upstream: Ensure that all returned SSHFP records for the specified host

name and hostkey type match instead of only one.  While there, simplify the
code somewhat and add some debugging.  Based on discussion in bz#3322, ok
djm@.

OpenBSD-Commit-ID: 0a6a0a476eb7f9dfe8fe2c05a1a395e3e9b22ee4

1 files changed, 26 insertions, 40 deletions

diff --git a/dns.c b/dns.c
index e8589f68..1cfc38e7 100644
--- a/dns.c
+++ b/dns.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: dns.c,v 1.40 2021/07/05 01:16:46 dtucker Exp $ */
+/* $OpenBSD: dns.c,v 1.41 2021/07/19 03:13:28 dtucker Exp $ */
 
 /*
  * Copyright (c) 2003 Wesley Griffin. All rights reserved.
@@ -75,6 +75,7 @@ dns_result_totext(unsigned int res)
 
 /*
  * Read SSHFP parameters from key buffer.
+ * Caller must free digest which is allocated by sshkey_fingerprint_raw().
  */
 static int
 dns_read_key(u_int8_t *algorithm, u_int8_t *digest_type,
@@ -86,32 +87,21 @@ dns_read_key(u_int8_t *algorithm, u_int8_t *digest_type,
 	switch (key->type) {
 	case KEY_RSA:
 		*algorithm = SSHFP_KEY_RSA;
-		if (!*digest_type)
-			*digest_type = SSHFP_HASH_SHA1;
 		break;
 	case KEY_DSA:
 		*algorithm = SSHFP_KEY_DSA;
-		if (!*digest_type)
-			*digest_type = SSHFP_HASH_SHA1;
 		break;
 	case KEY_ECDSA:
 		*algorithm = SSHFP_KEY_ECDSA;
-		if (!*digest_type)
-			*digest_type = SSHFP_HASH_SHA256;
 		break;
 	case KEY_ED25519:
 		*algorithm = SSHFP_KEY_ED25519;
-		if (!*digest_type)
-			*digest_type = SSHFP_HASH_SHA256;
 		break;
 	case KEY_XMSS:
 		*algorithm = SSHFP_KEY_XMSS;
-		if (!*digest_type)
-			*digest_type = SSHFP_HASH_SHA256;
 		break;
 	default:
 		*algorithm = SSHFP_KEY_RESERVED; /* 0 */
-		*digest_type = SSHFP_HASH_RESERVED; /* 0 */
 	}
 
 	switch (*digest_type) {
@@ -133,7 +123,6 @@ dns_read_key(u_int8_t *algorithm, u_int8_t *digest_type,
 	} else {
 		*digest = NULL;
 		*digest_len = 0;
-		success = 0;
 	}
 
 	return success;
@@ -212,7 +201,6 @@ verify_host_key_dns(const char *hostname, struct sockaddr *address,
 	struct rrsetinfo *fingerprints = NULL;
 
 	u_int8_t hostkey_algorithm;
-	u_int8_t hostkey_digest_type = SSHFP_HASH_RESERVED;
 	u_char *hostkey_digest;
 	size_t hostkey_digest_len;
 
@@ -248,14 +236,6 @@ verify_host_key_dns(const char *hostname, struct sockaddr *address,
 		    fingerprints->rri_nrdatas);
 	}
 
-	/* Initialize default host key parameters */
-	if (!dns_read_key(&hostkey_algorithm, &hostkey_digest_type,
-	    &hostkey_digest, &hostkey_digest_len, hostkey)) {
-		error("Error calculating host key fingerprint.");
-		freerrset(fingerprints);
-		return -1;
-	}
-
 	if (fingerprints->rri_nrdatas)
 		*flags |= DNS_VERIFY_FOUND;
 
@@ -271,35 +251,41 @@ verify_host_key_dns(const char *hostname, struct sockaddr *address,
 			verbose("Error parsing fingerprint from DNS.");
 			continue;
 		}
-
-		if (hostkey_digest_type != dnskey_digest_type) {
-			hostkey_digest_type = dnskey_digest_type;
-			free(hostkey_digest);
-
-			/* Initialize host key parameters */
-			if (!dns_read_key(&hostkey_algorithm,
-			    &hostkey_digest_type, &hostkey_digest,
-			    &hostkey_digest_len, hostkey)) {
-				error("Error calculating key fingerprint.");
-				freerrset(fingerprints);
-				return -1;
-			}
+		debug3_f("checking SSHFP type %d fptype %d", dnskey_algorithm,
+		    dnskey_digest_type);
+
+		/* Calculate host key fingerprint. */
+		if (!dns_read_key(&hostkey_algorithm, &dnskey_digest_type,
+		    &hostkey_digest, &hostkey_digest_len, hostkey)) {
+			error("Error calculating key fingerprint.");
+			freerrset(fingerprints);
+			return -1;
 		}
 
 		/* Check if the current key is the same as the given key */
 		if (hostkey_algorithm == dnskey_algorithm &&
-		    hostkey_digest_type == dnskey_digest_type) {
-			if (hostkey_digest_len == dnskey_digest_len &&
-			    timingsafe_bcmp(hostkey_digest, dnskey_digest,
-			    hostkey_digest_len) == 0)
+		    hostkey_digest_len == dnskey_digest_len) {
+			if (timingsafe_bcmp(hostkey_digest, dnskey_digest,
+			    hostkey_digest_len) == 0) {
+				debug_f("matched SSHFP type %d fptype %d",
+				    dnskey_algorithm, dnskey_digest_type);
 				*flags |= DNS_VERIFY_MATCH;
+			} else {
+				debug_f("failed SSHFP type %d fptype %d",
+				    dnskey_algorithm, dnskey_digest_type);
+				*flags |= DNS_VERIFY_FAILED;
+			}
 		}
 		free(dnskey_digest);
+		free(hostkey_digest); /* from sshkey_fingerprint_raw() */
 	}
 
-	free(hostkey_digest); /* from sshkey_fingerprint_raw() */
 	freerrset(fingerprints);
 
+	/* If any fingerprint failed to validate, return failure. */
+	if (*flags & DNS_VERIFY_FAILED)
+		*flags &= ~DNS_VERIFY_MATCH;
+
 	if (*flags & DNS_VERIFY_FOUND)
 		if (*flags & DNS_VERIFY_MATCH)
 			debug("matching host key fingerprint found in DNS");