[Stable-9.1.3 05/58] virtio-net: Fix size check in dhclient workaround

[Michael Tokarev]

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>
(cherry picked from commit a8575f7fb2f213e6690b23160b04271d47fdfaa8)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>

diff --git a/hw/net/virtio-net.c b/hw/net/virtio-net.c
index ed33a32877..cf27d5cbe5 100644
--- a/hw/net/virtio-net.c
+++ b/hw/net/virtio-net.c
@@ -1692,8 +1692,11 @@ 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 */
-- 
2.39.5