Re: [PATCH net] ip_gre: test csum_start instead of transport header

[Jakub Kicinski <kuba@kernel.org>]

On Mon,  6 Jun 2022 09:21:07 -0400 Willem de Bruijn wrote:
> From: Willem de Bruijn <willemb@google.com>
> 
> GRE with TUNNEL_CSUM will apply local checksum offload on
> CHECKSUM_PARTIAL packets.
> 
> ipgre_xmit must validate csum_start after an optional skb_pull,
> else lco_csum may trigger an overflow. The original check was
> 
> 	if (csum && skb_checksum_start(skb) < skb->data)
> 		return -EINVAL;
> 
> This had false positives when skb_checksum_start is undefined:
> when ip_summed is not CHECKSUM_PARTIAL. A discussed refinement
> was straightforward
> 
> 	if (csum && skb->ip_summed == CHECKSUM_PARTIAL &&
> 	    skb_checksum_start(skb) < skb->data)
> 		return -EINVAL;
> 
> But was eventually revised more thoroughly:
> - restrict the check to the only branch where needed, in an
>   uncommon GRE path that uses header_ops and calls skb_pull.
> - test skb_transport_header, which is set along with csum_start
>   in skb_partial_csum_set in the normal header_ops datapath.
> 
> Turns out skbs can arrive in this branch without the transport
> header set, e.g., through BPF redirection.
> 
> Revise the check back to check csum_start directly, and only if
> CHECKSUM_PARTIAL. Do leave the check in the updated location.
> Check field regardless of whether TUNNEL_CSUM is configured.
> 
> Link: https://lore.kernel.org/netdev/YS+h%2FtqCJJiQei+W@shredder/
> Link: https://lore.kernel.org/all/20210902193447.94039-2-willemdebruijn.kernel@gmail.com/T/#u
> Fixes: 8a0ed250f911 ("ip_gre: validate csum_start only on pull")
> Reported-by: syzbot <syzkaller@googlegroups.com>
> Signed-off-by: Willem de Bruijn <willemb@google.com>

Missing a CC for Alex, adding now.