slirp: udp: fix NULL pointer dereference because of uninitialized socket

When guest sends udp packet with source port and source addr 0,
uninitialized socket is picked up when looking for matching and already
created udp sockets, and later passed to sosendto() where NULL pointer
dereference is hit during so->slirp->vnetwork_mask.s_addr access.

Fix this by checking that the socket is not just a socket stub.

This is CVE-2014-3640.
Signed-off-by: NPetr Matousek <pmatouse@redhat.com>
Reported-by: NXavier Mehrenberger <xavier.mehrenberger@airbus.com>
Reported-by: NStephane Duverger <stephane.duverger@eads.net>
Reviewed-by: NJan Kiszka <jan.kiszka@siemens.com>
Reviewed-by: NMichael S. Tsirkin <mst@redhat.com>
Reviewed-by: NMichael Tokarev <mjt@tls.msk.ru>
Message-id: 20140918063537.GX9321@dhcp-25-225.brq.redhat.com
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>
(cherry picked from commit 01f7cecf)
Signed-off-by: NMichael Roth <mdroth@linux.vnet.ibm.com>

slirp/udp.c

@@ -152,7 +152,7 @@ udp_input(register struct mbuf *m, int iphlen)
 	 * Locate pcb for datagram.
 	 */
 	so = slirp->udp_last_so;
-	if (so->so_lport != uh->uh_sport ||
+	if (so == &slirp->udb || so->so_lport != uh->uh_sport ||
 	    so->so_laddr.s_addr != ip->ip_src.s_addr) {
 		struct socket *tmp;