It seems virtio_net could use more sanity checks. When PACKET_VNET_HDR
is used, it will end up calling:
tpacket_rcv() {
...
        if (do_vnet) {
                if (virtio_net_hdr_from_skb(skb, h.raw + macoff -
                                            sizeof(struct virtio_net_hdr),
                                            vio_le(), true)) {
                        spin_lock(&sk->sk_receive_queue.lock);
                        goto drop_n_account;
                }
        }

and virtio_net_hdr_from_skb does:
        if (skb_is_gso(skb)) {
...
                if (sinfo->gso_type & SKB_GSO_TCPV4)
                        hdr->gso_type = VIRTIO_NET_HDR_GSO_TCPV4;
                else if (sinfo->gso_type & SKB_GSO_TCPV6)
                        hdr->gso_type = VIRTIO_NET_HDR_GSO_TCPV6;
                else
                        return -EINVAL;

That is the receive path, but the send path is analogous. Just adds
UFO.

Meaning that any gso_type other than TCP would be rejected, but this
SCTP one got through. Seems the header contains a sctp header, but the
gso_type set was actually pointing to TCP (otherwise it would have
been rejected). AFAICT if this packet had an ESP header, for example,
it could have hit esp4_gso_segment. Can you please confirm this?

I have not tested this yet, but it certainly seems plausible.

There is nothing ensuring consistency between gso_type and
the actual packet contents that are parsed to look up gso callbacks.

I don't know of anywhere in the stack validating if the gso_type
matches the header that actually is in there.

The fix you mentioned is a good start, we want that one way or
another, but I'm afraid this bug is bigger than sctp.

Good point. Packet sockets require CAP_NET_RAW, but this is also
taken for virtio, so we probably want more stringent entry tests here.

The alternative to harden the segmentation code itself with a gso_type
sanity check in every gso callback is more work and fragile.

Need to figure out whether a brief check for just TCP or UDP is sufficient
or we need a full flow dissector step to support tunnel headers and such.