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

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Date:   Tue, 7 Jun 2022 10:33:24 -0700
From:   Eric Dumazet <edumazet@...gle.com>
To:     Willem de Bruijn <willemdebruijn.kernel@...il.com>
Cc:     netdev <netdev@...r.kernel.org>,
        David Miller <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Willem de Bruijn <willemb@...gle.com>,
        syzbot <syzkaller@...glegroups.com>
Subject: Re: [PATCH net] ip_gre: test csum_start instead of transport header

On Mon, Jun 6, 2022 at 6:21 AM Willem de Bruijn
<willemdebruijn.kernel@...il.com> wrote:
>
> From: Willem de Bruijn <willemb@...gle.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@...glegroups.com>
> Signed-off-by: Willem de Bruijn <willemb@...gle.com>

Reviewed-by: Eric Dumazet <edumazet@...gle.com>