netdev - Re: [PATCH net-next] net: fix up truesize of cloned skb in skb_prepare_for

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Date:   Tue, 2 Feb 2021 08:58:18 -0800
From:   Christoph Paasch <christoph.paasch@...il.com>
To:     Eric Dumazet <edumazet@...gle.com>
Cc:     Marco Elver <elver@...gle.com>,
        LKML <linux-kernel@...r.kernel.org>,
        kasan-dev <kasan-dev@...glegroups.com>,
        David Miller <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Jonathan Lemon <jonathan.lemon@...il.com>,
        Willem de Bruijn <willemb@...gle.com>,
        linmiaohe <linmiaohe@...wei.com>,
        Guillaume Nault <gnault@...hat.com>,
        Dongseok Yi <dseok.yi@...sung.com>,
        Yadu Kishore <kyk.segfault@...il.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        netdev <netdev@...r.kernel.org>,
        Alexander Potapenko <glider@...gle.com>,
        syzbot <syzbot+7b99aafdcc2eedea6178@...kaller.appspotmail.com>
Subject: Re: [PATCH net-next] net: fix up truesize of cloned skb in skb_prepare_for_shift()

On Mon, Feb 1, 2021 at 9:58 AM Eric Dumazet <edumazet@...gle.com> wrote:
>
> On Mon, Feb 1, 2021 at 6:34 PM Marco Elver <elver@...gle.com> wrote:
> >
> > On Mon, 1 Feb 2021 at 17:50, Christoph Paasch
>
> > > just a few days ago we found out that this also fixes a syzkaller
> > > issue on MPTCP (https://github.com/multipath-tcp/mptcp_net-next/issues/136).
> > > I confirmed that this patch fixes the issue for us as well:
> > >
> > > Tested-by: Christoph Paasch <christoph.paasch@...il.com>
> >
> > That's interesting, because according to your config you did not have
> > KFENCE enabled. Although it's hard to say what exactly caused the
> > truesize mismatch in your case, because it clearly can't be KFENCE
> > that caused ksize(kmalloc(S))!=ksize(kmalloc(S)) for you.
>
> Indeed, this seems strange. This might be a different issue.
>
> Maybe S != S ;)

Seems like letting syzkaller run for a few more days made it
eventually find the WARN again. As if Marco's change makes it harder
for us to trigger the issue.

Anyways, you can remove my "Tested-by" ;-)


Christoph