Re: [PATCH net] ipv4: ip_gre: Handle skb_pull() failure in ipgre_xmit()

[Eric Dumazet <edumazet@google.com>]

On Sun, Nov 26, 2023 at 4:17 PM Shigeru Yoshida <syoshida@redhat.com> wrote:
>
> In ipgre_xmit(), skb_pull() may fail even if pskb_inet_may_pull() returns
> true. For example, applications can create a malformed packet that causes
> this problem with PF_PACKET.
>
> This patch fixes the problem by dropping skb and returning from the
> function if skb_pull() fails.
>
> Fixes: c54419321455 ("GRE: Refactor GRE tunneling code.")
> Signed-off-by: Shigeru Yoshida <syoshida@redhat.com>
> ---
>  net/ipv4/ip_gre.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/net/ipv4/ip_gre.c b/net/ipv4/ip_gre.c
> index 22a26d1d29a0..95efa97cb84b 100644
> --- a/net/ipv4/ip_gre.c
> +++ b/net/ipv4/ip_gre.c
> @@ -643,7 +643,8 @@ static netdev_tx_t ipgre_xmit(struct sk_buff *skb,
>                 /* Pull skb since ip_tunnel_xmit() needs skb->data pointing
>                  * to gre header.
>                  */
> -               skb_pull(skb, tunnel->hlen + sizeof(struct iphdr));
> +               if (!skb_pull(skb, tunnel->hlen + sizeof(struct iphdr)))
> +                       goto free_skb;
>                 skb_reset_mac_header(skb);
>
>                 if (skb->ip_summed == CHECKSUM_PARTIAL &&
> --


I have syszbot reports with an actual repro for this one.

I do not think your patch is correct, something should be fixed
earlier (before we hit this point)