Re: [PATCH] xen: avoid race in p2m handling

From: David Vrabel
Date: Thu Oct 16 2014 - 11:56:10 EST


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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