Re: [PATCH v2 2/2] net: avoid infinite loop in mpls_gso_segment when mpls_hlen == 0

[Willem de Bruijn]

On Mon, Mar 8, 2021 at 5:32 AM Balazs Nemeth <bnemeth@xxxxxxxxxx> wrote:
>
> A packet with skb_inner_network_header(skb) == skb_network_header(skb)
> and ETH_P_MPLS_UC will prevent mpls_gso_segment from pulling any headers
> from the packet. Subsequently, the call to skb_mac_gso_segment will
> again call mpls_gso_segment with the same packet leading to an infinite
> loop.
>
> Signed-off-by: Balazs Nemeth <bnemeth@xxxxxxxxxx>
> ---
>  net/mpls/mpls_gso.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/net/mpls/mpls_gso.c b/net/mpls/mpls_gso.c
> index b1690149b6fa..cc1b6457fc93 100644
> --- a/net/mpls/mpls_gso.c
> +++ b/net/mpls/mpls_gso.c
> @@ -27,7 +27,7 @@ static struct sk_buff *mpls_gso_segment(struct sk_buff *skb,
>
>         skb_reset_network_header(skb);
>         mpls_hlen = skb_inner_network_header(skb) - skb_network_header(skb);
> -       if (unlikely(!pskb_may_pull(skb, mpls_hlen)))
> +       if (unlikely(!mpls_hlen || !pskb_may_pull(skb, mpls_hlen)))
>                 goto out;

Good cathc. Besides length zero, this can be more strict: a label is
4B, so mpls_hlen needs to be >= 4B.

Perhaps even aligned to 4B, too, but not if there may be other encap on top.

Unfortunately there is no struct or type definition that we can use a
sizeof instead of open coding the raw constant.