netdev - Re: BUG_ON in skb_segment, after bpf_skb_change

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Message-ID: <20190902164444.00f32685@pixies>
Date:   Mon, 2 Sep 2019 16:44:44 +0300
From:   Shmulik Ladkani <shmulik.ladkani@...il.com>
To:     Willem de Bruijn <willemdebruijn.kernel@...il.com>
Cc:     Daniel Borkmann <daniel@...earbox.net>,
        Eric Dumazet <eric.dumazet@...il.com>,
        netdev <netdev@...r.kernel.org>,
        Alexander Duyck <alexander.duyck@...il.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Yonghong Song <yhs@...com>,
        Steffen Klassert <steffen.klassert@...unet.com>,
        Shmulik Ladkani <shmulik@...anetworks.com>,
        eyal@...anetworks.com
Subject: Re: BUG_ON in skb_segment, after bpf_skb_change_proto was applied

On Sun, 1 Sep 2019 16:05:48 -0400
Willem de Bruijn <willemdebruijn.kernel@...il.com> wrote:

> One quick fix is to disable sg and thus revert to copying in this
> case. Not ideal, but better than a kernel splat:
> 
> @@ -3714,6 +3714,9 @@ struct sk_buff *skb_segment(struct sk_buff *head_skb,
>         sg = !!(features & NETIF_F_SG);
>         csum = !!can_checksum_protocol(features, proto);
> 
> +       if (list_skb && skb_headlen(list_skb) && !list_skb->head_frag)
> +               sg = false;
> +

Many thanks Willem for looking into this.

Indeed the above suggestion avoids the splat, at least with the
reproducer.

I'll look deeper into this, to make sure the generated skbs are indeed
legit and have correct content.

Shmulik