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Date:   Wed, 29 Dec 2021 10:23:07 +0100
From:   Michal Hocko <mhocko@suse.com>
To:     Andrew Morton <akpm@linux-foundation.org>
Cc:     syzbot <syzbot+bc9e2d2dbcb347dd215a@syzkaller.appspotmail.com>,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        syzkaller-bugs@googlegroups.com, mptcp@lists.linux.dev,
        netdev@vger.kernel.org
Subject: Re: [syzbot] WARNING in page_counter_cancel (3)
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On Tue 21-12-21 15:57:36, Andrew Morton wrote:
> On Sat, 18 Dec 2021 06:04:22 -0800 syzbot <syzbot+bc9e2d2dbcb347dd215a@syzkaller.appspotmail.com> wrote:
> 
> > syzbot has found a reproducer for the following issue on:
> > 
> > HEAD commit:    fbf252e09678 Add linux-next specific files for 20211216
> > git tree:       linux-next
> > console output: https://syzkaller.appspot.com/x/log.txt?x=1797de99b00000
> > kernel config:  https://syzkaller.appspot.com/x/.config?x=7fcbb9aa19a433c8
> > dashboard link: https://syzkaller.appspot.com/bug?extid=bc9e2d2dbcb347dd215a
> > compiler:       gcc (Debian 10.2.1-6) 10.2.1 20210110, GNU ld (GNU Binutils for Debian) 2.35.2
> > syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=135d179db00000
> > C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=113edb6db00000
> 
> Useful to have that, thanks.
> 
> I'm suspecting that mptcp is doing something strange.

Yes.

> Could I as the
> developers to please take a look?
> 
> 
> > IMPORTANT: if you fix the issue, please add the following tag to the commit:
> > Reported-by: syzbot+bc9e2d2dbcb347dd215a@syzkaller.appspotmail.com
> > 
> > R13: 00007ffdeb858640 R14: 00007ffdeb858680 R15: 0000000000000004
> >  </TASK>
> > ------------[ cut here ]------------
> > page_counter underflow: -4294966651 nr_pages=4294967295

__mptcp_mem_reclaim_partial is trying to uncharge (via
__sk_mem_reduce_allocated) negative amount. nr_pages has overflown when
converted from int to unsigned int (-1). I would say that
__mptcp_mem_reclaim_partial has evaluated
	reclaimable = mptcp_sk(sk)->rmem_fwd_alloc - sk_unused_reserved_mem(sk)
to 0 and __mptcp_rmem_reclaim(sk, reclaimable - 1) made it -1.
-- 
Michal Hocko
SUSE Labs