linux-kernel - Re: [PATCH] kobject: Don't trigger kobject_uevent(KOBJ

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Date:   Thu, 21 Feb 2019 21:31:05 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Kay Sievers <kay@...y.org>
Cc:     Dmitry Torokhov <dmitry.torokhov@...il.com>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        lkml <linux-kernel@...r.kernel.org>,
        syzbot <syzbot+f648cfb7e0b52bf7ae32@...kaller.appspotmail.com>
Subject: Re: [PATCH] kobject: Don't trigger kobject_uevent(KOBJ_REMOVE) twice.

On 2019/02/21 20:09, Greg Kroah-Hartman wrote:
> On Thu, Feb 21, 2019 at 07:40:20PM +0900, Tetsuo Handa wrote:
>> On 2019/02/21 4:52, Dmitry Torokhov wrote:
>>> On Wed, Feb 20, 2019 at 7:07 AM Greg Kroah-Hartman
>>> <gregkh@...uxfoundation.org> wrote:
>>>> But I would argue that this is not ok, as the remove uevent did NOT get
>>>> sent, and you are saying it did.
>>>
>>> "It is the thought that counts" here. The code was added to catch
>>> cases where nobody even attempted to send "remove" uevents. It does
>>> not guarantee that an event will ultimately be sent (we are at the
>>> point of no return as far as the rest of the kernel is concerned,
>>> there are no repeats or do-overs).
>>>
>>>>
>>>> What memory is being used-after-free here when we fail to properly send
>>>> a uevent?  Shouldn't we fix up that problem correctly?
>>
>> It is explained at https://lkml.kernel.org/r/20190219185558.GA210481@dtor-ws .
> 
> I agree with Dmitry here, it's not the input layer's job to do this.
> 
> But that didn't answer my question, what object is getting reused here?

"struct input_dev"->{name,phys} fields are accessed after they are kfree()d,
for uinput_destroy_device() sometimes kfree()s dev->{name,phys} at
uinput_destroy_device() before dev_uevent() is triggered by dropping the
refcount to 0. syzbot is hitting this bug from cdev_put() path when closing
a character file which drops the refcount to 0.

Therefore, I thought that we must not assume that kobject_uevent() won't be
called after uinput_destroy_device() called kfree(). And I wrote a patch
that explicitly sets "struct input_dev"->{name,phys} to NULL so that
input_dev_uevent() won't crash, for the timing of triggering last
input_put_device() is uncontrollable. But Dmitry Torokhov pointed out that
we should fix kobject side because input subsystem is not expecting that
kobject_uevent() is called after uinput_destroy_device().

> The input device is now dead at that point in time, no one else can
> touch it, but you are saying someone did, right?   Who is that someone?
> 
>>> This is the correct fix (in spirit, we may argue about whether it is
>>> valid to call the flag "state_add_uevent_sent" now or we should or
>>> collapse both it and "state_add_uevent_sent" into
>>> "need_send_remove_uevent"). Other subsystems are in their own right to
>>> not expect to get uvent callbacks past the point of calling
>>> device_del() as they are tearing down parts of the device environment
>>> (even though the device structure may stay in memory for a while).
>>>
>>> Thanks.
>>
>> Which subsystems benefit from commit 0f4dafc0563c6c49 ("Kobject: auto-cleanup
>> on final unref") ? If there is no such subsystem, it will be better to remove
>> state_add_uevent_sent and state_remove_uevent_sent logic.
> 
> You are asking me about a patch from 2007.  I have no idea, try removing
> it and see what happens :)

Waiting for response from Kay Sievers who wrote that patch. ;-)