netdev - Re: BUG_ON in skb_segment, after bpf_skb_change

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Date:   Tue, 3 Sep 2019 12:23:54 -0400
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     Shmulik Ladkani <shmulik@...anetworks.com>
Cc:     Willem de Bruijn <willemdebruijn.kernel@...il.com>,
        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>,
        eyal@...anetworks.com
Subject: Re: BUG_ON in skb_segment, after bpf_skb_change_proto was applied

On Tue, Sep 3, 2019 at 11:52 AM Shmulik Ladkani
<shmulik@...anetworks.com> wrote:
>
> 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;
> > +
>
> Thanks Willem.
>
> I followed this approach, and further refined it based on the conditions
> that lead to this BUG_ON:
>
>  - existance of frag_list
>  - mangled gso_size (using SKB_GSO_DODGY as a hint)
>  - some frag in the frag_list has a linear part that is NOT head_frag,
>    or length not equal to the requested gso_size
>
> BTW, doing so allowed me to refactor a loop that tests for similar
> conditions in the !(features & NETIF_F_GSO_PARTIAL) case, where an
> attempt to execute partial splitting at the frag_list pointer (see
> 07b26c9454a2 and 43170c4e0ba7).
>
> I've tested this using the reproducer, with various linear skbs in
> the frag_list and different gso_size mangling. All resulting 'segs'
> looked correct. Did not test on a live system yet.
>
> Comments are welcome.
>
> specifically, I would like to know whether we can
>  - better refine the condition where this "sg=false fallback" needs
>    to be applied
>  - consolidate my new 'list_skb && (type & SKB_GSO_DODGY)' case with
>    the existing '!(features & NETIF_F_GSO_PARTIAL)' case

This is a lot more code change. Especially for stable fixes that need
to be backported, a smaller patch is preferable.

My suggestion only tested the first frag_skb length. If a list can be
created where the first frag_skb is head_frag but a later one is not,
it will fail short. I kind of doubt that.

By default skb_gro_receive builds GSO skbs that can be segmented
along the original gso_size boundaries. We have so far only observed
this issue when messing with gso_size.

We can easily refine the test to fall back on to copying only if
skb_headlen(list_skb) != mss. Alternatively, only on SKB_GSO_DODGY is fine, too.

I suggest we stick with the two-liner.