1/* Verify the signature on a PKCS#7 message.
2 *
3 * Copyright (C) 2012 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) "PKCS7: "fmt
13#include <linux/kernel.h>
14#include <linux/export.h>
15#include <linux/slab.h>
16#include <linux/err.h>
17#include <linux/asn1.h>
18#include <crypto/hash.h>
19#include "public_key.h"
20#include "pkcs7_parser.h"
21
22/*
23 * Digest the relevant parts of the PKCS#7 data
24 */
25static int pkcs7_digest(struct pkcs7_message *pkcs7,
26			struct pkcs7_signed_info *sinfo)
27{
28	struct crypto_shash *tfm;
29	struct shash_desc *desc;
30	size_t digest_size, desc_size;
31	void *digest;
32	int ret;
33
34	kenter(",%u,%u", sinfo->index, sinfo->sig.pkey_hash_algo);
35
36	if (sinfo->sig.pkey_hash_algo >= PKEY_HASH__LAST ||
37	    !hash_algo_name[sinfo->sig.pkey_hash_algo])
38		return -ENOPKG;
39
40	/* Allocate the hashing algorithm we're going to need and find out how
41	 * big the hash operational data will be.
42	 */
43	tfm = crypto_alloc_shash(hash_algo_name[sinfo->sig.pkey_hash_algo],
44				 0, 0);
45	if (IS_ERR(tfm))
46		return (PTR_ERR(tfm) == -ENOENT) ? -ENOPKG : PTR_ERR(tfm);
47
48	desc_size = crypto_shash_descsize(tfm) + sizeof(*desc);
49	sinfo->sig.digest_size = digest_size = crypto_shash_digestsize(tfm);
50
51	ret = -ENOMEM;
52	digest = kzalloc(digest_size + desc_size, GFP_KERNEL);
53	if (!digest)
54		goto error_no_desc;
55
56	desc = digest + digest_size;
57	desc->tfm   = tfm;
58	desc->flags = CRYPTO_TFM_REQ_MAY_SLEEP;
59
60	/* Digest the message [RFC2315 9.3] */
61	ret = crypto_shash_init(desc);
62	if (ret < 0)
63		goto error;
64	ret = crypto_shash_finup(desc, pkcs7->data, pkcs7->data_len, digest);
65	if (ret < 0)
66		goto error;
67	pr_devel("MsgDigest = [%*ph]\n", 8, digest);
68
69	/* However, if there are authenticated attributes, there must be a
70	 * message digest attribute amongst them which corresponds to the
71	 * digest we just calculated.
72	 */
73	if (sinfo->msgdigest) {
74		u8 tag;
75
76		if (sinfo->msgdigest_len != sinfo->sig.digest_size) {
77			pr_debug("Sig %u: Invalid digest size (%u)\n",
78				 sinfo->index, sinfo->msgdigest_len);
79			ret = -EBADMSG;
80			goto error;
81		}
82
83		if (memcmp(digest, sinfo->msgdigest, sinfo->msgdigest_len) != 0) {
84			pr_debug("Sig %u: Message digest doesn't match\n",
85				 sinfo->index);
86			ret = -EKEYREJECTED;
87			goto error;
88		}
89
90		/* We then calculate anew, using the authenticated attributes
91		 * as the contents of the digest instead.  Note that we need to
92		 * convert the attributes from a CONT.0 into a SET before we
93		 * hash it.
94		 */
95		memset(digest, 0, sinfo->sig.digest_size);
96
97		ret = crypto_shash_init(desc);
98		if (ret < 0)
99			goto error;
100		tag = ASN1_CONS_BIT | ASN1_SET;
101		ret = crypto_shash_update(desc, &tag, 1);
102		if (ret < 0)
103			goto error;
104		ret = crypto_shash_finup(desc, sinfo->authattrs,
105					 sinfo->authattrs_len, digest);
106		if (ret < 0)
107			goto error;
108		pr_devel("AADigest = [%*ph]\n", 8, digest);
109	}
110
111	sinfo->sig.digest = digest;
112	digest = NULL;
113
114error:
115	kfree(digest);
116error_no_desc:
117	crypto_free_shash(tfm);
118	kleave(" = %d", ret);
119	return ret;
120}
121
122/*
123 * Find the key (X.509 certificate) to use to verify a PKCS#7 message.  PKCS#7
124 * uses the issuer's name and the issuing certificate serial number for
125 * matching purposes.  These must match the certificate issuer's name (not
126 * subject's name) and the certificate serial number [RFC 2315 6.7].
127 */
128static int pkcs7_find_key(struct pkcs7_message *pkcs7,
129			  struct pkcs7_signed_info *sinfo)
130{
131	struct x509_certificate *x509;
132	unsigned certix = 1;
133
134	kenter("%u", sinfo->index);
135
136	for (x509 = pkcs7->certs; x509; x509 = x509->next, certix++) {
137		/* I'm _assuming_ that the generator of the PKCS#7 message will
138		 * encode the fields from the X.509 cert in the same way in the
139		 * PKCS#7 message - but I can't be 100% sure of that.  It's
140		 * possible this will need element-by-element comparison.
141		 */
142		if (!asymmetric_key_id_same(x509->id, sinfo->signing_cert_id))
143			continue;
144		pr_devel("Sig %u: Found cert serial match X.509[%u]\n",
145			 sinfo->index, certix);
146
147		if (x509->pub->pkey_algo != sinfo->sig.pkey_algo) {
148			pr_warn("Sig %u: X.509 algo and PKCS#7 sig algo don't match\n",
149				sinfo->index);
150			continue;
151		}
152
153		sinfo->signer = x509;
154		return 0;
155	}
156
157	/* The relevant X.509 cert isn't found here, but it might be found in
158	 * the trust keyring.
159	 */
160	pr_debug("Sig %u: Issuing X.509 cert not found (#%*phN)\n",
161		 sinfo->index,
162		 sinfo->signing_cert_id->len, sinfo->signing_cert_id->data);
163	return 0;
164}
165
166/*
167 * Verify the internal certificate chain as best we can.
168 */
169static int pkcs7_verify_sig_chain(struct pkcs7_message *pkcs7,
170				  struct pkcs7_signed_info *sinfo)
171{
172	struct x509_certificate *x509 = sinfo->signer, *p;
173	int ret;
174
175	kenter("");
176
177	for (p = pkcs7->certs; p; p = p->next)
178		p->seen = false;
179
180	for (;;) {
181		pr_debug("verify %s: %*phN\n",
182			 x509->subject,
183			 x509->raw_serial_size, x509->raw_serial);
184		x509->seen = true;
185		ret = x509_get_sig_params(x509);
186		if (ret < 0)
187			goto maybe_missing_crypto_in_x509;
188
189		pr_debug("- issuer %s\n", x509->issuer);
190		if (x509->akid_skid)
191			pr_debug("- authkeyid %*phN\n",
192				 x509->akid_skid->len, x509->akid_skid->data);
193
194		if (!x509->akid_skid ||
195		    strcmp(x509->subject, x509->issuer) == 0) {
196			/* If there's no authority certificate specified, then
197			 * the certificate must be self-signed and is the root
198			 * of the chain.  Likewise if the cert is its own
199			 * authority.
200			 */
201			pr_debug("- no auth?\n");
202			if (x509->raw_subject_size != x509->raw_issuer_size ||
203			    memcmp(x509->raw_subject, x509->raw_issuer,
204				   x509->raw_issuer_size) != 0)
205				return 0;
206
207			ret = x509_check_signature(x509->pub, x509);
208			if (ret < 0)
209				goto maybe_missing_crypto_in_x509;
210			x509->signer = x509;
211			pr_debug("- self-signed\n");
212			return 0;
213		}
214
215		/* Look through the X.509 certificates in the PKCS#7 message's
216		 * list to see if the next one is there.
217		 */
218		pr_debug("- want %*phN\n",
219			 x509->akid_skid->len, x509->akid_skid->data);
220		for (p = pkcs7->certs; p; p = p->next) {
221			if (!p->skid)
222				continue;
223			pr_debug("- cmp [%u] %*phN\n",
224				 p->index, p->skid->len, p->skid->data);
225			if (asymmetric_key_id_same(p->skid, x509->akid_skid))
226				goto found_issuer;
227		}
228
229		/* We didn't find the root of this chain */
230		pr_debug("- top\n");
231		return 0;
232
233	found_issuer:
234		pr_debug("- subject %s\n", p->subject);
235		if (p->seen) {
236			pr_warn("Sig %u: X.509 chain contains loop\n",
237				sinfo->index);
238			return 0;
239		}
240		ret = x509_check_signature(p->pub, x509);
241		if (ret < 0)
242			return ret;
243		x509->signer = p;
244		if (x509 == p) {
245			pr_debug("- self-signed\n");
246			return 0;
247		}
248		x509 = p;
249		might_sleep();
250	}
251
252maybe_missing_crypto_in_x509:
253	/* Just prune the certificate chain at this point if we lack some
254	 * crypto module to go further.  Note, however, we don't want to set
255	 * sinfo->missing_crypto as the signed info block may still be
256	 * validatable against an X.509 cert lower in the chain that we have a
257	 * trusted copy of.
258	 */
259	if (ret == -ENOPKG)
260		return 0;
261	return ret;
262}
263
264/*
265 * Verify one signed information block from a PKCS#7 message.
266 */
267static int pkcs7_verify_one(struct pkcs7_message *pkcs7,
268			    struct pkcs7_signed_info *sinfo)
269{
270	int ret;
271
272	kenter(",%u", sinfo->index);
273
274	/* First of all, digest the data in the PKCS#7 message and the
275	 * signed information block
276	 */
277	ret = pkcs7_digest(pkcs7, sinfo);
278	if (ret < 0)
279		return ret;
280
281	/* Find the key for the signature if there is one */
282	ret = pkcs7_find_key(pkcs7, sinfo);
283	if (ret < 0)
284		return ret;
285
286	if (!sinfo->signer)
287		return 0;
288
289	pr_devel("Using X.509[%u] for sig %u\n",
290		 sinfo->signer->index, sinfo->index);
291
292	/* Verify the PKCS#7 binary against the key */
293	ret = public_key_verify_signature(sinfo->signer->pub, &sinfo->sig);
294	if (ret < 0)
295		return ret;
296
297	pr_devel("Verified signature %u\n", sinfo->index);
298
299	/* Verify the internal certificate chain */
300	return pkcs7_verify_sig_chain(pkcs7, sinfo);
301}
302
303/**
304 * pkcs7_verify - Verify a PKCS#7 message
305 * @pkcs7: The PKCS#7 message to be verified
306 *
307 * Verify a PKCS#7 message is internally consistent - that is, the data digest
308 * matches the digest in the AuthAttrs and any signature in the message or one
309 * of the X.509 certificates it carries that matches another X.509 cert in the
310 * message can be verified.
311 *
312 * This does not look to match the contents of the PKCS#7 message against any
313 * external public keys.
314 *
315 * Returns, in order of descending priority:
316 *
317 *  (*) -EKEYREJECTED if a signature failed to match for which we found an
318 *	appropriate X.509 certificate, or:
319 *
320 *  (*) -EBADMSG if some part of the message was invalid, or:
321 *
322 *  (*) -ENOPKG if none of the signature chains are verifiable because suitable
323 *	crypto modules couldn't be found, or:
324 *
325 *  (*) 0 if all the signature chains that don't incur -ENOPKG can be verified
326 *	(note that a signature chain may be of zero length), or:
327 */
328int pkcs7_verify(struct pkcs7_message *pkcs7)
329{
330	struct pkcs7_signed_info *sinfo;
331	struct x509_certificate *x509;
332	int enopkg = -ENOPKG;
333	int ret, n;
334
335	kenter("");
336
337	for (n = 0, x509 = pkcs7->certs; x509; x509 = x509->next, n++) {
338		ret = x509_get_sig_params(x509);
339		if (ret < 0)
340			return ret;
341	}
342
343	for (sinfo = pkcs7->signed_infos; sinfo; sinfo = sinfo->next) {
344		ret = pkcs7_verify_one(pkcs7, sinfo);
345		if (ret < 0) {
346			if (ret == -ENOPKG) {
347				sinfo->unsupported_crypto = true;
348				continue;
349			}
350			kleave(" = %d", ret);
351			return ret;
352		}
353		enopkg = 0;
354	}
355
356	kleave(" = %d", enopkg);
357	return enopkg;
358}
359EXPORT_SYMBOL_GPL(pkcs7_verify);
360