linux-kernel - Re: [PATCH 1/4] KVM: x86/mmu: Use -1 to flag an undefined spte in get_mmio

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Message-ID: <87tusjtrqp.fsf@vitty.brq.redhat.com>
Date:   Fri, 18 Dec 2020 09:58:54 +0100
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>
Cc:     Sean Christopherson <seanjc@...gle.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Ben Gardon <bgardon@...gle.com>,
        Richard Herbert <rherbert@...patico.ca>
Subject: Re: [PATCH 1/4] KVM: x86/mmu: Use -1 to flag an undefined spte in
 get_mmio_spte()

Sean Christopherson <seanjc@...gle.com> writes:

> Return -1 from the get_walk() helpers if the shadow walk doesn't fill at
> least one spte, which can theoretically happen if the walk hits a
> not-present PTPDR.  Returning the root level in such a case will cause

PDPTR

> get_mmio_spte() to return garbage (uninitialized stack data).  In
> practice, such a scenario should be impossible as KVM shouldn't get a
> reserved-bit page fault with a not-present PDPTR.
>
> Note, using mmu->root_level in get_walk() is wrong for other reasons,
> too, but that's now a moot point.
>
> Fixes: 95fb5b0258b7 ("kvm: x86/mmu: Support MMIO in the TDP MMU")
> Cc: Ben Gardon <bgardon@...gle.com>
> Cc: stable@...r.kernel.org
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> ---
>  arch/x86/kvm/mmu/mmu.c     | 7 ++++++-
>  arch/x86/kvm/mmu/tdp_mmu.c | 2 +-
>  2 files changed, 7 insertions(+), 2 deletions(-)
>
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index 7a6ae9e90bd7..a48cd12c01d7 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -3488,7 +3488,7 @@ static bool mmio_info_in_cache(struct kvm_vcpu *vcpu, u64 addr, bool direct)
>  static int get_walk(struct kvm_vcpu *vcpu, u64 addr, u64 *sptes)
>  {
>  	struct kvm_shadow_walk_iterator iterator;
> -	int leaf = vcpu->arch.mmu->root_level;
> +	int leaf = -1;
>  	u64 spte;
>  
>  
> @@ -3532,6 +3532,11 @@ static bool get_mmio_spte(struct kvm_vcpu *vcpu, u64 addr, u64 *sptep)
>  	else
>  		leaf = get_walk(vcpu, addr, sptes);
>  
> +	if (unlikely(leaf < 0)) {
> +		*sptep = 0ull;
> +		return reserved;
> +	}

When SPTE=0 is returned from get_mmio_spte(), handle_mmio_page_fault()
will return RET_PF_RETRY -- should it be RET_PF_INVALID instead?

> +
>  	rsvd_check = &vcpu->arch.mmu->shadow_zero_check;
>  
>  	for (level = root; level >= leaf; level--) {
> diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
> index 84c8f06bec26..50cec7a15ddb 100644
> --- a/arch/x86/kvm/mmu/tdp_mmu.c
> +++ b/arch/x86/kvm/mmu/tdp_mmu.c
> @@ -1152,8 +1152,8 @@ int kvm_tdp_mmu_get_walk(struct kvm_vcpu *vcpu, u64 addr, u64 *sptes)
>  {
>  	struct tdp_iter iter;
>  	struct kvm_mmu *mmu = vcpu->arch.mmu;
> -	int leaf = vcpu->arch.mmu->shadow_root_level;
>  	gfn_t gfn = addr >> PAGE_SHIFT;
> +	int leaf = -1;
>  
>  	tdp_mmu_for_each_pte(iter, mmu, gfn, gfn + 1) {
>  		leaf = iter.level;

-- 
Vitaly