rtl8139: Fix receive buffer overflow check

rtl8139_do_receive() tries to check for the overflow condition
by making sure that packet_size + 8 does not exceed the
available buffer space.  The issue here is that RxBuffAddr,
used to calculate available buffer space, is aligned to a
a 4 byte boundry after every update.  So it is possible that
every packet ends up being slightly padded when written
to the receive buffer.  This padding is not taken into
account when checking for overflow and we may end up missing
the overflow condition can causing buffer overwrite.

This patch takes alignment into consideration when
checking for overflow condition.

Signed-off-by: Vladislav Yasevich <vyasevic@redhat.com>
Reviewed-by: Jason Wang <jasowang@redhat.com>
Message-id: 1441121206-6997-2-git-send-email-vyasevic@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

hw/net/rtl8139.c

@@ -1146,7 +1146,9 @@ static ssize_t rtl8139_do_receive(NetClientState *nc, const uint8_t *buf, size_t
 
         /* if receiver buffer is empty then avail == 0 */
 
-        if (avail != 0 && size + 8 >= avail)
+#define RX_ALIGN(x) (((x) + 3) & ~0x3)
+
+        if (avail != 0 && RX_ALIGN(size + 8) >= avail)
         {
             DPRINTF("rx overflow: rx buffer length %d head 0x%04x "
                 "read 0x%04x === available 0x%04x need 0x%04x\n",
@@ -1174,7 +1176,7 @@ static ssize_t rtl8139_do_receive(NetClientState *nc, const uint8_t *buf, size_t
         rtl8139_write_buffer(s, (uint8_t *)&val, 4);
 
         /* correct buffer write pointer */
-        s->RxBufAddr = MOD2((s->RxBufAddr + 3) & ~0x3, s->RxBufferSize);
+        s->RxBufAddr = MOD2(RX_ALIGN(s->RxBufAddr), s->RxBufferSize);
 
         /* now we can signal we have received something */