[PATCH 4/5] tlshd: Make sure the client supports the PQ pk alg used by the server cert

[Scott Mayhew <smayhew@redhat.com>]

Store the public-key algorithm used by the server cert so we can later
compare it to the list of signing algorithms supported by the client in
the cert callback.

Link: https://github.com/oracle/ktls-utils/issues/113
Signed-off-by: Scott Mayhew <smayhew@redhat.com>
---
 src/tlshd/config.c | 12 +++++++-----
 src/tlshd/server.c | 49 ++++++++++++++++++++++++++++++++++------------
 src/tlshd/tlshd.h  |  2 +-
 3 files changed, 44 insertions(+), 19 deletions(-)

diff --git a/src/tlshd/config.c b/src/tlshd/config.c
index 6adacc0..735d24a 100644
--- a/src/tlshd/config.c
+++ b/src/tlshd/config.c
@@ -402,7 +402,8 @@ bool tlshd_config_get_server_crl(char **result)
 }
 
 #ifdef HAVE_GNUTLS_MLDSA
-static bool tlshd_cert_check_pk_alg(gnutls_datum_t *data)
+static bool tlshd_cert_check_pk_alg(gnutls_datum_t *data,
+				    gnutls_pk_algorithm_t *pkalg)
 {
 	gnutls_x509_crt_t cert;
 	gnutls_pk_algorithm_t pk_alg;
@@ -425,6 +426,7 @@ static bool tlshd_cert_check_pk_alg(gnutls_datum_t *data)
 	case GNUTLS_PK_MLDSA44:
 	case GNUTLS_PK_MLDSA65:
 	case GNUTLS_PK_MLDSA87:
+		*pkalg = pk_alg;
 		break;
 	default:
 		gnutls_x509_crt_deinit(cert);
@@ -441,7 +443,7 @@ static bool tlshd_cert_check_pk_alg(gnutls_datum_t *data)
  * @key: IN: the key field name from .conf
  * @certs: OUT: in-memory certificates
  * @certs_len: IN: maximum number of certs to get, OUT: number of certs found
- * @check_pq: IN: verify that the cert is using a PQ public-key alg
+ * @pkgalg: OUT: the PQ public-key alg that was used in the cert
  *
  * Return values:
  *   %true: certificate retrieved successfully
@@ -450,10 +452,10 @@ static bool tlshd_cert_check_pk_alg(gnutls_datum_t *data)
 bool tlshd_config_get_server_certs(const gchar *key,
 				   gnutls_pcert_st *certs,
 				   unsigned int *certs_len,
-				   bool check_pq)
+				   gnutls_pk_algorithm_t *pkalg)
 {
 #ifndef HAVE_GNUTLS_MLSDA
-	(void)check_pq;
+	(void)pkalg;
 #endif /* HAVE_GNUTLS_MLDSA */
 	gnutls_datum_t data;
 	gchar *pathname;
@@ -471,7 +473,7 @@ bool tlshd_config_get_server_certs(const gchar *key,
 	}
 
 #ifdef HAVE_GNUTLS_MLDSA
-	if (check_pq && !tlshd_cert_check_pk_alg(&data)) {
+	if (pkalg && !tlshd_cert_check_pk_alg(&data, pkalg)) {
 		tlshd_log_debug("%s: %s certificate not using a PQ public-key algorithm",
 				__func__, key);
 		free(data.data);
diff --git a/src/tlshd/server.c b/src/tlshd/server.c
index 210a09e..7a0ce24 100644
--- a/src/tlshd/server.c
+++ b/src/tlshd/server.c
@@ -50,6 +50,7 @@ static gnutls_pcert_st tlshd_server_certs[TLSHD_MAX_CERTS];
 static gnutls_privkey_t tlshd_server_pq_privkey;
 static unsigned int tlshd_server_pq_certs_len = TLSHD_MAX_CERTS;
 static gnutls_pcert_st tlshd_server_pq_certs[TLSHD_MAX_CERTS];
+static gnutls_pk_algorithm_t tlshd_server_pq_pkalg = GNUTLS_PK_UNKNOWN;
 #endif /* HAVE_GNUTLS_MLDSA */
 
 static bool tlshd_x509_server_get_certs(struct tlshd_handshake_parms *parms)
@@ -62,12 +63,12 @@ static bool tlshd_x509_server_get_certs(struct tlshd_handshake_parms *parms)
 	tlshd_config_get_server_certs("x509.pq.certificate",
 				      tlshd_server_pq_certs,
 				      &tlshd_server_pq_certs_len,
-				      true);
+				      &tlshd_server_pq_pkalg);
 #endif /* HAVE_GNUTLS_MLDSA */
 	return tlshd_config_get_server_certs("x509.certificate",
 					     tlshd_server_certs,
 					     &tlshd_server_certs_len,
-					     false);
+					     NULL);
 }
 
 static void tlshd_x509_server_put_certs(void)
@@ -158,20 +159,42 @@ tlshd_x509_retrieve_key_cb(gnutls_session_t session,
 		return -1;
 
 #ifdef HAVE_GNUTLS_MLDSA
-	for (i = 0; ; i++) {
-		ret = gnutls_sign_algorithm_get_requested(session, i, &client_alg);
-		if (ret != GNUTLS_E_SUCCESS)
-			break;
-		if (client_alg == GNUTLS_SIGN_MLDSA44
-				|| client_alg == GNUTLS_SIGN_MLDSA65
-				|| client_alg == GNUTLS_SIGN_MLDSA87) {
-			tlshd_log_debug("%s: Client supports ML-DSA", __func__);
-			use_pq_cert = true;
-			break;
+	/*
+ 	 * NB: Unfortunately when the callback function is invoked server-side,
+ 	 * pk_algos is NULL and pk_algos_length is 0. So we check the signature
+ 	 * algorithms the client supports and try to match one of them to the
+ 	 * public-key algorithm used by the server cert.
+ 	 */
+	if (tlshd_server_pq_pkalg != GNUTLS_PK_UNKNOWN) {
+		for (i = 0; ; i++) {
+			ret = gnutls_sign_algorithm_get_requested(session, i, &client_alg);
+			if (ret != GNUTLS_E_SUCCESS)
+				break;
+			switch (client_alg) {
+			case GNUTLS_SIGN_MLDSA44:
+				if (tlshd_server_pq_pkalg == GNUTLS_PK_MLDSA44)
+					use_pq_cert = true;
+				break;
+			case GNUTLS_SIGN_MLDSA65:
+				if (tlshd_server_pq_pkalg == GNUTLS_PK_MLDSA65)
+					use_pq_cert = true;
+				break;
+			case GNUTLS_SIGN_MLDSA87:
+				if (tlshd_server_pq_pkalg == GNUTLS_PK_MLDSA87)
+					use_pq_cert = true;
+				break;
+			default:
+				break;
+			}
+			if (use_pq_cert == true) {
+				tlshd_log_debug("%s: Client supports %s", __func__,
+						gnutls_sign_get_name(client_alg));
+				break;
+			}
 		}
 	}
 
-	if (use_pq_cert == true && tlshd_server_pq_certs_len > 0) {
+	if (use_pq_cert == true) {
 		tlshd_log_debug("%s: Selecting x509.pq.certificate from conf file", __func__);
 		*pcert_length = tlshd_server_pq_certs_len;
 		*pcert = tlshd_server_pq_certs;
diff --git a/src/tlshd/tlshd.h b/src/tlshd/tlshd.h
index 6bd55c7..ca84d8f 100644
--- a/src/tlshd/tlshd.h
+++ b/src/tlshd/tlshd.h
@@ -62,7 +62,7 @@ bool tlshd_config_get_server_crl(char **result);
 bool tlshd_config_get_server_certs(const gchar *key,
 				   gnutls_pcert_st *certs,
 				   unsigned int *certs_len,
-				   bool check_pq);
+				   gnutls_pk_algorithm_t *pkalg);
 bool tlshd_config_get_server_privkey(const gchar *key,
 				     gnutls_privkey_t *privkey);
 
-- 
2.50.1