From 94f70228e9b93cf84b8cb00e544d8a66dc108011 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Manuel=20P=C3=A9gouri=C3=A9-Gonnard?=
 <manuel.pegourie-gonnard@arm.com>
Date: Fri, 9 Aug 2024 11:26:25 +0200
Subject: [PATCH] Clean up mbedtls_ssl_check_cert_usage()
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Signed-off-by: Manuel Pégourié-Gonnard <manuel.pegourie-gonnard@arm.com>
---
 library/ssl_misc.h         | 10 +++++-----
 library/ssl_tls.c          | 11 +++++++----
 library/ssl_tls12_server.c |  2 +-
 3 files changed, 13 insertions(+), 10 deletions(-)

diff --git a/library/ssl_misc.h b/library/ssl_misc.h
index 120f8ca3ca..e00dcfcf49 100644
--- a/library/ssl_misc.h
+++ b/library/ssl_misc.h
@@ -1674,18 +1674,18 @@ static inline mbedtls_x509_crt *mbedtls_ssl_own_cert(mbedtls_ssl_context *ssl)
 }
 
 /*
- * Check usage of a certificate wrt extensions:
- * keyUsage, extendedKeyUsage (later), and nSCertType (later).
+ * Check usage of a certificate wrt usage extensions:
+ * keyUsage and extendedKeyUsage.
+ * (Note: nSCertType is deprecated and not standard, we don't check it.)
  *
- * Warning: cert_endpoint is the endpoint of the cert (ie, of our peer when we
- * check a cert we received from them)!
+ * Note: recv_endpoint is the receiver's endpoint.
  *
  * Return 0 if everything is OK, -1 if not.
  */
 MBEDTLS_CHECK_RETURN_CRITICAL
 int mbedtls_ssl_check_cert_usage(const mbedtls_x509_crt *cert,
                                  const mbedtls_ssl_ciphersuite_t *ciphersuite,
-                                 int cert_endpoint,
+                                 int recv_endpoint,
                                  uint32_t *flags);
 #endif /* MBEDTLS_X509_CRT_PARSE_C */
 
diff --git a/library/ssl_tls.c b/library/ssl_tls.c
index 018eb93d88..865f984b9a 100644
--- a/library/ssl_tls.c
+++ b/library/ssl_tls.c
@@ -6361,7 +6361,7 @@ const char *mbedtls_ssl_get_curve_name_from_tls_id(uint16_t tls_id)
 #if defined(MBEDTLS_X509_CRT_PARSE_C)
 int mbedtls_ssl_check_cert_usage(const mbedtls_x509_crt *cert,
                                  const mbedtls_ssl_ciphersuite_t *ciphersuite,
-                                 int cert_endpoint,
+                                 int recv_endpoint,
                                  uint32_t *flags)
 {
     int ret = 0;
@@ -6369,7 +6369,10 @@ int mbedtls_ssl_check_cert_usage(const mbedtls_x509_crt *cert,
     const char *ext_oid;
     size_t ext_len;
 
-    if (cert_endpoint == MBEDTLS_SSL_IS_SERVER) {
+    /* Note: don't guard this with MBEDTLS_SSL_CLI_C because the server wants
+     * to check what a compliant client will think while choosing which cert
+     * to send to the client. */
+    if (recv_endpoint == MBEDTLS_SSL_IS_CLIENT) {
         /* Server part of the key exchange */
         switch (ciphersuite->key_exchange) {
             case MBEDTLS_KEY_EXCHANGE_RSA:
@@ -6406,7 +6409,7 @@ int mbedtls_ssl_check_cert_usage(const mbedtls_x509_crt *cert,
         ret = -1;
     }
 
-    if (cert_endpoint == MBEDTLS_SSL_IS_SERVER) {
+    if (recv_endpoint == MBEDTLS_SSL_IS_CLIENT) {
         ext_oid = MBEDTLS_OID_SERVER_AUTH;
         ext_len = MBEDTLS_OID_SIZE(MBEDTLS_OID_SERVER_AUTH);
     } else {
@@ -8061,7 +8064,7 @@ static int ssl_parse_certificate_verify(mbedtls_ssl_context *ssl,
 
     if (mbedtls_ssl_check_cert_usage(chain,
                                      ciphersuite_info,
-                                     !ssl->conf->endpoint,
+                                     ssl->conf->endpoint,
                                      &ssl->session_negotiate->verify_result) != 0) {
         MBEDTLS_SSL_DEBUG_MSG(1, ("bad certificate (usage extensions)"));
         if (ret == 0) {
diff --git a/library/ssl_tls12_server.c b/library/ssl_tls12_server.c
index 81ee6002e1..e250fc03d3 100644
--- a/library/ssl_tls12_server.c
+++ b/library/ssl_tls12_server.c
@@ -756,7 +756,7 @@ static int ssl_pick_cert(mbedtls_ssl_context *ssl,
          * and decrypting with the same RSA key.
          */
         if (mbedtls_ssl_check_cert_usage(cur->cert, ciphersuite_info,
-                                         MBEDTLS_SSL_IS_SERVER, &flags) != 0) {
+                                         MBEDTLS_SSL_IS_CLIENT, &flags) != 0) {
             MBEDTLS_SSL_DEBUG_MSG(3, ("certificate mismatch: "
                                       "(extended) key usage extension"));
             continue;