linux-kernel - Possible race in dev_coredumpm()-del

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Date:   Wed, 13 Apr 2022 10:59:22 +0530
From:   Mukesh Ojha <quic_mojha@...cinc.com>
To:     <linux-kernel@...r.kernel.org>
CC:     <tglx@...utronix.de>, <sboyd@...nel.org>,
        <gregkh@...uxfoundation.org>, <johannes@...solutions.net>,
        "Rafael J. Wysocki" <rafael@...nel.org>
Subject: Possible race in dev_coredumpm()-del_timer() path

Hi All,

We are hitting one race due to which try_to_grab_pending() is stuck .
In following scenario, while running (p1)dev_coredumpm() devcd device is 
added to
the framework and uevent notification sent to userspace that result in 
the call to (p2) devcd_data_write()
which eventually try to delete the queued timer which in the racy 
scenario timer is not queued yet.
So, debug object report some warning and in the meantime timer is 
initialized and queued from p1 path.
and from p2 path it gets overriden again timer->entry.pprev=NULL and 
try_to_grab_pending() stuck
as del_timer() always return 0 as timer_pending() return false.

P1 P2(X)

      dev_coredumpm()

                                           Uevent notification sent to 
userspace
                                           for device addition

             device_add() ========================>                 
Process X reads this uevents
notification and do write call
that results in call to

devcd_data_write()
mod_delayed_work()
try_to_grab_pending()
del_timer()
debug_assert_init()

             INIT_DELAYED_WORK
                    (&devcd->del_wk, devcd_del);
             schedule_delayed_work(&devcd->del_wk,
                    DEVCD_TIMEOUT);

debug_object_fixup()
timer_fixup_assert_init()
timer_setup()
do_init_timer()   ==> reinitialized the timer to timer->entry.pprev=NULL

timer_pending()
!hlist_unhashed_lockless(&timer->entry)
!h->pprev

Thanks.
Mukesh