Re: [PATCH] x86/emulate: Check current->arch.vm_event in hvmemul_virtual_to_linear()

[Razvan Cojocaru <rcojocaru@bitdefender.com>]

On 04/07/16 20:27, Jan Beulich wrote:
>>>> On 07.04.16 at 10:39, <rcojocaru@bitdefender.com> wrote:
>> Theoretically it is possible for mem_access_emulate_each_rep to be
>> true even when current->arch.vm_event == NULL, so add an extra
>> check to hvmemul_virtual_to_linear().
> 
> Mind saying what those theoretical conditions are when this might
> happen?

This could happen if someone were to call xc_monitor_emulate_each_rep(),
but not xc_monitor_enable() (when current->arch.vm_event gets
allocated), or after someone called both, but afterwards called
xc_monitor_disable() (when current->arch.vm_event gets freed).

>> --- a/xen/arch/x86/hvm/emulate.c
>> +++ b/xen/arch/x86/hvm/emulate.c
>> @@ -514,7 +514,7 @@ static int hvmemul_virtual_to_linear(
>>       * vm_event being triggered for repeated writes to a whole page.
>>       */
>>      if ( unlikely(current->domain->arch.mem_access_emulate_each_rep) &&
>> -         current->arch.vm_event->emulate_flags != 0 )
>> +         current->arch.vm_event && current->arch.vm_event->emulate_flags != 0 )
> 
> That's then the third instance of "current" here - this needs
> latching into a local variable.

No problem.


Thanks,
Razvan


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