Don't memcpy the contents of an empty fragment

In DTLS if we have buffered a fragment for a zero length message (e.g.
ServerHelloDone) then, when we unbuffered the fragment, we were attempting
to memcpy the contents of the fragment which is zero length and a NULL
pointer. This is undefined behaviour. We should check first whether we
have a zero length fragment.

Fixes a travis issue.

[extended tests]

Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/6225)

diff --git a/ssl/d1_both.c b/ssl/d1_both.c
index e6bc761e8bf236568c35da7f949c3dd9d77b4ccc..8cf52fac8b1101a8d175455bd084e9db7dbe07f1 100644 (file)
--- a/ssl/d1_both.c
+++ b/ssl/d1_both.c
@@ -656,7 +656,8 @@ static int dtls1_retrieve_buffered_fragment(SSL *s, long max, int *ok)
 
         al = dtls1_preprocess_fragment(s, &frag->msg_header, max);
 
-        if (al == 0) {          /* no alert */
+        /* al will be 0 if no alert */
+        if (al == 0  && frag->msg_header.frag_len > 0) {
             unsigned char *p =
                 (unsigned char *)s->init_buf->data + DTLS1_HM_HEADER_LENGTH;
             memcpy(&p[frag->msg_header.frag_off], frag->fragment,