Re: [PATCH net] ip_gre: validate csum_start only on pull

[Ido Schimmel <idosch@idosch.org>]

On Sun, Sep 05, 2021 at 11:21:09AM -0400, Willem de Bruijn wrote:
> From: Willem de Bruijn <willemb@google.com>
> 
> The GRE tunnel device can pull existing outer headers in ipge_xmit.
> This is a rare path, apparently unique to this device. The below
> commit ensured that pulling does not move skb->data beyond csum_start.
> 
> But it has a false positive if ip_summed is not CHECKSUM_PARTIAL and
> thus csum_start is irrelevant.
> 
> Refine to exclude this. At the same time simplify and strengthen the
> test.
> 
> Simplify, by moving the check next to the offending pull, making it
> more self documenting and removing an unnecessary branch from other
> code paths.
> 
> Strengthen, by also ensuring that the transport header is correct and
> therefore the inner headers will be after skb_reset_inner_headers.
> The transport header is set to csum_start in skb_partial_csum_set.
> 
> Link: https://lore.kernel.org/netdev/YS+h%2FtqCJJiQei+W@shredder/
> Fixes: 1d011c4803c7 ("ip_gre: add validation for csum_start")
> Reported-by: Ido Schimmel <idosch@idosch.org>
> Suggested-by: Alexander Duyck <alexander.duyck@gmail.com>
> Signed-off-by: Willem de Bruijn <willemb@google.com>

FTR:

Tested-by: Ido Schimmel <idosch@nvidia.com>

Thanks