Re: [PATCH net-next] net/mpls: fix WARNING in mpls_gso_segment

[Eric Dumazet]

On Thu, Feb 22, 2024 at 5:00 AM Lizhi Xu <lizhi.xu@xxxxxxxxxxxxx> wrote:
>
> When the network header pointer is greater than the inner network header, the
> difference between the two can cause mpls_hlen overflow.
>
> Reported-and-tested-by: syzbot+99d15fcdb0132a1e1a82@syzkaller.appspotmailcom
> Signed-off-by: Lizhi Xu <lizhi.xu@xxxxxxxxxxxxx>
> ---
>  net/mpls/mpls_gso.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/net/mpls/mpls_gso.c b/net/mpls/mpls_gso.c
> index 533d082f0701..2ab24b2fd90f 100644
> --- a/net/mpls/mpls_gso.c
> +++ b/net/mpls/mpls_gso.c
> @@ -25,11 +25,11 @@ static struct sk_buff *mpls_gso_segment(struct sk_buff *skb,
>         netdev_features_t mpls_features;
>         u16 mac_len = skb->mac_len;
>         __be16 mpls_protocol;
> -       unsigned int mpls_hlen;
> +       int mpls_hlen;
>
>         skb_reset_network_header(skb);
>         mpls_hlen = skb_inner_network_header(skb) - skb_network_header(skb);
> -       if (unlikely(!mpls_hlen || mpls_hlen % MPLS_HLEN))
> +       if (unlikely(mpls_hlen <= 0 || mpls_hlen % MPLS_HLEN))
>                 goto out;
>         if (unlikely(!pskb_may_pull(skb, mpls_hlen)))
>                 goto out;
> --
> 2.43.0
>

I think Florian posted this patch, right ?

We must add a Fixes: tag

Also we should ask ourselves :
Why are we even looking at skb_inner_network_header(skb) if this was not set ?

Lets not hide a real bug, we need to understand the root cause.