linux-kernel - Re: [PATCH RFC] KVM: nSVM: Fix L1 state corruption upon return from SMM

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Date:   Thu, 24 Jun 2021 09:32:18 -0500
From:   Tom Lendacky <thomas.lendacky@....com>
To:     Paolo Bonzini <pbonzini@...hat.com>,
        Maxim Levitsky <mlevitsk@...hat.com>,
        Michael Roth <mdroth@...ux.vnet.ibm.com>
Cc:     Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Cathy Avery <cavery@...hat.com>,
        Emanuele Giuseppe Esposito <eesposit@...hat.com>,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        Sean Christopherson <seanjc@...gle.com>
Subject: Re: [PATCH RFC] KVM: nSVM: Fix L1 state corruption upon return from
 SMM

On 6/24/21 5:38 AM, Paolo Bonzini wrote:
> On 24/06/21 10:20, Maxim Levitsky wrote:
>> Something else to note, just for our information is that KVM
>> these days does vmsave/vmload to VM_HSAVE_PA to store/restore
>> the additional host state, something that is frowned upon in the spec,
>> but there is some justification of doing this in the commit message,
>> citing an old spec which allowed this.
> 
> True that.  And there is no mention in the specification for VMRUN that
> the host state-save area is a subset of the VMCB format (i.e., that it
> uses VMCB offsets for whatever subset of the state it saves in the VMCB),
> so the spec reference in the commit message is incorrect.  It would be
> nice if the spec guaranteed that.  Michael, Tom?

So that is (now) stated in APM volume 2, Appendix B in the paragraph after
Table B-3, where it starts "The format of the host save area is identical
to the guest save area described in the table below, except that ..."

Thanks,
Tom

> 
> In fact, Vitaly's patch *will* overwrite the vmsave/vmload parts of
> VM_HSAVE_PA, and it will store the L2 values rather than the L1 values,
> because KVM always does its vmload/vmrun/vmsave sequence using
> vmload(vmcs01) and vmsave(vmcs01)!  So that has to be changed to use code
> similar to svm_set_nested_state (which can be moved to a separate function
> and reused):
> 
>         dest->es = src->es;
>         dest->cs = src->cs;
>         dest->ss = src->ss;
>         dest->ds = src->ds;
>         dest->gdtr = src->gdtr;
>         dest->idtr = src->idtr;
>         dest->rflags = src->rflags | X86_EFLAGS_FIXED;
>         dest->efer = src->efer;
>         dest->cr0 = src->cr0;
>         dest->cr3 = src->cr3;
>         dest->cr4 = src->cr4;
>         dest->rax = src->rax;
>         dest->rsp = src->rsp;
>         dest->rip = src->rip;
>         dest->cpl = 0;
> 
> 
> Paolo
>