gso: fix udp gso fraglist segmentation after pull from frag_list

gso: fix udp gso fraglist segmentation after pull from frag_list

Detect 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 UDP, this
causes a NULL ptr deref in __udpv4_gso_segment_list_csum at
udp_hdr(seg->next)->dest.

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.

Link: https://lore.kernel.org/netdev/[email protected]/
Fixes: 9fd1ff5d2ac7 ("udp: Support UDP fraglist GRO/GSO.")
Signed-off-by: Willem de Bruijn <[email protected]>
Cc: [email protected]
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Jakub Kicinski <[email protected]>