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To:     Ben Gardon <bgardon@google.com>, linux-kernel@vger.kernel.org,
        kvm@vger.kernel.org
Cc:     Sean Christopherson <sean.j.christopherson@intel.com>,
        Peter Shier <pshier@google.com>,
        Jim Mattson <jmattson@google.com>,
        Zdenek Kaspar <zkaspar82@gmail.com>
References: <20201111185337.1237383-1-bgardon@google.com>
From:   Paolo Bonzini <pbonzini@redhat.com>
Subject: Re: [PATCH] kvm: x86/mmu: Fix is_tdp_mmu_check when using PAE
Message-ID: <6c4a78e8-8237-b604-d047-b1ea010cf655@redhat.com>
Date:   Fri, 13 Nov 2020 21:58:09 +0100
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On 11/11/20 19:53, Ben Gardon wrote:
> When PAE is in use, the root_hpa will not have a shadow page assoicated
> with it. In this case the kernel will crash with a NULL pointer
> dereference. Add checks to ensure is_tdp_mmu_root works as intended even
> when using PAE.
> 
> Tested: compiles
> 
> Fixes: 02c00b3a2f7e ("kvm: x86/mmu: Allocate and free TDP MMU roots")
> Reported-by: Zdenek Kaspar <zkaspar82@gmail.com>
> Signed-off-by: Ben Gardon <bgardon@google.com>
> ---
>   arch/x86/kvm/mmu/tdp_mmu.c | 10 ++++++++++
>   1 file changed, 10 insertions(+)
> 
> diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
> index 27e381c9da6c..13013f4d98ad 100644
> --- a/arch/x86/kvm/mmu/tdp_mmu.c
> +++ b/arch/x86/kvm/mmu/tdp_mmu.c
> @@ -49,8 +49,18 @@ bool is_tdp_mmu_root(struct kvm *kvm, hpa_t hpa)
>   {
>   	struct kvm_mmu_page *sp;
>   
> +	if (WARN_ON(!VALID_PAGE(hpa)))
> +		return false;
> +
>   	sp = to_shadow_page(hpa);
>   
> +	/*
> +	 * If this VM is being run with PAE, the TDP MMU will not be enabled
> +	 * and the root HPA will not have a shadow page associated with it.
> +	 */
> +	if (!sp)
> +		return false;
> +
>   	return sp->tdp_mmu_page && sp->root_count;
>   }
>   
> 

If this was just PAE, it would be easier to test "if (shadow_root_level 
 >= PT64_ROOT_4LEVEL)"---and more correct too, because using the 
page_private of __pa(vcpu->arch.mmu->pae_root) is a bit untidy; we 
should only use page_private for pages that we know have a shadow page.

In Jamie's case however, it is x86_64 (so kvm_mmu_get_tdp_level(vcpu) == 
4 and therefore the "if (shadow_root_level >= PT64_ROOT_4LEVEL)" would 
be true) but without EPT.  In that case we go through

	vcpu->arch.mmu->root_hpa = __pa(vcpu->arch.mmu->lm_root);

but lm_root is allocated with get_zeroed_page and therefore 
to_shadow_page is NULL.

I am thinking of testing simply "if (tdp_enabled)" so that we can see if 
there are other cases with to_shadow_page(hpa) == NULL and we don't 
sweep them under the rug.  Or test "if (tdp_enabled)" and also WARN if 
!sp.  What do you think?

Paolo