Re: KVM: MMU: bail out pagewalk on kvm_read_guest error

[Avi Kivity <avi@redhat.com>]

On 01/18/2010 07:05 PM, Marcelo Tosatti wrote:
> On Sun, Jan 17, 2010 at 10:23:37AM +0200, Avi Kivity wrote:
>    
>> On 01/14/2010 09:41 PM, Marcelo Tosatti wrote:
>>      
>>> Exit the guest pagetable walk loop if reading gpte failed. Otherwise its
>>> possible to enter an endless loop processing the previous present pte.
>>>
>>> Cc: stable@kernel.org
>>> Signed-off-by: Marcelo Tosatti<mtosatti@redhat.com>
>>>
>>> diff --git a/arch/x86/kvm/paging_tmpl.h b/arch/x86/kvm/paging_tmpl.h
>>> index 58a0f1e..ede2131 100644
>>> --- a/arch/x86/kvm/paging_tmpl.h
>>> +++ b/arch/x86/kvm/paging_tmpl.h
>>> @@ -150,7 +150,9 @@ walk:
>>>   		walker->table_gfn[walker->level - 1] = table_gfn;
>>>   		walker->pte_gpa[walker->level - 1] = pte_gpa;
>>>
>>> -		kvm_read_guest(vcpu->kvm, pte_gpa,&pte, sizeof(pte));
>>> +		if (kvm_read_guest(vcpu->kvm, pte_gpa,&pte, sizeof(pte)))
>>> +			goto not_present;
>>> +
>>>        
>> On real hardware, if you place a pte at non-existing memory, you
>> aren't guaranteed to get the present bit clear, so why is this
>> necessary?
>>
>> We should be able to survive any garbage the pte previously contained.
>>      
> The problem is the content of the previous pte is processed (which is
> valid), but the cmpxchg fails (see the loop), without level decreasing.
>
>    

Yes, you're right.  Patch is fine, then (we could triple-fault instead 
of #PF, but I don't think it matters).

-- 
error compiling committee.c: too many arguments to function