Re: [syzbot] WARNING in page_counter_cancel (3)

[Mat Martineau]

On Wed, 29 Dec 2021, Michal Hocko wrote:

> On Tue 21-12-21 15:57:36, Andrew Morton wrote:
> > On Sat, 18 Dec 2021 06:04:22 -0800 syzbot <syzbot+bc9e2d2dbcb347dd215a@xxxxxxxxxxxxxxxxxxxxxxxxx> 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?

Andrew -

Yes, we'll get a fix in to net-next soon - thanks for adding the mptcp & 
netdev lists.

> > > IMPORTANT: if you fix the issue, please add the following tag to the commit:
> > > Reported-by: syzbot+bc9e2d2dbcb347dd215a@xxxxxxxxxxxxxxxxxxxxxxxxx
> > >
> > > 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.

Thanks for the analysis Michal.

--
Mat Martineau
Intel