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 */
26static 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 */
126static 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
194check_len:
195	if (len <= ctx->sig_len) {
196		/* There may be padding */
197		ctx->sig_len = len;
198		return 0;
199	}
200not_pkcs7:
201	pr_debug("Signature data not PKCS#7\n");
202	return -ELIBBAD;
203}
204
205/*
206 * Compare two sections for canonicalisation.
207 */
208static 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 */
245static 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 */
322static 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(",%u", 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(hash_algo_name[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
383error:
384	kfree(desc);
385error_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_keyring: Signing certificates to use as starting points
396 * @_trusted: Set to true if trustworth, false otherwise
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 */
419int verify_pefile_signature(const void *pebuf, unsigned pelen,
420			    struct key *trusted_keyring, bool *_trusted)
421{
422	struct pkcs7_message *pkcs7;
423	struct pefile_context ctx;
424	const void *data;
425	size_t datalen;
426	int ret;
427
428	kenter("");
429
430	memset(&ctx, 0, sizeof(ctx));
431	ret = pefile_parse_binary(pebuf, pelen, &ctx);
432	if (ret < 0)
433		return ret;
434
435	ret = pefile_strip_sig_wrapper(pebuf, &ctx);
436	if (ret < 0)
437		return ret;
438
439	pkcs7 = pkcs7_parse_message(pebuf + ctx.sig_offset, ctx.sig_len);
440	if (IS_ERR(pkcs7))
441		return PTR_ERR(pkcs7);
442	ctx.pkcs7 = pkcs7;
443
444	ret = pkcs7_get_content_data(ctx.pkcs7, &data, &datalen, false);
445	if (ret < 0 || datalen == 0) {
446		pr_devel("PKCS#7 message does not contain data\n");
447		ret = -EBADMSG;
448		goto error;
449	}
450
451	ret = mscode_parse(&ctx);
452	if (ret < 0)
453		goto error;
454
455	pr_debug("Digest: %u [%*ph]\n",
456		 ctx.digest_len, ctx.digest_len, ctx.digest);
457
458	/* Generate the digest and check against the PKCS7 certificate
459	 * contents.
460	 */
461	ret = pefile_digest_pe(pebuf, pelen, &ctx);
462	if (ret < 0)
463		goto error;
464
465	ret = pkcs7_verify(pkcs7);
466	if (ret < 0)
467		goto error;
468
469	ret = pkcs7_validate_trust(pkcs7, trusted_keyring, _trusted);
470
471error:
472	pkcs7_free_message(ctx.pkcs7);
473	return ret;
474}
475