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: <87bkj03qmq.ffs@tglx>
Date:   Thu, 04 May 2023 20:20:29 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Frederic Weisbecker <frederic@...nel.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Anna-Maria Behnsen <anna-maria@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Sebastian Siewior <bigeasy@...utronix.de>,
        syzbot+5c54bd3eb218bb595aa9@...kaller.appspotmail.com,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Michael Kerrisk <mtk.manpages@...il.com>
Subject: Re: [patch 01/20] posix-timers: Prevent RT livelock in itimer_delete()

On Thu, May 04 2023 at 19:06, Frederic Weisbecker wrote:
> Le Tue, Apr 25, 2023 at 08:48:56PM +0200, Thomas Gleixner a écrit :
>> itimer_delete() has a retry loop when the timer is concurrently expired. On
>> non-RT kernels this just spin-waits until the timer callback has
>> completed. On RT kernels this is a potential livelock when the exiting task
>> preempted the hrtimer soft interrupt.
>> 
>> This only affects hrtimer based timers as Posix CPU timers cannot be
>> concurrently expired. For CONFIG_POSIX_CPU_TIMERS_TASK_WORK=y this is
>> obviously impossible as the task cannot run task work and exit at the same
>> time. The CONFIG_POSIX_CPU_TIMERS_TASK_WORK=n (only non-RT) is prevented
>> because interrupts are disabled.
>
> But the owner of the timer is not the same as the target of the timer, right?
>
> Though I seem to remember that we forbid setting a timer to a target outside
> the current process, in which case the owner and the target are the same at
> this exit stage. But I can't remember what enforces that permission in
> pid_for_clock()..

The owner of the timer is always the one which needs to find the entity
to synchronize on, whether that's the right hrtimer base or the task
which runs the expiry code.

wait_for_running_timer() is taking that into account:

  - The hrtimer timer based posix timers lock the hrtimer base expiry
    lock on the base to which the timer is currently associated

  - Posix CPU timers can be armed on a differnet process (only per
    thread timers are restricted to currents threadgroup) but the
    wait_for_running() callback "knows" how to find that process:

    When the timer is moved to the expiry list it gets:

         cputimer->firing = 1;
         rcu_assign_pointer(ctmr->handling, current);

   and the wait for running side does:

       rcu_read_lock()
       tsk = rcu_dereference(timr->it.cpu.handling);
       ....
       mutex_lock(&tsk->posix_cputimers_work.mutex);

   See collect_timerqueue(), handle_posix_cpu_timers() and
   posix_cpu_timer_wait_running() for details.

   commit f7abf14f0001 ("posix-cpu-timers: Implement the missing
   timer_wait_running callback") has quite some prose in the changelog.

Thanks,

        tglx

Powered by blists - more mailing lists

Powered by Openwall GNU/*/Linux Powered by OpenVZ