Assert that we do not exceed the DTLS MTU

Reviewed-by: Paul Dale <pauli@openssl.org>
Reviewed-by: Hugo Landau <hlandau@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/19516)

diff --git a/ssl/statem/statem_dtls.c b/ssl/statem/statem_dtls.c
index 2e71014ef8aced4c4ef7ea014016b993633f75af..4effc584160824c730d018e8af66cf699eb97bfb 100644 (file)
--- a/ssl/statem/statem_dtls.c
+++ b/ssl/statem/statem_dtls.c
@@ -7,6 +7,7 @@
  * https://www.openssl.org/source/license.html
  */
 
+#include <assert.h>
 #include <limits.h>
 #include <string.h>
 #include <stdio.h>
@@ -255,6 +256,16 @@ int dtls1_do_write(SSL_CONNECTION *s, int type)
             if (!ossl_assert(len == written))
                 return -1;
 
+            /*
+             * We should not exceed the MTU size. If compression is in use
+             * then the max record overhead calculation is unreliable so we do
+             * not check in that case. We use assert rather than ossl_assert
+             * because in a production build, if this assert were ever to fail,
+             * then the best thing to do is probably carry on regardless.
+             */
+            assert(s->s3.tmp.new_compression != NULL
+                   || BIO_wpending(s->wbio) <= (int)s->d1->mtu);
+
             if (type == SSL3_RT_HANDSHAKE && !s->d1->retransmitting) {
                 /*
                  * should not be done for 'Hello Request's, but in that case