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SubjectRe: [PATCH RFC] KVM: nSVM: Fix L1 state corruption upon return from SMM
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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
> >


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