Re: [PATCH] Fix SMP shadow instantiation race

[Avi Kivity <avi-atKUWr5tajBWk0Htik3J/w@public.gmane.org>]

Marcelo Tosatti wrote:
> On Mon, Dec 10, 2007 at 07:07:54PM +0200, Avi Kivity wrote:
>   
>> Marcelo Tosatti wrote:
>>     
>>> There is a race where VCPU0 is shadowing a pagetable entry while VCPU1
>>> is updating it, which results in a stale shadow copy.
>>>
>>> Fix that by comparing the contents of the cached guest pte with the
>>> current guest pte after write-protecting the guest pagetable.
>>>
>>> Attached program kvm_shadow_race.c demonstrates the problem.
>>>
>>>  
>>>       
>> Where is it?
>>     
>
> Attached.
>
>   

Can you explain what it does?  I get the same results on both host and 
guest (successful completion).

>>> diff --git a/drivers/kvm/paging_tmpl.h b/drivers/kvm/paging_tmpl.h
>>> index 72d4816..4fece01 100644
>>> --- a/drivers/kvm/paging_tmpl.h
>>> +++ b/drivers/kvm/paging_tmpl.h
>>> @@ -66,6 +66,7 @@ struct guest_walker {
>>> 	int level;
>>> 	gfn_t table_gfn[PT_MAX_FULL_LEVELS];
>>> 	pt_element_t pte;
>>> +	gpa_t pte_gpa;
>>>  
>>>       
>> I think this needs to be an array like table_gfn[].  The guest may play 
>> with the pde (and upper entries) as well as the pte.
>>     
>
> I was working under the assumption that the only significant bits of
> upper entries (WRITEABLE and PRESENT) that can be changed by the guest
> must be reflected first in the lower level pte's.
>
> Isnt that a fair assumption to make?
>
>   

The other bits (including the physical addresses) may change too.  There 
is no requirement that the changes to pde write/present bits be 
reflected on pte write/present bits.

Consider a unix kernel implementing fork() by write-protecting the pud 
tables.  It can write protect the entire user address space by clearing 
the write bit on the first 256 pgd entries.

(I don't think Linux does that; maybe that's a worthwhile optimization)

-- 
Any sufficiently difficult bug is indistinguishable from a feature.


-------------------------------------------------------------------------
SF.Net email is sponsored by:
Check out the new SourceForge.net Marketplace.
It's the best place to buy or sell services for
just about anything Open Source.
http://sourceforge.net/services/buy/index.php