client: Handle the MSG_EXT_INFO packet signalling supported extensions

RFC 8308: The extension negotiation in Secure Shell (SSH) Protocol

RFC 8332: Use of RSA Keys with SHA-256 and SHA-512
          in the Secure Shell (SSH) Protocol

Signed-off-by: Jakub Jelen <jjelen@redhat.com>
Reviewed-by: Andreas Schneider <asn@cryptomilk.org>

include/libssh/packet.h

@@ -51,6 +51,7 @@ SSH_PACKET_CALLBACK(ssh_packet_ignore_callback);
 SSH_PACKET_CALLBACK(ssh_packet_dh_reply);
 SSH_PACKET_CALLBACK(ssh_packet_newkeys);
 SSH_PACKET_CALLBACK(ssh_packet_service_accept);
+SSH_PACKET_CALLBACK(ssh_packet_ext_info);
 
 #ifdef WITH_SERVER
 SSH_PACKET_CALLBACK(ssh_packet_kexdh_init);

include/libssh/session.h

@@ -86,6 +86,11 @@ enum ssh_pending_call_e {
 #define SSH_OPT_FLAG_KBDINT_AUTH 0x4
 #define SSH_OPT_FLAG_GSSAPI_AUTH 0x8
 
+/* extensions flags */
+/* server-sig-algs extension */
+#define SSH_EXT_SIG_RSA_SHA256  0x01
+#define SSH_EXT_SIG_RSA_SHA512  0x02
+
 /* members that are common to ssh_session and ssh_bind */
 struct ssh_common_struct {
     struct error_struct error;
@@ -114,6 +119,9 @@ struct ssh_session_struct {
     /* session flags (SSH_SESSION_FLAG_*) */
     int flags;
 
+    /* Extensions negotiated using RFC 8308 */
+    uint32_t extensions;
+
     ssh_string banner; /* that's the issue banner from
                        the server */
     char *discon_msg; /* disconnect message from

include/libssh/ssh2.h

@@ -7,6 +7,7 @@
 #define SSH2_MSG_DEBUG	4
 #define SSH2_MSG_SERVICE_REQUEST	5
 #define SSH2_MSG_SERVICE_ACCEPT 6
+#define SSH2_MSG_EXT_INFO 7
 
 #define SSH2_MSG_KEXINIT	 20
 #define SSH2_MSG_NEWKEYS 21

src/packet.c

@@ -59,8 +59,9 @@ static ssh_packet_callback default_packet_handlers[]= {
   NULL,
 #endif
   ssh_packet_service_accept,               // SSH2_MSG_SERVICE_ACCEPT             6
-  NULL, NULL, NULL, NULL, NULL, NULL, NULL,
+  ssh_packet_ext_info,                     // SSH2_MSG_EXT_INFO                   7
-  NULL, NULL, NULL,	NULL, NULL, NULL,      //                                     7-19
+  NULL, NULL, NULL, NULL, NULL, NULL,
+  NULL, NULL, NULL, NULL, NULL, NULL,      //                                     8-19
   ssh_packet_kexinit,                      // SSH2_MSG_KEXINIT	                  20
   ssh_packet_newkeys,                      // SSH2_MSG_NEWKEYS                    21
   NULL, NULL, NULL, NULL, NULL, NULL, NULL,

src/packet_cb.c

@@ -270,3 +270,55 @@ SSH_PACKET_CALLBACK(ssh_packet_service_accept){
 
 	return SSH_PACKET_USED;
 }
+
+/**
+ * @internal
+ * @brief handles a SSH2_MSG_EXT_INFO packet defined in RFC 8308
+ *
+ */
+SSH_PACKET_CALLBACK(ssh_packet_ext_info)
+{
+    int rc;
+    uint32_t nr_extensions = 0;
+    uint32_t i;
+    (void)type;
+    (void)user;
+
+    SSH_LOG(SSH_LOG_PACKET, "Received SSH_MSG_EXT_INFO");
+
+    rc = ssh_buffer_get_u32(packet, &nr_extensions);
+    if (rc == 0) {
+        SSH_LOG(SSH_LOG_PACKET, "Failed to read number of extensions");
+        return SSH_PACKET_USED;
+    }
+    nr_extensions = ntohl(nr_extensions);
+    SSH_LOG(SSH_LOG_PACKET, "Follows %u extensions", nr_extensions);
+
+    for (i = 0; i < nr_extensions; i++) {
+        char *name = NULL;
+        char *value = NULL;
+        int cmp;
+
+        rc = ssh_buffer_unpack(packet, "ss", &name, &value);
+        if (rc != SSH_OK) {
+            SSH_LOG(SSH_LOG_PACKET, "Error reading extension name-value pair");
+            return SSH_PACKET_USED;
+        }
+
+        cmp = strcmp(name, "server-sig-algs");
+        if (cmp == 0) {
+            /* TODO check for NULL bytes */
+            SSH_LOG(SSH_LOG_PACKET, "Extension: %s=<%s>", name, value);
+            if (ssh_match_group(value, "rsa-sha2-512")) {
+                session->extensions |= SSH_EXT_SIG_RSA_SHA512;
+            }
+            if (ssh_match_group(value, "rsa-sha2-256")) {
+                session->extensions |= SSH_EXT_SIG_RSA_SHA256;
+            }
+        }
+        free(name);
+        free(value);
+    }
+
+    return SSH_PACKET_USED;
+}