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

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Date: Wed, 13 Mar 2024 16:55:04 +0800
From: Yan Zhao <yan.y.zhao@...el.com>
To: "Tian, Kevin" <kevin.tian@...el.com>
CC: Sean Christopherson <seanjc@...gle.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

> 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.

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.