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

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Date:   Thu, 24 Jun 2021 18:36:12 +0300
From:   Maxim Levitsky <mlevitsk@...hat.com>
To:     Tom Lendacky <thomas.lendacky@....com>,
        Paolo Bonzini <pbonzini@...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 Thu, 2021-06-24 at 09:32 -0500, Tom Lendacky wrote:
> 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 ..."

This is a very good find! I wish it was written in the commit message
of commit that added that vmsave/vmload to VM_HSAVE_PA area.

Maybe we should add a comment to the code pointing to this location of the APM.

Thanks,
	Best regards,
		Maxim Levitsky

> 
> 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
> >