Re: [PATCH v3] x86/speculation, KVM: only IBPB for switch_mm_always_ibpb on vCPU load

[Jon Kohler]

> On May 12, 2022, at 9:44 AM, Borislav Petkov <bp@xxxxxxxxx> wrote:
> 
> On Tue, May 10, 2022 at 03:50:31PM +0000, Sean Christopherson wrote:
>>>    x86/speculation, KVM: remove IBPB on vCPU load
>>> 
>>>    Remove IBPB that is done on KVM vCPU load, as the guest-to-guest
>>>    attack surface is already covered by switch_mm_irqs_off() ->
>>>    cond_mitigation().
>>> 
>>>    The original 15d45071523d ("KVM/x86: Add IBPB support") was simply wrong in
>>>    its guest-to-guest design intention. There are three scenarios at play
>>>    here:
>>> 
>>>    1. If the vCPUs belong to the same VM, they are in the same security 
>>>    domain and do not need an IPBP.
>>>    2. If the vCPUs belong to different VMs, and each VM is in its own mm_struct,
>>>    switch_mm_irqs_off() will handle IBPB as an mm switch is guaranteed to
>>>    occur prior to loading a vCPU belonging to a different VMs.
>>>    3. If the vCPUs belong to different VMs, but multiple VMs share an mm_struct,
>>>    then the security benefits of an IBPB when switching vCPUs are dubious, 
>>>    at best.
>>> 
>>>    Issuing IBPB from KVM vCPU load would only cover #3, but there are no
>> 
>> Just to hedge, there are no _known_ use cases.
>> 
>>>    real world tangible use cases for such a configuration.
>> 
>> and I would further qualify this with:
>> 
>>      but there are no known real world, tangible use cases for running multiple
>>      VMs belonging to different security domains in a shared address space.
>> 
>> Running multiple VMs in a single address space is plausible and sane, _if_ they
>> are all in the same security domain or security is not a concern.  That way the
>> statement isn't invalidated if someone pops up with a use case for running multiple
>> VMs but has no security story.
>> 
>> Other than that, LGTM.
>> 
>>>    If multiple VMs
>>>    are sharing an mm_structs, prediction attacks are the least of their
>>>    security worries.
>>> 
>>>    Fixes: 15d45071523d ("KVM/x86: Add IBPB support")
>>>    (Reviewedby/signed of by people here)
>>>    (Code change simply whacks IBPB in KVM vmx/svm and thats it)
> 
> I agree with all that I've read so far - the only thing that's missing is:
> 
> 	(Documentation in Documentation/admin-guide/hw-vuln/spectre.rst about what the use
> 	 cases are and what we're protecting against and what we're *not* protecting
> 	 against because <raisins>).
> 
> Thx.

Ok Thanks, Boris. I’ll review that doc and make modifications on v4, and make sure
that you are cc’d.

Thanks again,
Jon

> 
> -- 
> Regards/Gruss,
>    Boris.
> 
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