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From: Sean Christopherson <seanjc@google.com>
To: Paolo Bonzini <pbonzini@redhat.com>
Cc: Vitaly Kuznetsov <vkuznets@redhat.com>,
	Wanpeng Li <wanpengli@tencent.com>,
	Jim Mattson <jmattson@google.com>, Joerg Roedel <joro@8bytes.org>,
	kvm@vger.kernel.org, linux-kernel@vger.kernel.org,
	Babu Moger <babu.moger@amd.com>,
	Joao Martins <joao.m.martins@oracle.com>,
	David Woodhouse <dwmw@amazon.co.uk>
Subject: Re: [PATCH v2 2/9] KVM: x86: Check CR3 GPA for validity regardless of vCPU mode
Date: Thu, 22 Apr 2021 16:55:42 +0000	[thread overview]
Message-ID: <YIGqjoHwG+7rHWyp@google.com> (raw)
In-Reply-To: <8716951d-cddb-d5f9-e7e2-b651120a51e7@redhat.com>

On Thu, Apr 22, 2021, Paolo Bonzini wrote:
> On 22/04/21 04:21, Sean Christopherson wrote:
> > Check CR3 for an invalid GPA even if the vCPU isn't in long mode.  For
> > bigger emulation flows, notably RSM, the vCPU mode may not be accurate
> > if CR0/CR4 are loaded after CR3.  For MOV CR3 and similar flows, the
> > caller is responsible for truncating the value.
> > 
> > Note, SMRAM.CR3 is read-only, so this is mostly a theoretical bug since
> > KVM will not have stored an illegal CR3 into SMRAM during SMI emulation.
> 
> Well, the guest could have changed it...

That's what I tried to address with "SMRAM.CR3 is read-only".  Both Intel and
AMD state that modifying read-only fields will result in unpredictable behavior,
i.e. KVM going into the weeds would be within spec.  IIRC, there's no real
danger to the host, it'll "just" fail VM-Enter.

SDM:
  Some register images are read-only, and must not be modified (modifying these
  registers will result in unpredictable behavior)

APM:

  Software should not modify offsets specified as read-only or reserved,
  otherwise unpredictable results can occur.

  reply	other threads:[~2021-04-22 16:55 UTC|newest]

Thread overview: 17+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-04-22  2:21 [PATCH v2 0/9] KVM: x86: Fixes for (benign?) truncation bugs Sean Christopherson
2021-04-22  2:21 ` [PATCH v2 1/9] KVM: x86: Remove emulator's broken checks on CR0/CR3/CR4 loads Sean Christopherson
2021-04-22  6:50   ` Paolo Bonzini
2021-04-22  2:21 ` [PATCH v2 2/9] KVM: x86: Check CR3 GPA for validity regardless of vCPU mode Sean Christopherson
2021-04-22  6:51   ` Paolo Bonzini
2021-04-22 16:55     ` Sean Christopherson [this message]
2021-04-22  2:21 ` [PATCH v2 3/9] KVM: SVM: Truncate GPR value for DR and CR accesses in !64-bit mode Sean Christopherson
2021-04-22  2:21 ` [PATCH v2 4/9] KVM: VMX: Truncate GPR value for DR and CR reads " Sean Christopherson
2021-04-22  2:21 ` [PATCH v2 5/9] KVM: nVMX: Truncate bits 63:32 of VMCS field on nested check in !64-bit Sean Christopherson
2021-04-22  2:21 ` [PATCH v2 6/9] KVM: nVMX: Truncate base/index GPR value on address calc " Sean Christopherson
2021-04-22  2:21 ` [PATCH v2 7/9] KVM: x86/xen: Drop RAX[63:32] when processing hypercall Sean Christopherson
2021-04-22  9:51   ` Vitaly Kuznetsov
2021-04-22 10:35     ` Paolo Bonzini
2021-04-22 10:49       ` Vitaly Kuznetsov
2021-04-22  2:21 ` [PATCH v2 8/9] KVM: SVM: Use default rAX size for INVLPGA emulation Sean Christopherson
2021-04-22  2:21 ` [PATCH v2 9/9] KVM: x86: Rename GPR accessors to make mode-aware variants the defaults Sean Christopherson
2021-04-22  6:55 ` [PATCH v2 0/9] KVM: x86: Fixes for (benign?) truncation bugs Paolo Bonzini

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