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From:   Schspa Shi <schspa@gmail.com>
To:     Luis Chamberlain <mcgrof@kernel.org>
Cc:     linux-kernel@vger.kernel.org,
        syzbot+10d19d528d9755d9af22@syzkaller.appspotmail.com,
        syzbot+70d5d5d83d03db2c813d@syzkaller.appspotmail.com,
        syzbot+83cb0411d0fcf0a30fc1@syzkaller.appspotmail.com,
        syzbot+c92c6a251d49ceceb625@syzkaller.appspotmail.com
Subject: Re: [PATCH v2] umh: fix out of scope usage when the process is
 being killed
Date:   Thu, 15 Dec 2022 13:11:39 +0800
In-reply-to: <Y5orL90JpH6LUd4D@bombadil.infradead.org>
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Luis Chamberlain <mcgrof@kernel.org> writes:

> On Wed, Dec 14, 2022 at 09:46:56PM +0800, Schspa Shi wrote:
>> When the process is killed, wait_for_completion_state will return with
>> -ERESTARTSYS, and the completion variable in the stack will be unavailable,
>> even freed. If the user-mode thread is complete at the same time, there
>> will be a race to use a unavailable variable.
>> 
>> Please refer to the following scenarios.
>>             T1                                  T2
>> ------------------------------------------------------------------
>> call_usermodehelper_exec
>>                                    call_usermodehelper_exec_async
>>                                    << do something >>
>>                                    umh_complete(sub_info);
>>                                    comp = xchg(&sub_info->complete, NULL);
>>                                    /* we got the completion */
>>                                    << context switch >>
>> 
>>     << Being killed >>
>> 	retval = wait_for_completion_state(sub_info->complete, state);
>> 	if (!retval)
>> 		goto wait_done;
>> 
>> 	if (wait & UMH_KILLABLE) {
>> 		/* umh_complete() will see NULL and free sub_info */
>> 		if (xchg(&sub_info->complete, NULL))
>> 			goto unlock;
>>         << we can't got the completion, because T2 take it already >>
>> 	}
>> 	....
>> 	return retval;
>> }
>> 
>> /**
>>  * the completion variable in stack is end of life cycle.
>>  * and maybe freed due to process is recycled.
>>  */
>>                                    -------- BUG here----------
>>                                    if (comp)
>>                                        complete(comp);
>> 
>> To fix it, we can add an additional wait_for_completion to ensure the
>> completion object is completely unused. And this is what
>> kthread_create_on_node does to handle this race.
>> 
>> Reported-by: syzbot+10d19d528d9755d9af22@syzkaller.appspotmail.com
>> Reported-by: syzbot+70d5d5d83d03db2c813d@syzkaller.appspotmail.com
>> Reported-by: syzbot+83cb0411d0fcf0a30fc1@syzkaller.appspotmail.com
>> Reported-by: syzbot+c92c6a251d49ceceb625@syzkaller.appspotmail.com
>> Signed-off-by: Schspa Shi <schspa@gmail.com>
>> ---
>
> Please fix the commit log a bit more with the cotext I provided, *if*
> on the other thread the community agrees with the approach to be
> compartamentalized. I am still not sure why this would fix the
> UAF after thinking about it some more, and the issue would mean
> there likely could be a generic fix / issue to consider.
>

I think a syntactic sugar for a complete api can be added here for a
generic usage.

> So for now I'd like more review of this race and the proposed fix
> as I mentioned in the follow-up threaad in your v1 patch. Let's
> follow up there and see how that discussion goes.
>

Ok, let's talk about this on the v1 patch's thread.

>   Luis

-- 
BRs
Schspa Shi