Fix overread when verifying SERVER_HELLO in DTLS

ChangeLog

@@ -29,6 +29,8 @@ Bugfix
      a contribution from Tobias Tangemann. #541
    * Fixed cert_app sample program for debug output and for use when no root
      certificates are provided.
+   * Fix potential byte overread when verifying malformed SERVER_HELLO in
+     ssl_parse_hello_verify_request() for DTLS. Found by Guido Vranken.
 
 Changes
    * Extended test coverage of special cases, and added new timing test suite.

library/ssl_cli.c

@@ -1355,6 +1355,15 @@ static int ssl_parse_hello_verify_request( mbedtls_ssl_context *ssl )
     cookie_len = *p++;
     MBEDTLS_SSL_DEBUG_BUF( 3, "cookie", p, cookie_len );
 
+    if( ( ssl->in_msg + ssl->in_msglen ) - p < cookie_len )
+    {
+        MBEDTLS_SSL_DEBUG_MSG( 1,
+            ( "cookie length does not match incoming message size" ) );
+        mbedtls_ssl_send_alert_message( ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
+                                    MBEDTLS_SSL_ALERT_MSG_DECODE_ERROR );
+        return( MBEDTLS_ERR_SSL_BAD_HS_SERVER_HELLO );
+    }
+
     mbedtls_free( ssl->handshake->verify_cookie );
 
     ssl->handshake->verify_cookie = mbedtls_calloc( 1, cookie_len );