linux-kernel - Re: [PATCH 5/5] KVM: VMX: Always honor guest PAT on CPUs that support self-snoop

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Date: Thu, 14 Mar 2024 08:12:12 +0800
From: Yan Zhao <yan.y.zhao@...el.com>
To: Sean Christopherson <seanjc@...gle.com>
CC: Kevin Tian <kevin.tian@...el.com>, "jgg@...dia.com" <jgg@...dia.com>,
	Paolo Bonzini <pbonzini@...hat.com>, Lai Jiangshan <jiangshanlai@...il.com>,
	"Paul E. McKenney" <paulmck@...nel.org>, Josh Triplett
	<josh@...htriplett.org>, "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
	"rcu@...r.kernel.org" <rcu@...r.kernel.org>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>, Yiwei Zhang <zzyiwei@...gle.com>
Subject: Re: [PATCH 5/5] KVM: VMX: Always honor guest PAT on CPUs that
 support self-snoop

On Wed, Mar 13, 2024 at 08:09:28AM -0700, Sean Christopherson wrote:
> On Wed, Mar 13, 2024, Yan Zhao wrote:
> > > We'll certain fix the security hole on CPUs w/ self-snoop. In this case
> > > CPU accesses are guaranteed to be coherent and the vulnerability can
> > > only be exposed via non-coherent DMA which is supposed to be fixed
> > > by your coming series. 
> > > 
> > > But for old CPUs w/o self-snoop the hole can be exploited using either CPU
> > > or non-coherent DMA once the guest PAT is honored. As long as nobody
> > > is willing to actually fix the CPU path (is it possible?) I'm kind of convinced
> > We can cook a patch to check CPU self-snoop and force WB in EPT even for
> > non-coherent DMA if no self-snoop. Then back porting such a patch together
> > with the IOMMU side mitigation for non-coherent DMA.
> 
> Please don't.  This is a "let sleeping dogs lie" situation.
> 
>   let sleeping dogs lie - avoid interfering in a situation that is currently
>   causing no problems but might do so as a result of such interference.
> 
> Yes, there is technically a flaw, but we have zero evidence that anyone cares or
> that it is actually problematic in practice.  On the other hand, any functional
> change we make has a non-zero changes of breaking existing setups that have worked
> for many years. 
> 
> > Otherwise, IOMMU side mitigation alone is meaningless for platforms of CPU of
> > no self-snoop.
> > 
> > > by Sean that sustaining the old behavior is probably the best option...
> > Yes, as long as we think exposing secuirty hole on those platforms is acceptable. 
> 
> Yes, I think it's acceptable.  Obviously not ideal, but given the alternatives,
> I think it is a reasonable risk.
> 
> Being 100% secure is simply not possible.  Security is often about balancing the
> risk/threat against the cost.  In this case, the risk is low (old hardware,
> uncommon setup for untrusted guests, small window of opportunity, and limited
> data exposure), whereas the cost is high (decent chance of breaking existing VMs).
Ok, thanks for explanation!
I still have one last question: if in future there are CPUs with no selfsnoop
(for some unknown reason, or just paranoid), do we allow this unsafe honoring of
guest memory type for non-coherent DMAs?