Re: [Xen-devel] [PATCH] vm_event: make sure the domain is paused in key domctls

[Razvan Cojocaru <rcojocaru@xxxxxxxxxxxxxxx>]

On 01/28/2016 11:47 PM, Tamas K Lengyel wrote:
> 
> 
> On Thu, Jan 28, 2016 at 2:05 PM, Razvan Cojocaru
> <rcojocaru@xxxxxxxxxxxxxxx <mailto:rcojocaru@xxxxxxxxxxxxxxx>> wrote:
> 
>     On 01/28/2016 10:09 PM, Tamas K Lengyel wrote:
>     > On Thu, Jan 28, 2016 at 6:52 AM, Razvan Cojocaru
>     > <rcojocaru@xxxxxxxxxxxxxxx <mailto:rcojocaru@xxxxxxxxxxxxxxx>
>     <mailto:rcojocaru@xxxxxxxxxxxxxxx
>     <mailto:rcojocaru@xxxxxxxxxxxxxxx>>> wrote:
>     >
>     >     This patch pauses the domain for all writes through the 'ad'
>     >     pointer in monitor_domctl(), defers a domain_unpause() call until
>     >     after the CRs are updated for the MONITOR_EVENT_WRITE_CTRLREG
>     >     case, and makes sure that the domain is paused for both vm_event
>     >     enable and disable cases in vm_event_domctl().
>     >     Thanks go to Andrew Cooper for his review and suggestions.
>     >
>     >
>     > For vm_event_enable the domain is already paused by libxc before the
>     > domctl is issued. I don't see a problem in doing another pause in Xen,
>     > but given we have XSA-99, just doing this pause in Xen would not be
>     > enough. So is it really necessary/fixes anything?
> 
>     This isn't about XSA-99, the problem here is related to my previous
>     patch "x86 vm_event: reset monitor in vm_event_cleanup_domain()". While
>     that improves matters and greatly reduces the chances of crashes due to
>     hvm_msr_write_intercept() or hvm_set_crX() dereferencing a NULL
>     v->arch.vm_event that's assumed to be OK, when the corresponding
>     v->domain->arch.monitor is non-zero, the foolproof way is to make sure
>     that functions such as vm_event_cleanup_domain() are always being called
>     only while the domain has been paused. So there should be a
>     domain_pause() call somewhere on the call path before that.
> 
> 
> Sure, but that's the disable case. I was only wondering about the enable
> case where the domain is already paused.

Yes, for the disable case it is functionally redundant. I just thought
it would be consistent to use domain_pause() throughout and didn't think
it would hurt anything. But I have nothing against removing the
domain_pause() for the enable case (perhaps replacing it with a comment
that libxc already pauses externally), unless Andrew has any objections.


Thanks,
Razvan

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