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 	kfree(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 	kfree(ctx.digest);
454 	return ret;
455 }
456