Re: [PATCH] x86/monitor: Disallow setting mem_access_emulate_each_rep when vm_event is NULL

[Razvan Cojocaru <rcojocaru@bitdefender.com>]

On 04/08/16 11:17, Andrew Cooper wrote:
> On 08/04/2016 08:16, Razvan Cojocaru wrote:
>> On 04/08/16 10:10, Razvan Cojocaru wrote:
>>> It is meaningless (and potentially dangerous - see hvmemul_virtual_to_linear())
>>> to set mem_access_emulate_each_rep before xc_monitor_enable() (which allocates
>>> vcpu->arch.vm_event) has been called, so return an error from the
>>> XEN_DOMCTL_MONITOR_OP_EMULATE_EACH_REP hypercall when that is the case.
>>>
>>> Signed-off-by: Razvan Cojocaru <rcojocaru@bitdefender.com>
>>> ---
>>>  xen/include/asm-x86/monitor.h | 15 +++++++++++++--
>>>  1 file changed, 13 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/xen/include/asm-x86/monitor.h b/xen/include/asm-x86/monitor.h
>>> index 0954b59..0544836 100644
>>> --- a/xen/include/asm-x86/monitor.h
>>> +++ b/xen/include/asm-x86/monitor.h
>>> @@ -35,11 +35,22 @@ int arch_monitor_domctl_op(struct domain *d, struct xen_domctl_monitor_op *mop)
>>>      switch ( mop->op )
>>>      {
>>>      case XEN_DOMCTL_MONITOR_OP_EMULATE_EACH_REP:
>>> +    {
>>> +        bool_t value = !!mop->event;
> 
> While you are at at, one extra newline here please.
> 
>>>          domain_pause(d);
>>> -        d->arch.mem_access_emulate_each_rep = !!mop->event;
>>> +        /*
>>> +         * Enabling emulate_each_rep without a vm_event subscriber
>>> +         * is meaningless.
>>> +         */
>>> +        if ( !d->vcpu || !d->vcpu[0]->arch.vm_event )
>> Sorry, this is a bit convoluted, and wrong: if d->vcpu != NULL it will
>> return -EINVAL even if d->vcpu[0]->arch.vm_event is NULL. I'll rework
>> it. Apologies for the noise.
> 
> The issue comes about because d->arch.mem_access_emulate_each_rep is a
> per-domain variable, whereas vm_event listeners are per-vcpu.
> 
> In principle, it would be legitimate for an introspection engine to only
> want to introspect a single vcpu.
> 
> Either we need to enforce that every vcpu has vm_event infrastructure
> set up (and allow for delivery of events is disabled on a per-vcpu
> basis), or settings such as emulate_each_rep needs to be made per-vcpu.

Xc_monitor_enable() does set vm_event up for all of the domain's VCPUs,
so at least for now it is a fair assumption that if it is set for the
first one, it's set for all of them. In other words, your first
requirement is currently being implicitly enforced.

Introspecting a single VCPU is an interesting idea, but not terribly
useful at this point.


Thanks,
Razvan

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