linux-kernel - Re: [PATCH 2/2] kvm: x86: fix stale mmio cache bug

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Date:	Thu, 14 Aug 2014 09:25:49 -0700
From:	David Matlack <dmatlack@...gle.com>
To:	Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com>
Cc:	Gleb Natapov <gleb@...nel.org>, Avi Kivity <avi.kivity@...il.com>,
	mtosatti@...hat.com, Paolo Bonzini <pbonzini@...hat.com>,
	linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
	stable@...r.kernel.org
Subject: Re: [PATCH 2/2] kvm: x86: fix stale mmio cache bug

On Thu, Aug 14, 2014 at 12:01 AM, Xiao Guangrong
<xiaoguangrong@...ux.vnet.ibm.com> wrote:
> From: David Matlack <dmatlack@...gle.com>
>
> The following events can lead to an incorrect KVM_EXIT_MMIO bubbling
> up to userspace:
>
> (1) Guest accesses gpa X without a memory slot. The gfn is cached in
> struct kvm_vcpu_arch (mmio_gfn). On Intel EPT-enabled hosts, KVM sets
> the SPTE write-execute-noread so that future accesses cause
> EPT_MISCONFIGs.
>
> (2) Host userspace creates a memory slot via KVM_SET_USER_MEMORY_REGION
> covering the page just accessed.
>
> (3) Guest attempts to read or write to gpa X again. On Intel, this
> generates an EPT_MISCONFIG. The memory slot generation number that
> was incremented in (2) would normally take care of this but we fast
> path mmio faults through quickly_check_mmio_pf(), which only checks
> the per-vcpu mmio cache. Since we hit the cache, KVM passes a
> KVM_EXIT_MMIO up to userspace.
>
> This patch fixes the issue by using the memslot generation number
> to validate the mmio cache.
>
> [ xiaoguangrong: adjust the code to make it simpler for stable-tree fix. ]

The adjustments look good. Thanks!
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