linux-kernel - Re: [PATCH v4 2/5] locking/rwsem: Protect all writes to owner by WRITE

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <573F7302.6040609@hpe.com>
Date:	Fri, 20 May 2016 16:26:42 -0400
From:	Waiman Long <waiman.long@....com>
To:	Jason Low <jason.low2@....com>
CC:	<jason.low2@...com>, Davidlohr Bueso <dave@...olabs.net>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>, <linux-kernel@...r.kernel.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Scott J Norton <scott.norton@....com>,
	Douglas Hatch <doug.hatch@....com>
Subject: Re: [PATCH v4 2/5] locking/rwsem: Protect all writes to owner by
 WRITE_ONCE

On 05/19/2016 06:21 PM, Jason Low wrote:
> On Wed, 2016-05-18 at 12:58 -0700, Jason Low wrote:
>> On Wed, 2016-05-18 at 14:29 -0400, Waiman Long wrote:
>>> On 05/18/2016 01:21 PM, Jason Low wrote:
>>>> On Wed, 2016-05-18 at 07:04 -0700, Davidlohr Bueso wrote:
>>>>> On Tue, 17 May 2016, Waiman Long wrote:
>>>>>
>>>>>> Without using WRITE_ONCE(), the compiler can potentially break a
>>>>>> write into multiple smaller ones (store tearing). So a read from the
>>>>>> same data by another task concurrently may return a partial result.
>>>>>> This can result in a kernel crash if the data is a memory address
>>>>>> that is being dereferenced.
>>>>>>
>>>>>> This patch changes all write to rwsem->owner to use WRITE_ONCE()
>>>>>> to make sure that store tearing will not happen. READ_ONCE() may
>>>>>> not be needed for rwsem->owner as long as the value is only used for
>>>>>> comparison and not dereferencing.
>>>> It might be okay to leave out READ_ONCE() for reading rwsem->owner, but
>>>> couldn't we include it to at least document that we're performing a
>>>> "special" lockless read?
>>>>
>>> Using READ_ONCE() does have a bit of cost as it limits compiler
>>> optimization. If we changes all access to rwsem->owner to READ_ONCE()
>>> and WRITE_ONCE(), we may as well change its type to volatile and be done
>>> with.
>> Right, although there are still places like the init function where
>> WRITE_ONCE isn't necessary.
>>
>>> I am not against doing that, but it feels a bit over-reach for me.
>>> On the other hand, we may define a do-nothing macro that designates the
>>> owner as a special variable for documentation purpose, but don't need
>>> protection at that particular call site.
>> It should be fine to use the standard READ_ONCE here, even if it's just
>> for documentation, as it's probably not going to cost anything in
>> practice. It would be better to avoid adding any special macros for this
>> which may just add more complexity.
> By the way, this potential "partial write" issue may also apply to
> mutexes as well, so we should also make a similar change to
> mutex_set_owner() and mutex_clear_owner().
>
> Jason
>
  Yes, I am aware of that. I just don't have the time to to do a mutex 
patch yet. As you have sent out a patch on that, this is now covered.

Cheers,
Longman