Check for alerts while waiting for a dry event

At a couple of points in a DTLS/SCTP handshake we need to wait for a dry
event before continuing. However if an alert has been sent by the peer
then we will never receive that dry event and an infinite loop results.

This commit changes things so that we attempt to read a message if we
are waiting for a dry event but haven't got one yet. This should never
succeed, but any alerts will be processed.

Fixes #4763
Reviewed-by: NRich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/5084)

ssl/statem/statem_dtls.c

@@ -922,9 +922,14 @@ int dtls_construct_change_cipher_spec(SSL *s, WPACKET *pkt)
 }
 
 #ifndef OPENSSL_NO_SCTP
+/*
+ * Wait for a dry event. Should only be called at a point in the handshake
+ * where we are not expecting any data from the peer except an alert.
+ */
 WORK_STATE dtls_wait_for_dry(SSL *s)
 {
-    int ret;
+    int ret, errtype;
+    size_t len;
 
     /* read app data until dry event */
     ret = BIO_dgram_sctp_wait_for_dry(SSL_get_wbio(s));
@@ -935,6 +940,19 @@ WORK_STATE dtls_wait_for_dry(SSL *s)
     }
 
     if (ret == 0) {
+        /*
+         * We're not expecting any more messages from the peer at this point -
+         * but we could get an alert. If an alert is waiting then we will never
+         * return successfully. Therefore we attempt to read a message. This
+         * should never succeed but will process any waiting alerts.
+         */
+        if (dtls_get_reassembled_message(s, &errtype, &len)) {
+            /* The call succeeded! This should never happen */
+            SSLfatal(s, SSL_AD_UNEXPECTED_MESSAGE, SSL_F_DTLS_WAIT_FOR_DRY,
+                     SSL_R_UNEXPECTED_MESSAGE);
+            return WORK_ERROR;
+        }
+
         s->s3->in_read_app_data = 2;
         s->rwstate = SSL_READING;
         BIO_clear_retry_flags(SSL_get_rbio(s));