netdev - Re: [PATCH net] ip6_tunnel: be careful when accessing the inner header

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Message-ID: <CAM_iQpX5ySGZqKTsaQO6WHaFLMqG6Q2ZAUT=ctGijL-7KU_1uA@mail.gmail.com>
Date:   Fri, 21 Sep 2018 11:51:23 -0700
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     Paolo Abeni <pabeni@...hat.com>
Cc:     Linux Kernel Network Developers <netdev@...r.kernel.org>,
        David Miller <davem@...emloft.net>,
        Alexander Potapenko <glider@...gle.com>
Subject: Re: [PATCH net] ip6_tunnel: be careful when accessing the inner header

On Wed, Sep 19, 2018 at 6:04 AM Paolo Abeni <pabeni@...hat.com> wrote:
> diff --git a/net/ipv6/ip6_tunnel.c b/net/ipv6/ip6_tunnel.c
> index 419960b0ba16..a0b6932c3afd 100644
> --- a/net/ipv6/ip6_tunnel.c
> +++ b/net/ipv6/ip6_tunnel.c
> @@ -1234,7 +1234,7 @@ static inline int
>  ip4ip6_tnl_xmit(struct sk_buff *skb, struct net_device *dev)
>  {
>         struct ip6_tnl *t = netdev_priv(dev);
> -       const struct iphdr  *iph = ip_hdr(skb);
> +       const struct iphdr  *iph;
>         int encap_limit = -1;
>         struct flowi6 fl6;
>         __u8 dsfield;
> @@ -1242,6 +1242,11 @@ ip4ip6_tnl_xmit(struct sk_buff *skb, struct net_device *dev)
>         u8 tproto;
>         int err;
>
> +       /* ensure we can access the full inner ip header */
> +       if (!pskb_may_pull(skb, sizeof(struct iphdr)))
> +               return -1;
> +
> +       iph = ip_hdr(skb);

Hmm...

How do IPv4 tunnels ensure they have the right inner header to access?
ip_tunnel_xmit() uses skb_inner_network_header() to access inner header
which doesn't have any check either AFAIK.