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From:   Dmitry Vyukov <dvyukov@google.com>
Date:   Thu, 30 Jul 2020 07:58:16 +0200
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To:     Vinicius Costa Gomes <vinicius.gomes@intel.com>
Cc:     "Zhang, Qiang" <Qiang.Zhang@windriver.com>,
        syzbot <syzbot+9f78d5c664a8c33f4cce@syzkaller.appspotmail.com>,
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On Wed, Jul 29, 2020 at 9:13 PM Vinicius Costa Gomes
<vinicius.gomes@intel.com> wrote:
>
> Hi,
>
> "Zhang, Qiang" <Qiang.Zhang@windriver.com> writes:
>
> > ________________________________________
> > =E5=8F=91=E4=BB=B6=E4=BA=BA: linux-kernel-owner@vger.kernel.org <linux-=
kernel-owner@vger.kernel.org> =E4=BB=A3=E8=A1=A8 syzbot <syzbot+9f78d5c664a=
8c33f4cce@syzkaller.appspotmail.com>
> > =E5=8F=91=E9=80=81=E6=97=B6=E9=97=B4: 2020=E5=B9=B47=E6=9C=8829=E6=97=
=A5 13:53
> > =E6=94=B6=E4=BB=B6=E4=BA=BA: davem@davemloft.net; fweisbec@gmail.com; j=
hs@mojatatu.com; jiri@resnulli.us; linux-kernel@vger.kernel.org; mingo@kern=
el.org; netdev@vger.kernel.org; syzkaller-bugs@googlegroups.com; tglx@linut=
ronix.de; vinicius.gomes@intel.com; xiyou.wangcong@gmail.com
> > =E4=B8=BB=E9=A2=98: 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_ke=
rn()
> > git tree:       net
> > console output: https://syzkaller.appspot.com/x/log.txt?x=3D12925e38900=
000
> > kernel config:  https://syzkaller.appspot.com/x/.config?x=3Df87a5e4232f=
db267
> > dashboard link: https://syzkaller.appspot.com/bug?extid=3D9f78d5c664a8c=
33f4cce
> > compiler:       gcc (GCC) 10.1.0-syz 20200507
> > syz repro:
> > https://syzkaller.appspot.com/x/repro.syz?x=3D16587f8c900000
>
> 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)?