Re: Possible race in dev_coredumpm()-del_timer() path

From: Thomas Gleixner
Date: Thu Apr 14 2022 - 06:38:19 EST


On Wed, Apr 13 2022 at 12:58, Greg KH wrote:
> On Wed, Apr 13, 2022 at 03:46:39PM +0530, Mukesh Ojha wrote:
>> p1 p2(X)
>>
>> dev_coredump() uevent sent to userspace
>> device_add() =========================> userspace process X reads the uevents
>> writes to devcd fd which
>> results into writes to
>>
>> devcd_data_write()
>> mod_delayed_work()
>> try_to_grab_pending()
>> del_timer()
>> debug_assert_init()
>> INIT_DELAYED_WORK
>> schedule_delayed_work
>> debug_object_fixup()
>
> Why do you have object debugging enabled? That's going to take a LONG
> time, and will find bugs in your code. Perhaps like this one?

It's not finding bugs in his code. It finds bug in the upstream
dev_coredump code.

> And if you turn object debugging off, what happens?

The debugobject splat goes away, but the problem persists.

device_add() -> uevent

Preemption or concurrency:

devcd_data_write()
mod_delayed_work(..., w, 0); <- Uninitialized.

The dev_coredump code exposes the device before it is fully initialized
and a write ending up in devcd_data_write() touches uninitialized work.

It does not help to move the initialization before device_add() as that
creates another problem:

INIT_DELAYED_WORK(w)
...
device_add() -> uevent

Preemption or concurrency:

devcd_data_write()
mod_delayed_work(..., w, 0); <- Schedules work immediately

work_queue_runs()
devcd_del(w)
device_del()
put_device() <- Drops the last reference

initialization continues...

So, yes this needs serialization of some sort.

Same problem vs. disabled_store().

Thanks,

tglx