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

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?


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