net: mipsnet: check transmit buffer size before sending

When processing MIPSnet I/O port write operation, it uses a
transmit buffer tx_buffer[MAX_ETH_FRAME_SIZE=1514]. Two indices
's->tx_written' and 's->tx_count' are used to control data written
to this buffer. If the two were to be equal before writing, it'd
lead to an OOB write access beyond tx_buffer. Add check to avoid it.

Reported-by: Li Qiang <qiang6-s@360.cn>
Signed-off-by: Prasad J Pandit <pjp@fedoraproject.org>
Signed-off-by: Jason Wang <jasowang@redhat.com>

hw/net/mipsnet.c

@@ -183,10 +183,12 @@ static void mipsnet_ioport_write(void *opaque, hwaddr addr,
         break;
     case MIPSNET_TX_DATA_BUFFER:
         s->tx_buffer[s->tx_written++] = val;
-        if (s->tx_written == s->tx_count) {
+        if ((s->tx_written >= MAX_ETH_FRAME_SIZE)
+            || (s->tx_written == s->tx_count)) {
             /* Send buffer. */
-            trace_mipsnet_send(s->tx_count);
-            qemu_send_packet(qemu_get_queue(s->nic), s->tx_buffer, s->tx_count);
+            trace_mipsnet_send(s->tx_written);
+            qemu_send_packet(qemu_get_queue(s->nic),
+                                s->tx_buffer, s->tx_written);
             s->tx_count = s->tx_written = 0;
             s->intctl |= MIPSNET_INTCTL_TXDONE;
             s->busy = 1;