linux-kernel - Re: [PATCH] ip_gre/ip6_gre: add check for invalid csum

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Date:   Thu, 19 Aug 2021 12:56:59 -0400
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     Shreyansh Chouhan <chouhan.shreyansh630@...il.com>
Cc:     davem@...emloft.net, yoshfuji@...ux-ipv6.org, dsahern@...nel.org,
        kuba@...nel.org, willemdebruijn.kernel@...il.com,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        syzbot+ff8e1b9f2f36481e2efc@...kaller.appspotmail.com
Subject: Re: [PATCH] ip_gre/ip6_gre: add check for invalid csum_start

On Thu, Aug 19, 2021 at 10:35 AM Shreyansh Chouhan
<chouhan.shreyansh630@...il.com> wrote:
>
> If we get a ip gre packet with TUNNEL_CSUM set, an invalid csum_start
> value causes skb->csum_start offset to be less than the offset for
> skb->data after we pull the ip header from the packet during the
> ipgre_xmit call.
>
> This patch adds a sanity check to gre_handle_offloads, which checks the
> validity of skb->csum_start after we have pulled the ip header from the
> packet in the ipgre_xmit call.
>
> Reported-by: syzbot+ff8e1b9f2f36481e2efc@...kaller.appspotmail.com
> Signed-off-by: Shreyansh Chouhan <chouhan.shreyansh630@...il.com>

For the ipv4 portion:

Fixes: c54419321455 ("GRE: Refactor GRE tunneling code.")

For the ipv6 portion:

Fixes: b05229f44228 ("gre6: Cleanup GREv6 transmit path, call common
GRE functions")

It's possible that a similar bug exists before those, but the patch
wouldn't apply anyway.

Technically, for backporting purposes, the patch needs to be split
into two, each with their own Fixes tag. And target [PATCH net]