Re: [PATCH] KVM: x86/mmu: Don't WARN on a NULL shadow page in TDP MMU check

[David Matlack]

On Tue, Jun 22, 2021 at 12:24 AM Sean Christopherson <seanjc@xxxxxxxxxx> wrote:
>
> Treat a NULL shadow page in the "is a TDP MMU" check as valid, non-TDP
> root.  KVM uses a "direct" PAE paging MMU when TDP is disabled and the
> guest is running with paging disabled.  In that case, root_hpa points at
> the pae_root page (of which only 32 bytes are used), not a standard
> shadow page, and the WARN fires (a lot).
>
> Fixes: 0b873fd7fb53 ("KVM: x86/mmu: Remove redundant is_tdp_mmu_enabled check")
> Cc: David Matlack <dmatlack@xxxxxxxxxx>
> Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>

Thanks for the fix. I was able to reproduce the issue by running a
kvm-unit-test with EPT=N. I'll add that to my "pre-send-email"
workflow in the future.

Reviewed-by: David Matlack <dmatlack@xxxxxxxxxx>

> ---
>  arch/x86/kvm/mmu/tdp_mmu.h | 10 ++++++----
>  1 file changed, 6 insertions(+), 4 deletions(-)
>
> diff --git a/arch/x86/kvm/mmu/tdp_mmu.h b/arch/x86/kvm/mmu/tdp_mmu.h
> index b981a044ab55..1cae4485b3bc 100644
> --- a/arch/x86/kvm/mmu/tdp_mmu.h
> +++ b/arch/x86/kvm/mmu/tdp_mmu.h
> @@ -94,11 +94,13 @@ static inline bool is_tdp_mmu(struct kvm_mmu *mmu)
>         if (WARN_ON(!VALID_PAGE(hpa)))
>                 return false;
>
> +       /*
> +        * A NULL shadow page is legal when shadowing a non-paging guest with
> +        * PAE paging, as the MMU will be direct with root_hpa pointing at the
> +        * pae_root page, not a shadow page.
> +        */
>         sp = to_shadow_page(hpa);
> -       if (WARN_ON(!sp))
> -               return false;
> -
> -       return is_tdp_mmu_page(sp) && sp->root_count;
> +       return sp && is_tdp_mmu_page(sp) && sp->root_count;
>  }
>  #else
>  static inline bool kvm_mmu_init_tdp_mmu(struct kvm *kvm) { return false; }
> --
> 2.32.0.288.g62a8d224e6-goog
>