[v2] net: fix segmentation of forwarding fraglist GRO

[Jakub Kicinski]

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---
net: fix segmentation of forwarding fraglist GRO

This patch adds checks for frag_list presence and protocol consistency
when handling SKB_GSO_FRAGLIST packets in GSO segmentation. This ensures
XLAT-converted packets fall back to skb_segment instead of skb_segment_list.

> diff --git a/net/ipv4/tcp_offload.c b/net/ipv4/tcp_offload.c
> index fdda18b1abda..6c2c10f37f87 100644
> --- a/net/ipv4/tcp_offload.c
> +++ b/net/ipv4/tcp_offload.c
> @@ -107,7 +107,9 @@ static struct sk_buff *tcp4_gso_segment(struct sk_buff *skb,
>  	if (skb_shinfo(skb)->gso_type & SKB_GSO_FRAGLIST) {
>  		struct tcphdr *th = tcp_hdr(skb);
>
> -		if (skb_pagelen(skb) - th->doff * 4 == skb_shinfo(skb)->gso_size)
> +		if ((skb_pagelen(skb) - th->doff * 4 == skb_shinfo(skb)->gso_size) &&
> +		    skb_has_frag_list(skb) &&
> +		    (skb->protocol == skb_shinfo(skb)->frag_list->protocol))
>  			return __tcp4_gso_segment_list(skb, features);

Should tcp6_gso_segment() in net/ipv6/tcpv6_offload.c receive the same
treatment? Looking at that function, it has the identical pattern before
this patch:

    if (skb_shinfo(skb)->gso_type & SKB_GSO_FRAGLIST) {
        struct tcphdr *th = tcp_hdr(skb);

        if (skb_pagelen(skb) - th->doff * 4 == skb_shinfo(skb)->gso_size)
            return __tcp6_gso_segment_list(skb, features);

        skb->ip_summed = CHECKSUM_NONE;
    }

If an IPv4 packet is GRO-aggregated with fraglist and then converted by
XLAT to IPv6, the same protocol mismatch could occur when it reaches
tcp6_gso_segment(). Paolo raised this point in the v1 review:
https://lore.kernel.org/all/aab6c515-12e4-48ca-8220-c0797dae781f@redhat.com/

[ ... ]
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