linux-kernel - Re: [Xen-devel] [PATCH] xen: avoid race in p2m handling

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Message-ID: <5440EC45.1020205@citrix.com>
Date:	Fri, 17 Oct 2014 11:15:33 +0100
From:	David Vrabel <david.vrabel@...rix.com>
To:	Juergen Gross <jgross@...e.com>,
	David Vrabel <david.vrabel@...rix.com>,
	<linux-kernel@...r.kernel.org>, <xen-devel@...ts.xensource.com>,
	<konrad.wilk@...cle.com>
Subject: Re: [Xen-devel] [PATCH] xen: avoid race in p2m handling

On 17/10/14 05:23, Juergen Gross wrote:
> On 10/16/2014 05:50 PM, David Vrabel wrote:
>> On 16/10/14 07:13, Juergen Gross wrote:
>>> When a new p2m leaf is allocated this leaf is linked into the p2m tree
>>> via cmpxchg. Unfortunately the compare value for checking the success
>>> of the update is read after checking for the need of a new leaf. It is
>>> possible that a new leaf has been linked into the tree concurrently
>>> in between. This could lead to a leaked memory page and to the loss of
>>> some p2m entries.
>>>
>>> Avoid the race by using the read compare value for checking the need
>>> of a new p2m leaf.
>> [...]
>>> @@ -579,11 +580,10 @@ static bool alloc_p2m(unsigned long pfn)
>>>           }
>>>       }
>>>
>>> -    if (p2m_top[topidx][mididx] == p2m_identity ||
>>> -        p2m_top[topidx][mididx] == p2m_missing) {
>>> +    p2m_orig = p2m_top[topidx][mididx];
>>
>> Do you need to use ACCESS_ONCE() here?
> 
> Yes, you are probably right. Should I send a new patch or do you want
> to modify it?

Can you go through and see if there are any other places in the p2m code
that also need ACCESS_ONCE()? And then repost, thanks!

David
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