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

[Borislav Petkov]

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.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette