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

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Date:   Thu, 21 Feb 2019 19:40:20 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     Dmitry Torokhov <dmitry.torokhov@...il.com>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        lkml <linux-kernel@...r.kernel.org>, Kay Sievers <kay@...y.org>,
        syzbot <syzbot+f648cfb7e0b52bf7ae32@...kaller.appspotmail.com>
Subject: Re: [PATCH] kobject: Don't trigger kobject_uevent(KOBJ_REMOVE) twice.

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 .

> 
> 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.