loop++) {
275 		i = canon[loop];
276 		if (ctx->secs[i].raw_data_size == 0)
277 			continue;
278 		ret = crypto_shash_update(desc,
279 					  pebuf + ctx->secs[i].data_addr,
280 					  ctx->secs[i].raw_data_size);
281 		if (ret < 0) {
282 			kfree(canon);
283 			return ret;
284 		}
285 		hashed_bytes += ctx->secs[i].raw_data_size;
286 	}
287 	kfree(canon);
288 
289 	if (pelen > hashed_bytes) {
290 		tmp = hashed_bytes + ctx->certs_size;
291 		ret = crypto_shash_update(desc,
292 					  pebuf + hashed_bytes,
293 					  pelen - tmp);
294 		if (ret < 0)
295 			return ret;
296 	}
297 
298 	return 0;
299 }
300 
301 /*
302  * Digest the contents of the PE binary, leaving out the image checksum and the
303  * certificate data block.
304  */
305 static int pefile_digest_pe(const void *pebuf, unsigned int pelen,
306 			    struct pefile_context *ctx)
307 {
308 	struct crypto_shash *tfm;
309 	struct shash_desc *desc;
310 	size_t digest_size, desc_size;
311 	void *digest;
312 	int ret;
313 
314 	kenter(",%u", ctx->digest_algo);
315 
316 	/* Allocate the hashing algorithm we're going to need and find out how
317 	 * big the hash operational data will be.
318 	 */
319 	tfm = crypto_alloc_shash(hash_algo_name[ctx->digest_algo], 0, 0);
320 	if (IS_ERR(tfm))
321 		return (PTR_ERR(tfm) == -ENOENT) ? -ENOPKG : PTR_ERR(tfm);
322 
323 	desc_size = crypto_shash_descsize(tfm) + sizeof(*desc);
324 	digest_size = crypto_shash_digestsize(tfm);
325 
326 	if (digest_size != ctx->digest_len) {
327 		pr_debug("Digest size mismatch (%zx != %x)\n",
328 			 digest_size, ctx->digest_len);
329 		ret = -EBADMSG;
330 		goto error_no_desc;
331 	}
332 	pr_debug("Digest: desc=%zu size=%zu\n", desc_size, digest_size);
333 
334 	ret = -ENOMEM;
335 	desc = kzalloc(desc_size + digest_size, GFP_KERNEL);
336 	if (!desc)
337 		goto error_no_desc;
338 
339 	desc->tfm   = tfm;
340 	desc->flags = CRYPTO_TFM_REQ_MAY_SLEEP;
341 	ret = crypto_shash_init(desc);
342 	if (ret < 0)
343 		goto error;
344 
345 	ret = pefile_digest_pe_contents(pebuf, pelen, ctx, desc);
346 	if (ret < 0)
347 		goto error;
348 
349 	digest = (void *)desc + desc_size;
350 	ret = crypto_shash_final(desc, digest);
351 	if (ret < 0)
352 		goto error;
353 
354 	pr_debug("Digest calc = [%*ph]\n", ctx->digest_len, digest);
355 
356 	/* Check that the PE file digest matches that in the MSCODE part of the
357 	 * PKCS#7 certificate.
358 	 */
359 	if (memcmp(digest, ctx->digest, ctx->digest_len) != 0) {
360 		pr_debug("Digest mismatch\n");
361 		ret = -EKEYREJECTED;
362 	} else {
363 		pr_debug("The digests match!\n");
364 	}
365 
366 error:
367 	kfree(desc);
368 error_no_desc:
369 	crypto_free_shash(tfm);
370 	kleave(" = %d", ret);
371 	return ret;
372 }
373 
374 /**
375  * verify_pefile_signature - Verify the signature on a PE binary image
376  * @pebuf: Buffer containing the PE binary image
377  * @pelen: Length of the binary image
378  * @trust_keyring: Signing certificates to use as starting points
379  * @_trusted: Set to true if trustworth, false otherwise
380  *
381  * Validate that the certificate chain inside the PKCS#7 message inside the PE
382  * binary image intersects keys we already know and trust.
383  *
384  * Returns, in order of descending priority:
385  *
386  *  (*) -ELIBBAD if the image cannot be parsed, or:
387  *
388  *  (*) -EKEYREJECTED if a signature failed to match for which we have a valid
389  *	key, or:
390  *
391  *  (*) 0 if at least one signature chain intersects with the keys in the trust
392  *	keyring, or:
393  *
394  *  (*) -ENOPKG if a suitable crypto module couldn't be found for a check on a
395  *	chain.
396  *
397  *  (*) -ENOKEY if we couldn't find a match for any of the signature chains in
398  *	the message.
399  *
400  * May also return -ENOMEM.
401  */
402 int verify_pefile_signature(const void *pebuf, unsigned pelen,
403 			    struct key *trusted_keyring, bool *_trusted)
404 {
405 	struct pkcs7_message *pkcs7;
406 	struct pefile_context ctx;
407 	const void *data;
408 	size_t datalen;
409 	int ret;
410 
411 	kenter("");
412 
413 	memset(&ctx, 0, sizeof(ctx));
414 	ret = pefile_parse_binary(pebuf, pelen, &ctx);
415 	if (ret < 0)
416 		return ret;
417 
418 	ret = pefile_strip_sig_wrapper(pebuf, &ctx);
419 	if (ret < 0)
420 		return ret;
421 
422 	pkcs7 = pkcs7_parse_message(pebuf + ctx.sig_offset, ctx.sig_len);
423 	if (IS_ERR(pkcs7))
424 		return PTR_ERR(pkcs7);
425 	ctx.pkcs7 = pkcs7;
426 
427 	ret = pkcs7_get_content_data(ctx.pkcs7, &data, &datalen, false);
428 	if (ret < 0 || datalen == 0) {
429 		pr_devel("PKCS#7 message does not contain data\n");
430 		ret = -EBADMSG;
431 		goto error;
432 	}
433 
434 	ret = mscode_parse(&ctx);
435 	if (ret < 0)
436 		goto error;
437 
438 	pr_debug("Digest: %u [%*ph]\n",
439 		 ctx.digest_len, ctx.digest_len, ctx.digest);
440 
441 	/* Generate the digest and check against the PKCS7 certificate
442 	 * contents.
443 	 */
444 	ret = pefile_digest_pe(pebuf, pelen, &ctx);
445 	if (ret < 0)
446 		goto error;
447 
448 	ret = pkcs7_verify(pkcs7);
449 	if (ret < 0)
450 		goto error;
451 
452 	ret = pkcs7_validate_trust(pkcs7, trusted_keyring, _trusted);
453 
454 error:
455 	pkcs7_free_message(ctx.pkcs7);
456 	return ret;
457 }
458