linux-kernel - Re: timers: Move clearing of base::timer

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <87a6up7kpt.fsf@nanos.tec.linutronix.de>
Date:   Mon, 07 Dec 2020 15:29:50 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Marco Elver <elver@...gle.com>,
        kasan-dev <kasan-dev@...glegroups.com>,
        Peter Zijlstra <peterz@...radead.org>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Anna-Maria Behnsen <anna-maria@...utronix.de>
Subject: Re: timers: Move clearing of base::timer_running under base::lock

On Mon, Dec 07 2020 at 14:07, Sebastian Andrzej Siewior wrote:
> On 2020-12-06 22:40:07 [+0100], Thomas Gleixner wrote:
>> syzbot reported KCSAN data races vs. timer_base::timer_running being set to
>> NULL without holding base::lock in expire_timers().
>> 
>> This looks innocent and most reads are clearly not problematic but for a
>> non-RT kernel it's completely irrelevant whether the store happens before
>> or after taking the lock. For an RT kernel moving the store under the lock
>> requires an extra unlock/lock pair in the case that there is a waiter for
>> the timer. But that's not the end of the world and definitely not worth the
>> trouble of adding boatloads of comments and annotations to the code. Famous
>> last words...
>> 
>> Reported-by: syzbot+aa7c2385d46c5eba0b89@...kaller.appspotmail.com
>> Reported-by: syzbot+abea4558531bae1ba9fe@...kaller.appspotmail.com
>> Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
>
> One thing I noticed while testing it is that the "corner" case in
> timer_sync_wait_running() is quite reliably hit by rcu_preempt
> rcu_gp_fqs_loop() -> swait_event_idle_timeout_exclusive() invocation.

I assume it's something like this:

     timeout -> wakeup

->preemption
        del_timer_sync()
                .....

Thanks,

        tglx