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

Sean Christopherson
2021-04-22T16:55:50Z

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.