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From:   Willem de Bruijn <willemdebruijn.kernel@gmail.com>
Date:   Mon, 8 Mar 2021 11:07:10 -0500
Message-ID: <CA+FuTSdWSCzkB7sDn+_0Oxy8JqmqL=nsQXP_3bnb4Xdd=0A=KQ@mail.gmail.com>
Subject: Re: [PATCH v2 2/2] net: avoid infinite loop in mpls_gso_segment when
 mpls_hlen == 0
To:     Balazs Nemeth <bnemeth@redhat.com>
Cc:     Network Development <netdev@vger.kernel.org>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        "Michael S. Tsirkin" <mst@redhat.com>,
        Jason Wang <jasowang@redhat.com>,
        David Miller <davem@davemloft.net>,
        virtualization@lists.linux-foundation.org
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On Mon, Mar 8, 2021 at 5:32 AM Balazs Nemeth <bnemeth@redhat.com> 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@redhat.com>
> ---
>  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.