linux-kernel - Re: [PATCH] KVM: nSVM: vmentry ignores EFER.LMA and possibly RFLAGS.VM

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Date:   Thu, 9 Jul 2020 11:28:25 -0700
From:   Jim Mattson <jmattson@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        kvm list <kvm@...r.kernel.org>,
        Maxim Levitsky <mlevitsk@...hat.com>
Subject: Re: [PATCH] KVM: nSVM: vmentry ignores EFER.LMA and possibly RFLAGS.VM

On Thu, Jul 9, 2020 at 10:25 AM Paolo Bonzini <pbonzini@...hat.com> wrote:
>
> On 09/07/20 19:12, Jim Mattson wrote:
> >> +
> >> +       /* The processor ignores EFER.LMA, but svm_set_efer needs it.  */
> >> +       efer &= ~EFER_LMA;
> >> +       if ((nested_vmcb->save.cr0 & X86_CR0_PG)
> >> +           && (nested_vmcb->save.cr4 & X86_CR4_PAE)
> >> +           && (efer & EFER_LME))
> >> +               efer |= EFER_LMA;
> > The CR4.PAE check is unnecessary, isn't it? The combination CR0.PG=1,
> > EFER.LMA=1, and CR4.PAE=0 is not a legal processor state.

Oops, I meant EFER.LME above.

Krish pointed out that I was quoting from Intel's documentation. The
same constraints are covered in Table 14-5 of AMD's APM, volume 2.

> Yeah, I was being a bit cautious because this is the nested VMCB and it
> can be filled in with invalid state, but indeed that condition was added
> just yesterday by myself in nested_vmcb_checks (while reviewing Krish's
> CR0/CR3/CR4 reserved bit check series).

>From Canonicalization and Consistency Checks of section 15.5 in AMD's
APM, volume 2:

The following conditions are considered illegal state combinations:
...
EFER.LME and CR0.PG are both set and CR4.PAE is zero.

This VMCB state should result in an immediate #VMEXIT with exit code -1.

> That said, the VMCB here is guest memory and it can change under our
> feet between nested_vmcb_checks and nested_prepare_vmcb_save.  Copying
> the whole save area is overkill, but we probably should copy at least
> EFER/CR0/CR3/CR4 in a struct at the beginning of nested_svm_vmrun; this
> way there'd be no TOC/TOU issues between nested_vmcb_checks and
> nested_svm_vmrun.  This would also make it easier to reuse the checks in
> svm_set_nested_state.  Maybe Maxim can look at it while I'm on vacation,
> as he's eager to do more nSVM stuff. :D

I fear that nested SVM is rife with TOCTTOU issues.