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Message-ID: <76dfc0b8-f892-c445-084c-62672293ca92@redhat.com>
Date:   Wed, 27 Apr 2022 18:16:54 +0200
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Subject: Re: [PATCH MANUALSEL 5.17 5/7] KVM: x86/mmu: avoid NULL-pointer
 dereference on page freeing bugs
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To:     Sasha Levin <sashal@kernel.org>, linux-kernel@vger.kernel.org,
        stable@vger.kernel.org
Cc:     Sean Christopherson <seanjc@google.com>, tglx@linutronix.de,
        mingo@redhat.com, bp@alien8.de, dave.hansen@linux.intel.com,
        x86@kernel.org, kvm@vger.kernel.org
References: <20220427155408.19352-1-sashal@kernel.org>
 <20220427155408.19352-5-sashal@kernel.org>
From:   Paolo Bonzini <pbonzini@redhat.com>
In-Reply-To: <20220427155408.19352-5-sashal@kernel.org>
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On 4/27/22 17:54, Sasha Levin wrote:
> From: Paolo Bonzini <pbonzini@redhat.com>
> 
> [ Upstream commit 9191b8f0745e63edf519e4a54a4aaae1d3d46fbd ]
> 
> WARN and bail if KVM attempts to free a root that isn't backed by a shadow
> page.  KVM allocates a bare page for "special" roots, e.g. when using PAE
> paging or shadowing 2/3/4-level page tables with 4/5-level, and so root_hpa
> will be valid but won't be backed by a shadow page.  It's all too easy to
> blindly call mmu_free_root_page() on root_hpa, be nice and WARN instead of
> crashing KVM and possibly the kernel.
> 
> Reviewed-by: Sean Christopherson <seanjc@google.com>
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> Signed-off-by: Sasha Levin <sashal@kernel.org>
> ---
>   arch/x86/kvm/mmu/mmu.c | 2 ++
>   1 file changed, 2 insertions(+)
> 
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index 7f009ebb319a..e7cd16e1e0a0 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -3239,6 +3239,8 @@ static void mmu_free_root_page(struct kvm *kvm, hpa_t *root_hpa,
>   		return;
>   
>   	sp = to_shadow_page(*root_hpa & PT64_BASE_ADDR_MASK);
> +	if (WARN_ON(!sp))
> +		return;
>   
>   	if (is_tdp_mmu_page(sp))
>   		kvm_tdp_mmu_put_root(kvm, sp, false);

Acked-by: Paolo Bonzini <pbonzini@redhat.com>