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 -ENODATA;
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  *  (*) -ENODATA if there is no signature present.
407  *
408  *  (*) -ENOPKG if a suitable crypto module couldn't be found for a check on a
409  *	chain.
410  *
411  *  (*) -ENOKEY if we couldn't find a match for any of the signature chains in
412  *	the message.
413  *
414  * May also return -ENOMEM.
415  */
416 int verify_pefile_signature(const void *pebuf, unsigned pelen,
417 			    struct key *trusted_keys,
418 			    enum key_being_used_for usage)
419 {
420 	struct pefile_context ctx;
421 	int ret;
422 
423 	kenter("");
424 
425 	memset(&ctx, 0, sizeof(ctx));
426 	ret = pefile_parse_binary(pebuf, pelen, &ctx);
427 	if (ret < 0)
428 		return ret;
429 
430 	ret = pefile_strip_sig_wrapper(pebuf, &ctx);
431 	if (ret < 0)
432 		return ret;
433 
434 	ret = verify_pkcs7_signature(NULL, 0,
435 				     pebuf + ctx.sig_offset, ctx.sig_len,
436 				     trusted_keys, usage,
437 				     mscode_parse, &ctx);
438 	if (ret < 0)
439 		goto error;
440 
441 	pr_debug("Digest: %u [%*ph]\n",
442 		 ctx.digest_len, ctx.digest_len, ctx.digest);
443 
444 	/* Generate the digest and check against the PKCS7 certificate
445 	 * contents.
446 	 */
447 	ret = pefile_digest_pe(pebuf, pelen, &ctx);
448 
449 error:
450 	kzfree(ctx.digest);
451 	return ret;
452 }
453