pcnet: fix rx buffer overflow(CVE-2015-7512)

Backends could provide a packet whose length is greater than buffer
size. Check for this and truncate the packet to avoid rx buffer
overflow in this case.

Cc: Prasad J Pandit <pjp@fedoraproject.org>
Cc: qemu-stable@nongnu.org
Signed-off-by: Jason Wang <jasowang@redhat.com>
[BR: BSC#957162]
Signed-off-by: Bruce Rogers <brogers@suse.com>

hw/net/pcnet.c

@@ -1086,6 +1086,12 @@ ssize_t pcnet_receive(NetClientState *nc, const uint8_t *buf, size_t size_)
             int pktcount = 0;
 
             if (!s->looptest) {
+                if (size > 4092) {
+#ifdef PCNET_DEBUG_RMD
+                    fprintf(stderr, "pcnet: truncates rx packet.\n");
+#endif
+                    size = 4092;
+                }
                 memcpy(src, buf, size);
                 /* no need to compute the CRC */
                 src[size] = 0;