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_warn("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_warn("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_warn("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 	/* sbsign rounds up the length of certificate table (in optional
139 	 * header data directories) to 8 byte alignment.  However, the PE
140 	 * specification states that while entries are 8-byte aligned, this is
141 	 * not included in their length, and as a result, pesign has not
142 	 * rounded up since 0.110.
143 	 */
144 	if (wrapper.length > ctx->sig_len) {
145 		pr_warn("Signature wrapper bigger than sig len (%x > %x)\n",
146 			ctx->sig_len, wrapper.length);
147 		return -ELIBBAD;
148 	}
149 	if (wrapper.revision != WIN_CERT_REVISION_2_0) {
150 		pr_warn("Signature is not revision 2.0\n");
151 		return -ENOTSUPP;
152 	}
153 	if (wrapper.cert_type != WIN_CERT_TYPE_PKCS_SIGNED_DATA) {
154 		pr_warn("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_warn("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_warn("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_warn("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 	ret = crypto_shash_init(desc);
358 	if (ret < 0)
359 		goto error;
360 
361 	ret = pefile_digest_pe_contents(pebuf, pelen, ctx, desc);
362 	if (ret < 0)
363 		goto error;
364 
365 	digest = (void *)desc + desc_size;
366 	ret = crypto_shash_final(desc, digest);
367 	if (ret < 0)
368 		goto error;
369 
370 	pr_debug("Digest calc = [%*ph]\n", ctx->digest_len, digest);
371 
372 	/* Check that the PE file digest matches that in the MSCODE part of the
373 	 * PKCS#7 certificate.
374 	 */
375 	if (memcmp(digest, ctx->digest, ctx->digest_len) != 0) {
376 		pr_warn("Digest mismatch\n");
377 		ret = -EKEYREJECTED;
378 	} else {
379 		pr_debug("The digests match!\n");
380 	}
381 
382 error:
383 	kfree_sensitive(desc);
384 error_no_desc:
385 	crypto_free_shash(tfm);
386 	kleave(" = %d", ret);
387 	return ret;
388 }
389 
390 /**
391  * verify_pefile_signature - Verify the signature on a PE binary image
392  * @pebuf: Buffer containing the PE binary image
393  * @pelen: Length of the binary image
394  * @trust_keys: Signing certificate(s) to use as starting points
395  * @usage: The use to which the key is being put.
396  *
397  * Validate that the certificate chain inside the PKCS#7 message inside the PE
398  * binary image intersects keys we already know and trust.
399  *
400  * Returns, in order of descending priority:
401  *
402  *  (*) -ELIBBAD if the image cannot be parsed, or:
403  *
404  *  (*) -EKEYREJECTED if a signature failed to match for which we have a valid
405  *	key, or:
406  *
407  *  (*) 0 if at least one signature chain intersects with the keys in the trust
408  *	keyring, or:
409  *
410  *  (*) -ENODATA if there is no signature present.
411  *
412  *  (*) -ENOPKG if a suitable crypto module couldn't be found for a check on a
413  *	chain.
414  *
415  *  (*) -ENOKEY if we couldn't find a match for any of the signature chains in
416  *	the message.
417  *
418  * May also return -ENOMEM.
419  */
420 int verify_pefile_signature(const void *pebuf, unsigned pelen,
421 			    struct key *trusted_keys,
422 			    enum key_being_used_for usage)
423 {
424 	struct pefile_context ctx;
425 	int ret;
426 
427 	kenter("");
428 
429 	memset(&ctx, 0, sizeof(ctx));
430 	ret = pefile_parse_binary(pebuf, pelen, &ctx);
431 	if (ret < 0)
432 		return ret;
433 
434 	ret = pefile_strip_sig_wrapper(pebuf, &ctx);
435 	if (ret < 0)
436 		return ret;
437 
438 	ret = verify_pkcs7_signature(NULL, 0,
439 				     pebuf + ctx.sig_offset, ctx.sig_len,
440 				     trusted_keys, usage,
441 				     mscode_parse, &ctx);
442 	if (ret < 0)
443 		goto error;
444 
445 	pr_debug("Digest: %u [%*ph]\n",
446 		 ctx.digest_len, ctx.digest_len, ctx.digest);
447 
448 	/* Generate the digest and check against the PKCS7 certificate
449 	 * contents.
450 	 */
451 	ret = pefile_digest_pe(pebuf, pelen, &ctx);
452 
453 error:
454 	kfree_sensitive(ctx.digest);
455 	return ret;
456 }
457