Re: [PATCH 1/7] KVM: x86: Retry page fault if MMU reload is pending and root has no sp

[Paolo Bonzini]

On 12/9/21 07:05, Sean Christopherson wrote:
> Play nice with a NULL shadow page when checking for an obsolete root in
> the page fault handler by flagging the page fault as stale if there's no
> shadow page associated with the root and KVM_REQ_MMU_RELOAD is pending.
> Invalidating memslots, which is the only case where _all_ roots need to
> be reloaded, requests all vCPUs to reload their MMUs while holding
> mmu_lock for lock.
>
> The "special" roots, e.g. pae_root when KVM uses PAE paging, are not
> backed by a shadow page.  Running with TDP disabled or with nested NPT
> explodes spectaculary due to dereferencing a NULL shadow page pointer.
>
> Skip the KVM_REQ_MMU_RELOAD check if there is a valid shadow page for the
> root.  Zapping shadow pages in response to guest activity, e.g. when the
> guest frees a PGD, can trigger KVM_REQ_MMU_RELOAD even if the current
> vCPU isn't using the affected root.  I.e. KVM_REQ_MMU_RELOAD can be seen
> with a completely valid root shadow page.  This is a bit of a moot point
> as KVM currently unloads all roots on KVM_REQ_MMU_RELOAD, but that will
> be cleaned up in the future.
>
> Fixes: a955cad84cda ("KVM: x86/mmu: Retry page fault if root is invalidated by memslot update")
> Cc: stable@xxxxxxxxxxxxxxx
> Cc: Maxim Levitsky <mlevitsk@xxxxxxxxxx>
> Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> ---
>   arch/x86/kvm/mmu/mmu.c | 16 +++++++++++++++-
>   1 file changed, 15 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index 1ccee4d17481..1d275e9d76b5 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -3971,7 +3971,21 @@ static bool kvm_faultin_pfn(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault,
>   static bool is_page_fault_stale(struct kvm_vcpu *vcpu,
>   				struct kvm_page_fault *fault, int mmu_seq)
>   {
> -	if (is_obsolete_sp(vcpu->kvm, to_shadow_page(vcpu->arch.mmu->root_hpa)))
> +	struct kvm_mmu_page *sp = to_shadow_page(vcpu->arch.mmu->root_hpa);
> +
> +	/* Special roots, e.g. pae_root, are not backed by shadow pages. */
> +	if (sp && is_obsolete_sp(vcpu->kvm, sp))
> +		return true;
> +
> +	/*
> +	 * Roots without an associated shadow page are considered invalid if
> +	 * there is a pending request to free obsolete roots.  The request is
> +	 * only a hint that the current root _may_ be obsolete and needs to be
> +	 * reloaded, e.g. if the guest frees a PGD that KVM is tracking as a
> +	 * previous root, then __kvm_mmu_prepare_zap_page() signals all vCPUs
> +	 * to reload even if no vCPU is actively using the root.
> +	 */
> +	if (!sp && kvm_test_request(KVM_REQ_MMU_RELOAD, vcpu))
>   		return true;
>   
>   	return fault->slot &&

Queued this one for 5.16, thanks.

Paolo