X.509: Support X.509 lookup by Issuer+Serial form AuthorityKeyIdentifier

If an X.509 certificate has an AuthorityKeyIdentifier extension that provides
an issuer and serialNumber, then make it so that these are used in preference
to the keyIdentifier field also held therein for searching for the signing
certificate.

If both the issuer+serialNumber and the keyIdentifier are supplied, then the
certificate is looked up by the former but the latter is checked as well.  If
the latter doesn't match the subjectKeyIdentifier of the parent certificate,
EKEYREJECTED is returned.

This makes it possible to chain X.509 certificates based on the issuer and
serialNumber fields rather than on subjectKeyIdentifier.  This is necessary as
we are having to deal with keys that are represented by X.509 certificates
that lack a subjectKeyIdentifier.

Signed-off-by: David Howells <dhowells@redhat.com>
Tested-by: Vivek Goyal <vgoyal@redhat.com>

1 files changed, 36 insertions, 11 deletions

diff --git a/crypto/asymmetric_keys/pkcs7_verify.c b/crypto/asymmetric_keys/pkcs7_verify.c
index a4d083f7e9e1..42bfc9de0d79 100644
--- a/crypto/asymmetric_keys/pkcs7_verify.c
+++ b/crypto/asymmetric_keys/pkcs7_verify.c
@@ -170,6 +170,7 @@ static int pkcs7_verify_sig_chain(struct pkcs7_message *pkcs7,
 				  struct pkcs7_signed_info *sinfo)
 {
 	struct x509_certificate *x509 = sinfo->signer, *p;
+	struct asymmetric_key_id *auth;
 	int ret;
 
 	kenter("");
@@ -187,11 +188,14 @@ static int pkcs7_verify_sig_chain(struct pkcs7_message *pkcs7,
 			goto maybe_missing_crypto_in_x509;
 
 		pr_debug("- issuer %s\n", x509->issuer);
+		if (x509->akid_id)
+			pr_debug("- authkeyid.id %*phN\n",
+				 x509->akid_id->len, x509->akid_id->data);
 		if (x509->akid_skid)
-			pr_debug("- authkeyid %*phN\n",
+			pr_debug("- authkeyid.skid %*phN\n",
 				 x509->akid_skid->len, x509->akid_skid->data);
 
-		if (!x509->akid_skid ||
+		if ((!x509->akid_id && !x509->akid_skid) ||
 		    strcmp(x509->subject, x509->issuer) == 0) {
 			/* If there's no authority certificate specified, then
 			 * the certificate must be self-signed and is the root
@@ -215,21 +219,42 @@ static int pkcs7_verify_sig_chain(struct pkcs7_message *pkcs7,
 		/* Look through the X.509 certificates in the PKCS#7 message's
 		 * list to see if the next one is there.
 		 */
-		pr_debug("- want %*phN\n",
-			 x509->akid_skid->len, x509->akid_skid->data);
-		for (p = pkcs7->certs; p; p = p->next) {
-			if (!p->skid)
-				continue;
-			pr_debug("- cmp [%u] %*phN\n",
-				 p->index, p->skid->len, p->skid->data);
-			if (asymmetric_key_id_same(p->skid, x509->akid_skid))
-				goto found_issuer;
+		auth = x509->akid_id;
+		if (auth) {
+			pr_debug("- want %*phN\n", auth->len, auth->data);
+			for (p = pkcs7->certs; p; p = p->next) {
+				pr_debug("- cmp [%u] %*phN\n",
+					 p->index, p->id->len, p->id->data);
+				if (asymmetric_key_id_same(p->id, auth))
+					goto found_issuer_check_skid;
+			}
+		} else {
+			auth = x509->akid_skid;
+			pr_debug("- want %*phN\n", auth->len, auth->data);
+			for (p = pkcs7->certs; p; p = p->next) {
+				if (!p->skid)
+					continue;
+				pr_debug("- cmp [%u] %*phN\n",
+					 p->index, p->skid->len, p->skid->data);
+				if (asymmetric_key_id_same(p->skid, auth))
+					goto found_issuer;
+			}
 		}
 
 		/* We didn't find the root of this chain */
 		pr_debug("- top\n");
 		return 0;
 
+	found_issuer_check_skid:
+		/* We matched issuer + serialNumber, but if there's an
+		 * authKeyId.keyId, that must match the CA subjKeyId also.
+		 */
+		if (x509->akid_skid &&
+		    !asymmetric_key_id_same(p->skid, x509->akid_skid)) {
+			pr_warn("Sig %u: X.509 chain contains auth-skid nonmatch (%u->%u)\n",
+				sinfo->index, x509->index, p->index);
+			return -EKEYREJECTED;
+		}
 	found_issuer:
 		pr_debug("- subject %s\n", p->subject);
 		if (p->seen) {