netdev - Re: 回复: INFO: rcu detected stall in tc_modify

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Date:   Thu, 30 Jul 2020 07:58:16 +0200
From:   Dmitry Vyukov <dvyukov@...gle.com>
To:     Vinicius Costa Gomes <vinicius.gomes@...el.com>
Cc:     "Zhang, Qiang" <Qiang.Zhang@...driver.com>,
        syzbot <syzbot+9f78d5c664a8c33f4cce@...kaller.appspotmail.com>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "fweisbec@...il.com" <fweisbec@...il.com>,
        "jhs@...atatu.com" <jhs@...atatu.com>,
        "jiri@...nulli.us" <jiri@...nulli.us>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "mingo@...nel.org" <mingo@...nel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "syzkaller-bugs@...glegroups.com" <syzkaller-bugs@...glegroups.com>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "xiyou.wangcong@...il.com" <xiyou.wangcong@...il.com>
Subject: Re: 回复: INFO: rcu detected stall in tc_modify_qdisc

On Wed, Jul 29, 2020 at 9:13 PM Vinicius Costa Gomes
<vinicius.gomes@...el.com> wrote:
>
> Hi,
>
> "Zhang, Qiang" <Qiang.Zhang@...driver.com> writes:
>
> > ________________________________________
> > 发件人: linux-kernel-owner@...r.kernel.org <linux-kernel-owner@...r.kernel.org> 代表 syzbot <syzbot+9f78d5c664a8c33f4cce@...kaller.appspotmail.com>
> > 发送时间: 2020年7月29日 13:53
> > 收件人: davem@...emloft.net; fweisbec@...il.com; jhs@...atatu.com; jiri@...nulli.us; linux-kernel@...r.kernel.org; mingo@...nel.org; netdev@...r.kernel.org; syzkaller-bugs@...glegroups.com; tglx@...utronix.de; vinicius.gomes@...el.com; xiyou.wangcong@...il.com
> > 主题: 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)?