net: gso: fix tcp fraglist segmentation after pull from frag_list

In the Linux kernel, the following vulnerability has been resolved:

net: gso: fix tcp fraglist segmentation after pull from frag_list

Detect tcp gso fraglist skbs with corrupted geometry (see below) and pass these to skb_segment instead of skb_segment_list, as the first can segment them correctly.

Valid SKB_GSO_FRAGLIST skbs - consist of two or more segments - the head_skb holds the protocol headers plus first gso_size - one or more frag_list skbs hold exactly one segment - all but the last must be gso_size

Optional datapath hooks such as NAT and BPF (bpf_skb_pull_data) can modify these skbs, breaking these invariants.

In extreme cases they pull all data into skb linear. For TCP, this causes a NULL ptr deref in __tcpv4_gso_segment_list_csum at tcp_hdr(seg->next).

Detect invalid geometry due to pull, by checking head_skb size. Don't just drop, as this may blackhole a destination. Convert to be able to pass to regular skb_segment.

Approach and description based on a patch by Willem de Bruijn.