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Date: Mon, 15 Aug 2016 11:49:12 +0200
From: Michal Hocko <mhocko@kernel.org>
Subject: Re: [PATCH 09/10] vhost, mm: make sure that oom_reaper doesn't reap
 memory read by vhost
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To: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Oleg Nesterov <oleg@redhat.com>, "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>, linux-mm@kvack.org, Andrew Morton <akpm@linux-foundation.org>, Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>, David Rientjes <rientjes@google.com>, Vladimir Davydov <vdavydov@parallels.com>

On Mon 15-08-16 02:06:31, Michael S. Tsirkin wrote:
[...]
> So fundamentally, won't the following make copy to/from user
> return EFAULT?  If yes, vhost is already prepared to handle that.
> 
> 
> diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
> index dc80230..e5dbee5 100644
> --- a/arch/x86/mm/fault.c
> +++ b/arch/x86/mm/fault.c
> @@ -1309,6 +1309,11 @@ retry:
>  		might_sleep();
>  	}
>  
> +	if (unlikely(test_bit(MMF_UNSTABLE, &mm->flags))) {
> +		bad_area(regs, error_code, address);
> +		return;
> +	}
> +
>  	vma = find_vma(mm, address);
>  	if (unlikely(!vma)) {
>  		bad_area(regs, error_code, address);

This would be racy but even if we did the check _after_ the #PF is
handled then I am not very happy to touch the #PF path which is quite
hot for something as rare as OOM and which only has one user which needs
a special handling. That is the primary reason why I prefer the specific
API.

-- 
Michal Hocko
SUSE Labs

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