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

[David Vrabel <david.vrabel@xxxxxxxxxx>]

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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