RE: [PATCH v1 0/7] Implement support for external IPT monitoring

Kang, Luwei posted 7 patches 3 weeks ago
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RE: [PATCH v1 0/7] Implement support for external IPT monitoring

Posted by Kang, Luwei 3 weeks ago
> > > How does KVM deal with this, do they insert/modify trace packets on
> > > trapped and emulated instructions by the VMM?
> >
> > The KVM includes instruction decoder and
> emulator(arch/x86/kvm/emulate.c), and the guest's memory can be set to
> write-protect as well. But it doesn't support Intel PT packets software emulator.
> For KVM, the Intel PT feature will be exposed to KVM guest and KVM guest can
> use Intel PT feature like native.
> 
> But if such feature is exposed to the guest for it's own usage, won't it be
> missing packets for instructions emulated by the VMM?

If setting the guest's memory write-protect, I think yes. 

Thanks,
Luwei Kang

> 
> Thanks, Roger.

Re: [PATCH v1 0/7] Implement support for external IPT monitoring

Posted by Michał Leszczyński 3 weeks ago
----- 18 cze 2020 o 1:29, Kang, Luwei luwei.kang@intel.com napisał(a):

>> > > How does KVM deal with this, do they insert/modify trace packets on
>> > > trapped and emulated instructions by the VMM?
>> >
>> > The KVM includes instruction decoder and
>> emulator(arch/x86/kvm/emulate.c), and the guest's memory can be set to
>> write-protect as well. But it doesn't support Intel PT packets software
>> emulator.
>> For KVM, the Intel PT feature will be exposed to KVM guest and KVM guest can
>> use Intel PT feature like native.
>> 
>> But if such feature is exposed to the guest for it's own usage, won't it be
>> missing packets for instructions emulated by the VMM?
> 
> If setting the guest's memory write-protect, I think yes.


Thus, I propose to leave it as it is right now. If somebody is purposely altering the VM state then he/she should consult not only the IPT but also understand what was done "in the meantime" by additional features, e.g. when something was altered by vm_event callback. As Tamas said previously, we usually just want to see certain path leading to vmexit.

Please also note that there is a PTWRITE instruction that could be used in the future in order to add custom payloads/hints to the PT trace, when needed.


> 
> Thanks,
> Luwei Kang
> 
>> 
> > Thanks, Roger.

Re: [PATCH v1 0/7] Implement support for external IPT monitoring

Posted by Roger Pau Monné 3 weeks ago
On Thu, Jun 18, 2020 at 02:56:17AM +0200, Michał Leszczyński wrote:
> ----- 18 cze 2020 o 1:29, Kang, Luwei luwei.kang@intel.com napisał(a):
> 
> >> > > How does KVM deal with this, do they insert/modify trace packets on
> >> > > trapped and emulated instructions by the VMM?
> >> >
> >> > The KVM includes instruction decoder and
> >> emulator(arch/x86/kvm/emulate.c), and the guest's memory can be set to
> >> write-protect as well. But it doesn't support Intel PT packets software
> >> emulator.
> >> For KVM, the Intel PT feature will be exposed to KVM guest and KVM guest can
> >> use Intel PT feature like native.
> >> 
> >> But if such feature is exposed to the guest for it's own usage, won't it be
> >> missing packets for instructions emulated by the VMM?
> > 
> > If setting the guest's memory write-protect, I think yes.
> 
> 
> Thus, I propose to leave it as it is right now. If somebody is purposely altering the VM state then he/she should consult not only the IPT but also understand what was done "in the meantime" by additional features, e.g. when something was altered by vm_event callback. As Tamas said previously, we usually just want to see certain path leading to vmexit.
> 
> Please also note that there is a PTWRITE instruction that could be used in the future in order to add custom payloads/hints to the PT trace, when needed.

Yes, I think the usage of IPT by a third party against a guest is
fine, as such third party can also use introspection and get the
information about the emulated instructions.

OTOH exposing the feature to the guest itself for it's own usage seems
wrong without adding the packets related to the instructions emulated.

I understand the current series only cares about the first option, so
that's perfectly fine.

Roger.