Re: åå: INFO: rcu detected stall in tc_modify_qdisc

[Dmitry Vyukov]

On Wed, Jul 29, 2020 at 9:13 PM Vinicius Costa Gomes
<vinicius.gomes@xxxxxxxxx> wrote:
>
> Hi,
>
> "Zhang, Qiang" <Qiang.Zhang@xxxxxxxxxxxxx> writes:
>
> > ________________________________________
> > åää: linux-kernel-owner@xxxxxxxxxxxxxxx <linux-kernel-owner@xxxxxxxxxxxxxxx> äè syzbot <syzbot+9f78d5c664a8c33f4cce@xxxxxxxxxxxxxxxxxxxxxxxxx>
> > åéæé: 2020å7æ29æ 13:53
> > æää: davem@xxxxxxxxxxxxx; fweisbec@xxxxxxxxx; jhs@xxxxxxxxxxxx; jiri@xxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx; mingo@xxxxxxxxxx; netdev@xxxxxxxxxxxxxxx; syzkaller-bugs@xxxxxxxxxxxxxxxx; tglx@xxxxxxxxxxxxx; vinicius.gomes@xxxxxxxxx; xiyou.wangcong@xxxxxxxxx
> > äé: INFO: rcu detected stall in tc_modify_qdisc
> >
> > Hello,
> >
> > syzbot found the following issue on:
> >
> > HEAD commit:    181964e6 fix a braino in cmsghdr_from_user_compat_to_kern()
> > git tree:       net
> > console output: https://syzkaller.appspot.com/x/log.txt?x=12925e38900000
> > kernel config:  https://syzkaller.appspot.com/x/.config?x=f87a5e4232fdb267
> > dashboard link: https://syzkaller.appspot.com/bug?extid=9f78d5c664a8c33f4cce
> > compiler:       gcc (GCC) 10.1.0-syz 20200507
> > syz repro:
> > https://syzkaller.appspot.com/x/repro.syz?x=16587f8c900000
>
> It seems that syzkaller is generating an schedule with too small
> intervals (3ns in this case) which causes a hrtimer busy-loop which
> starves other kernel threads.
>
> We could put some limits on the interval when running in software mode,
> but I don't like this too much, because we are talking about users with
> CAP_NET_ADMIN and they have easier ways to do bad things to the system.

Hi Vinicius,

Could you explain why you don't like the argument if it's for CAP_NET_ADMIN?
Good code should check arguments regardless I think and it's useful to
protect root from, say, programming bugs rather than kill the machine
on any bug and misconfiguration. What am I missing?

Also are we talking about CAP_NET_ADMIN in a user ns as well
(effectively nobody)?