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

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Message-ID: <543FE940.60901@citrix.com>
Date:	Thu, 16 Oct 2014 16:50:24 +0100
From:	David Vrabel <david.vrabel@...rix.com>
To:	Juergen Gross <jgross@...e.com>, <linux-kernel@...r.kernel.org>,
	<xen-devel@...ts.xensource.com>, <konrad.wilk@...cle.com>
Subject: Re: [PATCH] xen: avoid race in p2m handling

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?

David

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