virtio-net: fix guest-triggerable buffer overrun

When VM guest programs multicast addresses for
a virtio net card, it supplies a 32 bit
entries counter for the number of addresses.
These addresses are read into tail portion of
a fixed macs array which has size MAC_TABLE_ENTRIES,
at offset equal to in_use.

To avoid overflow of this array by guest, qemu attempts
to test the size as follows:
-    if (in_use + mac_data.entries <= MAC_TABLE_ENTRIES) {

however, as mac_data.entries is uint32_t, this sum
can overflow, e.g. if in_use is 1 and mac_data.entries
is 0xffffffff then in_use + mac_data.entries will be 0.

Qemu will then read guest supplied buffer into this
memory, overflowing buffer on heap.

CVE-2014-0150

Cc: qemu-stable@nongnu.org
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>
Message-id: 1397218574-25058-1-git-send-email-mst@redhat.com
Reviewed-by: NMichael Tokarev <mjt@tls.msk.ru>
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>

hw/net/virtio-net.c

@@ -677,7 +677,7 @@ static int virtio_net_handle_mac(VirtIONet *n, uint8_t cmd,
         goto error;
     }
 
-    if (in_use + mac_data.entries <= MAC_TABLE_ENTRIES) {
+    if (mac_data.entries <= MAC_TABLE_ENTRIES - in_use) {
         s = iov_to_buf(iov, iov_cnt, 0, &macs[in_use * ETH_ALEN],
                        mac_data.entries * ETH_ALEN);
         if (s != mac_data.entries * ETH_ALEN) {