virtio-net: Fix size check in dhclient workaround

work_around_broken_dhclient() accesses IP and UDP headers to detect
relevant packets and to calculate checksums, but it didn't check if
the packet has size sufficient to accommodate them, causing out-of-bound
access hazards. Fix this by correcting the size requirement.

Fixes: 1d41b0c1ec66 ("Work around dhclient brokenness")
Cc: qemu-stable@nongnu.org
Signed-off-by: Akihiko Odaki <akihiko.odaki@daynix.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>

diff --git a/hw/net/virtio-net.c b/hw/net/virtio-net.c
index 75b4a28fb3ae5ad245ba07edceb45fe7768b73a9..a2a8d6b07bcce4596baf7d820ee03a513abdc24b 100644 (file)
--- a/hw/net/virtio-net.c
+++ b/hw/net/virtio-net.c
@@ -1697,8 +1697,11 @@ static void virtio_net_hdr_swap(VirtIODevice *vdev, struct virtio_net_hdr *hdr)
 static void work_around_broken_dhclient(struct virtio_net_hdr *hdr,
                                         uint8_t *buf, size_t size)
 {
+    size_t csum_size = ETH_HLEN + sizeof(struct ip_header) +
+                       sizeof(struct udp_header);
+
     if ((hdr->flags & VIRTIO_NET_HDR_F_NEEDS_CSUM) && /* missing csum */
-        (size > 27 && size < 1500) && /* normal sized MTU */
+        (size >= csum_size && size < 1500) && /* normal sized MTU */
         (buf[12] == 0x08 && buf[13] == 0x00) && /* ethertype == IPv4 */
         (buf[23] == 17) && /* ip.protocol == UDP */
         (buf[34] == 0 && buf[35] == 67)) { /* udp.srcport == bootps */