linux-kernel - Re: [PATCH v2 2/9] KVM: x86: Check CR3 GPA for validity regardless of vCPU mode

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Date:   Thu, 22 Apr 2021 16:55:42 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Babu Moger <babu.moger@....com>,
        Joao Martins <joao.m.martins@...cle.com>,
        David Woodhouse <dwmw@...zon.co.uk>
Subject: Re: [PATCH v2 2/9] KVM: x86: Check CR3 GPA for validity regardless
 of vCPU mode

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.