Re: [PATCH] KVM: x86/mmu: avoid accidentally go to shadow path for 0 count tdp root

[Yan Zhao]

On Thu, Nov 10, 2022 at 04:54:20PM +0000, Sean Christopherson wrote:
> On Thu, Nov 10, 2022, Yan Zhao wrote:
> > kvm mmu uses "if (is_tdp_mmu(vcpu->arch.mmu))" to choose between tdp mmu
> > and shadow path.
> > If a root is a tdp mmu page while its root_count is 0, it's not valid to
> > go to the shadow path.
> > 
> > So, return true and add a warn on zero root count.
> > 
> > Signed-off-by: Yan Zhao <yan.y.zhao@xxxxxxxxx>
> > ---
> >  arch/x86/kvm/mmu/tdp_mmu.h | 6 +++++-
> >  1 file changed, 5 insertions(+), 1 deletion(-)
> > 
> > diff --git a/arch/x86/kvm/mmu/tdp_mmu.h b/arch/x86/kvm/mmu/tdp_mmu.h
> > index c163f7cc23ca..58b4881654a9 100644
> > --- a/arch/x86/kvm/mmu/tdp_mmu.h
> > +++ b/arch/x86/kvm/mmu/tdp_mmu.h
> > @@ -74,6 +74,7 @@ static inline bool is_tdp_mmu(struct kvm_mmu *mmu)
> >  {
> >  	struct kvm_mmu_page *sp;
> >  	hpa_t hpa = mmu->root.hpa;
> > +	bool is_tdp;
> >  
> >  	if (WARN_ON(!VALID_PAGE(hpa)))
> >  		return false;
> > @@ -84,7 +85,10 @@ static inline bool is_tdp_mmu(struct kvm_mmu *mmu)
> >  	 * pae_root page, not a shadow page.
> >  	 */
> >  	sp = to_shadow_page(hpa);
> > -	return sp && is_tdp_mmu_page(sp) && sp->root_count;
> > +	is_tdp = sp && is_tdp_mmu_page(sp);
> > +	WARN_ON(is_tdp && !refcount_read(&sp->tdp_mmu_root_count));
> > +
> > +	return is_tdp;
> 
> I have a series/patch that drops this code entirely, I would rather just go that
> route directly.
> 
> https://lore.kernel.org/all/20221012181702.3663607-9-seanjc@xxxxxxxxxx
Thanks for pointing me to this link.
Yes, it's a much better fix!

Thanks
Yan