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

[Juergen Gross <jgross@suse.com>]

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?

Juergen