Re: [Devel] [PATCH VZ10] KVM: x86: Check for invalid/obsolete root *after* making MMU pages available

[Vasileios Almpanis <vasileios.almpanis@virtuozzo.com>]

Reviewed-by: Vasileios Almpanis <vasileios.almpanis@virtuozzo.com>

On 8/7/26 3:47 PM, Pavel Tikhomirov wrote:
> From: Sean Christopherson <seanjc@google.com>
>
> Check for a "stale" page fault, i.e. for an invalid and/or obsolete root,
> after making MMU pages available for the shadow MMU.  If reclaiming shadow
> pages zaps an in-use root, i.e. marks it invalid, then KVM will attempt to
> map memory into an invalid root.  On its own, populating an invalid root is
> "fine", but because child shadow pages inherit their parent's role, any
> children created during the map/fetch will be created as invalid pages,
> thus violating KVM's invariant that invalid pages are never on the list of
> active MMU pages.
>
> Note, the underlying flaw has existed since KVM first started tracking
> invalid roots in 2008 (commit 2e53d63acba7, "KVM: MMU: ignore zapped root
> pagetables"), but the true badness only came along in 2020 (Linux 5.9)
> with the invariant that invalid shadow pages can't be on the list of
> active pages.
>
> Note #2, inheriting role.invalid when creating child shadow pages is also
> far from ideal; that flaw will be addressed separately.
>
> Reported-by: Hyunwoo Kim <imv4bel@gmail.com>
> Fixes: f95eec9bed76 ("KVM: x86/mmu: Don't put invalid SPs back on the list of active pages")
> Cc: stable at vger.kernel.org
> Signed-off-by: Sean Christopherson <seanjc@google.com>
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
>
> CVE-2026-64561 [Zapscape]
> https://virtuozzo.atlassian.net/browse/VSTOR-140678
> (cherry picked from commit 2abd5287f08319fa35764566b15c6e22cb1068db)
> Signed-off-by: Pavel Tikhomirov <ptikhomirov@virtuozzo.com>
> ---
>   arch/x86/kvm/mmu/mmu.c         |  9 +++++----
>   arch/x86/kvm/mmu/paging_tmpl.h | 10 ++++++----
>   2 files changed, 11 insertions(+), 8 deletions(-)
>
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index 6472b0e1b45db..04591cfe4f425 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -4788,16 +4788,17 @@ static int direct_page_fault(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault
>   	if (r != RET_PF_CONTINUE)
>   		return r;
>   
> -	r = RET_PF_RETRY;
>   	write_lock(&vcpu->kvm->mmu_lock);
>   
> -	if (is_page_fault_stale(vcpu, fault))
> -		goto out_unlock;
> -
>   	r = make_mmu_pages_available(vcpu);
>   	if (r)
>   		goto out_unlock;
>   
> +	if (is_page_fault_stale(vcpu, fault)) {
> +		r = RET_PF_RETRY;
> +		goto out_unlock;
> +	}
> +
>   	r = direct_map(vcpu, fault);
>   
>   out_unlock:
> diff --git a/arch/x86/kvm/mmu/paging_tmpl.h b/arch/x86/kvm/mmu/paging_tmpl.h
> index ed762bb4b007b..af220c9c8ab8d 100644
> --- a/arch/x86/kvm/mmu/paging_tmpl.h
> +++ b/arch/x86/kvm/mmu/paging_tmpl.h
> @@ -827,15 +827,17 @@ static int FNAME(page_fault)(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault
>   	}
>   #endif
>   
> -	r = RET_PF_RETRY;
>   	write_lock(&vcpu->kvm->mmu_lock);
>   
> -	if (is_page_fault_stale(vcpu, fault))
> -		goto out_unlock;
> -
>   	r = make_mmu_pages_available(vcpu);
>   	if (r)
>   		goto out_unlock;
> +
> +	if (is_page_fault_stale(vcpu, fault)) {
> +		r = RET_PF_RETRY;
> +		goto out_unlock;
> +	}
> +
>   	r = FNAME(fetch)(vcpu, fault, &walker);
>   
>   out_unlock:

-- 
Best regards, Vasileios Almpanis
Software Developer, Virtuozzo.