netdev - Re: [PATCH net] ip_gre: validate csum

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Date:   Sun, 5 Sep 2021 08:47:16 -0700
From:   Alexander Duyck <alexander.duyck@...il.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>,
        Ido Schimmel <idosch@...sch.org>,
        chouhan.shreyansh630@...il.com,
        Willem de Bruijn <willemb@...gle.com>
Subject: Re: [PATCH net] ip_gre: validate csum_start only on pull

On Sun, Sep 5, 2021 at 8:21 AM Willem de Bruijn
<willemdebruijn.kernel@...il.com> wrote:
>
> From: Willem de Bruijn <willemb@...gle.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@...sch.org>
> Suggested-by: Alexander Duyck <alexander.duyck@...il.com>
> Signed-off-by: Willem de Bruijn <willemb@...gle.com>
> ---
>  net/ipv4/ip_gre.c | 9 ++++++---
>  1 file changed, 6 insertions(+), 3 deletions(-)
>
> diff --git a/net/ipv4/ip_gre.c b/net/ipv4/ip_gre.c
> index 177d26d8fb9c..0fe6c936dc54 100644
> --- a/net/ipv4/ip_gre.c
> +++ b/net/ipv4/ip_gre.c
> @@ -473,8 +473,6 @@ static void __gre_xmit(struct sk_buff *skb, struct net_device *dev,
>
>  static int gre_handle_offloads(struct sk_buff *skb, bool csum)
>  {
> -       if (csum && skb_checksum_start(skb) < skb->data)
> -               return -EINVAL;
>         return iptunnel_handle_offloads(skb, csum ? SKB_GSO_GRE_CSUM : SKB_GSO_GRE);
>  }
>
> @@ -632,15 +630,20 @@ static netdev_tx_t ipgre_xmit(struct sk_buff *skb,
>         }
>
>         if (dev->header_ops) {
> +               const int pull_len = tunnel->hlen + sizeof(struct iphdr);
> +
>                 if (skb_cow_head(skb, 0))
>                         goto free_skb;
>
>                 tnl_params = (const struct iphdr *)skb->data;
>
> +               if (pull_len > skb_transport_offset(skb))
> +                       goto free_skb;
> +
>                 /* Pull skb since ip_tunnel_xmit() needs skb->data pointing
>                  * to gre header.
>                  */
> -               skb_pull(skb, tunnel->hlen + sizeof(struct iphdr));
> +               skb_pull(skb, pull_len);
>                 skb_reset_mac_header(skb);
>         } else {
>                 if (skb_cow_head(skb, dev->needed_headroom))
> --
> 2.33.0.153.gba50c8fa24-goog
>

Looks good to me.

Reviewed-by: Alexander Duyck <alexanderduyck@...com>