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

[Razvan Cojocaru <rcojocaru@xxxxxxxxxxxxxxx>]

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>> 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.

In fact, any writes to domain-scope variables that can affect the way a
guest runs (while the guest is running) should ideally be done with the
domain paused, hence the rest of the changes in the patch.


Thanks,
Razvan

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