# Re: \[PATCH net v3 1/8\] net: virtio: validate outer protocol of UDP tunnel GSO packets

- From: sashiko-bot@kernel.org
- Date: 2026-11-06 08:16:40 
- List: virtualization
- Page: https://ratatoskr.run/virtualization/2026/11/17876504
- Thread: https://ratatoskr.run/virtualization/2026/11/17870111/t.md

```text
> virtio_net_hdr_tnl_to_skb() trusts the virtio header to describe a
> UDP tunnel over IPv4 or IPv6, but never checks that the frame actually
> carries such outer headers. Because VIRTIO_NET_HDR_F_NEEDS_CSUM is
> mandatory for tunnel GSO, __virtio_net_hdr_to_skb() skips the flow
> dissector, so virtio_net_hdr_match_proto() is never applied either.
> 
> A guest (or a tap writer) can then inject a frame with an arbitrary
> outer ethertype, for example ETH_P_NSH, with SKB_GSO_UDP_TUNNEL set.
> The GSO stack then runs nsh_gso_segment() on top of the UDP tunnel
> segmentation, which assumes a different header layout. The result is
> segments with skb->data before skb->head, and a host crash in
> skb_under_panic() as soon as a later layer pushes a header:
> 
>   skbuff: skb_under_panic: ... put:4 ... dev:gretap1
>   kernel BUG at net/core/skbuff.c:215!
> [ ... ]
> Co-developed-by: Michael S. Tsirkin <mst@redhat.com>
> Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
> Reviewed-by: Willem de Bruijn <willemb@google.com>
> Acked-by: Michael S. Tsirkin <mst@redhat.com>
> Signed-off-by: Eric Dumazet <edumazet@kernel.org>

Sashiko has reviewed this patch and found no issues.

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20261105115715.1466924-1-edumazet@kernel.org?part=1

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