Re: [Xen-devel] [PATCH] xen: avoid writing to freed memory after race in p2m handling

[Juergen Gross <jgross@suse.com>]

On 10/14/2014 11:30 AM, David Vrabel wrote:
> On 14/10/14 10:00, Juergen Gross wrote:
>> In case a race was detected during allocation of a new p2m tree
>> element in alloc_p2m() the new allocated mid_mfn page is freed without
>> updating the pointer to the found value in the tree. This will result
>> in overwriting the just freed page with the mfn of the p2m leaf.
>
> Can this race actually happen?  i.e., does this need tagging for stable?

Good question. I just stumbled over it while writing the linear p2m-list
patch.

Is it possible for gnttab_map_refs() to call set_foreign_p2m_mapping()
specifying a pfn which has been invalid before? In this case the race
could happen in dom0.

I think ballooning alone can't trigger this race, as it is calling
set_phys_to_machine() under lock only.

Juergen

>
> David
>
>> --- a/arch/x86/xen/p2m.c
>> +++ b/arch/x86/xen/p2m.c
>> @@ -566,6 +566,7 @@ static bool alloc_p2m(unsigned long pfn)
>>   		/* Separately check the mid mfn level */
>>   		unsigned long missing_mfn;
>>   		unsigned long mid_mfn_mfn;
>> +		unsigned long old_mfn;
>>
>>   		mid_mfn = alloc_p2m_page();
>>   		if (!mid_mfn)
>> @@ -575,10 +576,13 @@ static bool alloc_p2m(unsigned long pfn)
>>
>>   		missing_mfn = virt_to_mfn(p2m_mid_missing_mfn);
>>   		mid_mfn_mfn = virt_to_mfn(mid_mfn);
>> -		if (cmpxchg(top_mfn_p, missing_mfn, mid_mfn_mfn) != missing_mfn)
>> +		old_mfn = cmpxchg(top_mfn_p, missing_mfn, mid_mfn_mfn);
>> +		if (old_mfn != missing_mfn) {
>>   			free_p2m_page(mid_mfn);
>> -		else
>> +			mid_mfn = mfn_to_virt(old_mfn);
>> +		} else {
>>   			p2m_top_mfn_p[topidx] = mid_mfn;
>> +		}
>>   	}
>>
>>   	if (p2m_top[topidx][mididx] == p2m_identity ||
>>
>
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