Re: [PATCH net-next] net: fix up truesize of cloned skb in skb_prepare_for_shift()

From: Christoph Paasch <christoph.paasch@gmail.com>
To: Eric Dumazet <edumazet@google.com>
Cc: Marco Elver <elver@google.com>,
	LKML <linux-kernel@vger.kernel.org>,
	kasan-dev <kasan-dev@googlegroups.com>,
	David Miller <davem@davemloft.net>,
	Jakub Kicinski <kuba@kernel.org>,
	Jonathan Lemon <jonathan.lemon@gmail.com>,
	Willem de Bruijn <willemb@google.com>,
	linmiaohe <linmiaohe@huawei.com>,
	Guillaume Nault <gnault@redhat.com>,
	Dongseok Yi <dseok.yi@samsung.com>,
	Yadu Kishore <kyk.segfault@gmail.com>,
	Al Viro <viro@zeniv.linux.org.uk>,
	netdev <netdev@vger.kernel.org>,
	Alexander Potapenko <glider@google.com>,
	syzbot <syzbot+7b99aafdcc2eedea6178@syzkaller.appspotmail.com>
Subject: Re: [PATCH net-next] net: fix up truesize of cloned skb in skb_prepare_for_shift()
Date: Tue, 2 Feb 2021 08:58:18 -0800	[thread overview]
Message-ID: <CALMXkpbpB7AWNvtH4dbgP_uFi0hV8Zg0JfPkkdOLFwLRvxGMPg@mail.gmail.com> (raw)
In-Reply-To: <CANn89iJbAQU7U61RD2pyZfcXah0P5huqK3W92OEP513pqGT_wA@mail.gmail.com>

On Mon, Feb 1, 2021 at 9:58 AM Eric Dumazet <edumazet@google.com> wrote:
>
> On Mon, Feb 1, 2021 at 6:34 PM Marco Elver <elver@google.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@gmail.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