Fix mbedtls_ssl_get_record_expansion() for CBC modes

`mbedtls_ssl_get_record_expansion()` is supposed to return the maximum
difference between the size of a protected record and the size of the
encapsulated plaintext.

Previously, it did not correctly estimate the maximum record expansion
in case of CBC ciphersuites in (D)TLS versions 1.1 and higher, in which
case the ciphertext is prefixed by an explicit IV.

This commit fixes this bug. Fixes #1914.

library/ssl_tls.c

@@ -6686,6 +6686,7 @@ int mbedtls_ssl_get_record_expansion( const mbedtls_ssl_context *ssl )
 {
     size_t transform_expansion;
     const mbedtls_ssl_transform *transform = ssl->transform_out;
+    unsigned block_size;
 
 #if defined(MBEDTLS_ZLIB_SUPPORT)
     if( ssl->session_out->compression != MBEDTLS_SSL_COMPRESS_NULL )
@@ -6704,8 +6705,27 @@ int mbedtls_ssl_get_record_expansion( const mbedtls_ssl_context *ssl )
             break;
 
         case MBEDTLS_MODE_CBC:
-            transform_expansion = transform->maclen
+
-                      + mbedtls_cipher_get_block_size( &transform->cipher_ctx_enc );
+            block_size = mbedtls_cipher_get_block_size(
+                &transform->cipher_ctx_enc );
+
+#if defined(MBEDTLS_SSL_PROTO_TLS1_1) || defined(MBEDTLS_SSL_PROTO_TLS1_2)
+            if( ssl->minor_ver >= MBEDTLS_SSL_MINOR_VERSION_2 )
+            {
+                /* Expansion due to addition of
+                 * - MAC
+                 * - CBC padding (theoretically up to 256 bytes, but
+                 *                we never use more than block_size)
+                 * - explicit IV
+                 */
+                transform_expansion = transform->maclen + 2 * block_size;
+            }
+            else
+#endif /* MBEDTLS_SSL_PROTO_TLS1_1 || MBEDTLS_SSL_PROTO_TLS1_2 */
+            {
+                /* No explicit IV prior to TLS 1.1. */
+                transform_expansion = transform->maclen + block_size;
+            }
             break;
 
         default: