CVE-2024-49979

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 skbsegment instead of skbsegment_list, as the first can segment them correctly.

Valid SKBGSOFRAGLIST skbs - consist of two or more segments - the headskb holds the protocol headers plus first gsosize - one or more fraglist skbs hold exactly one segment - all but the last must be gsosize

Optional datapath hooks such as NAT and BPF (bpfskbpull_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 _tcpv4gsosegmentlistcsum at tcphdr(seg->next).

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

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