linux-kernel - Re: KASAN: use-after-free Read in tcf_block

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Message-ID: <CAM_iQpUpe5J30_aOzDUHrxiC7hXyQfH6-R7TM_L==gkj_XTu5w@mail.gmail.com>
Date:   Thu, 27 Sep 2018 10:50:00 -0700
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     Dmitry Vyukov <dvyukov@...gle.com>
Cc:     Eric Dumazet <eric.dumazet@...il.com>,
        syzbot+37b8770e6d5a8220a039@...kaller.appspotmail.com,
        David Miller <davem@...emloft.net>,
        Jamal Hadi Salim <jhs@...atatu.com>,
        Jiri Pirko <jiri@...nulli.us>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>,
        syzkaller-bugs@...glegroups.com
Subject: Re: KASAN: use-after-free Read in tcf_block_find

On Thu, Sep 27, 2018 at 1:11 AM Dmitry Vyukov <dvyukov@...gle.com> wrote:
>
> Would a stack trace for call_rcu be helpful here? I have this idea for
> a long time, but never get around to implementing it:
> https://bugzilla.kernel.org/show_bug.cgi?id=198437

Yes. Generally speaking, showing backtrace of call_rcu()
or schedule_work(0 etc. is very helpful, we are more interested
in who calls call_rcu() than what that RCU callback does.

BTW, yesterday I asked syzbot to test this:
https://github.com/congwang/linux/commit/b7815584cf1c0bbb79e8f6fe3e4b66ba10375560
I still don't get any result.

For this specific bug, we should hold a refcnt in dev->qdisc, I don't
even see how call_rcu() could be invoked, unless of course we mess
up with qdisc refcnt.