net: pcnet: add check to validate receive data size(CVE-2015-7504)

In loopback mode, pcnet_receive routine appends CRC code to the
receive buffer. If the data size given is same as the buffer size,
the appended CRC code overwrites 4 bytes after s->buffer. Added a
check to avoid that.

Reported by: Qinghao Tang <luodalongde@gmail.com>
Cc: qemu-stable@nongnu.org
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Prasad J Pandit <pjp@fedoraproject.org>
Signed-off-by: Jason Wang <jasowang@redhat.com>

hw/net/pcnet.c

@@ -1084,7 +1084,7 @@ ssize_t pcnet_receive(NetClientState *nc, const uint8_t *buf, size_t size_)
                 uint32_t fcs = ~0;
                 uint8_t *p = src;
 
-                while (p != &src[size-4])
+                while (p != &src[size])
                     CRC(fcs, *p++);
                 crc_err = (*(uint32_t *)p != htonl(fcs));
             }
@@ -1233,8 +1233,10 @@ static void pcnet_transmit(PCNetState *s)
         bcnt = 4096 - GET_FIELD(tmd.length, TMDL, BCNT);
 
         /* if multi-tmd packet outsizes s->buffer then skip it silently.
-           Note: this is not what real hw does */
-        if (s->xmit_pos + bcnt > sizeof(s->buffer)) {
+         * Note: this is not what real hw does.
+         * Last four bytes of s->buffer are used to store CRC FCS code.
+         */
+        if (s->xmit_pos + bcnt > sizeof(s->buffer) - 4) {
             s->xmit_pos = -1;
             goto txdone;
         }