linux-kernel - Re: [PATCH] Yama: remove locking from delete path

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Message-ID: <CAGXu5jLOcQnmfxu7yYUVtUsTbLWrL+7uVi=fPqAKmfbb=CGEAw@mail.gmail.com>
Date:	Tue, 20 Nov 2012 08:07:44 -0800
From:	Kees Cook <keescook@...omium.org>
To:	Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
Cc:	linux-kernel@...r.kernel.org, james.l.morris@...cle.com,
	john.johansen@...onical.com, serge.hallyn@...ntu.com,
	eparis@...hat.com, linux-security-module@...r.kernel.org
Subject: Re: [PATCH] Yama: remove locking from delete path

On Tue, Nov 20, 2012 at 4:24 AM, Tetsuo Handa
<penguin-kernel@...ove.sakura.ne.jp> wrote:
> Kees Cook wrote:
>> Instead of locking the list during a delete, mark entries as invalid
>> and trigger a workqueue to clean them up. This lets us easily handle
>> task_free from interrupt context.
>
>> @@ -57,9 +80,12 @@ static int yama_ptracer_add(struct task_struct *tracer,
>>
>>       added->tracee = tracee;
>>       added->tracer = tracer;
>> +     added->invalid = false;
>>
>> -     spin_lock_bh(&ptracer_relations_lock);
>> +     spin_lock(&ptracer_relations_lock);
>
> Can't you use
>         spin_lock_irqsave(&ptracer_relations_lock, flags);
>         spin_unlock_irqrestore(&ptracer_relations_lock, flags);
> instead of adding ->invalid ?

The _bh was sufficient originally, but looking at Sasha's deadlock, it
seems like I should get rid of locking entirely on this path. What do
you think of this report:
https://lkml.org/lkml/2012/10/17/600

I'm concerned that blocking interrupts would be an even more expensive
solution, since every task_free() is forced to block interrupts
briefly. Most systems will have either an empty relations list, or a
very short one, so it seemed better to avoid any locking at all on the
task_free() path. Now the locking contention would be moved to being
between the workqueue and any add calls.

-Kees

-Kees

-- 
Kees Cook
Chrome OS Security
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