Re: [PATCH MANUALSEL 5.15 6/7] KVM: x86/mmu: avoid NULL-pointer dereference on page freeing bugs

[Paolo Bonzini <pbonzini@redhat.com>]

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 34e828badc51..806f9d42bcce 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -3314,6 +3314,8 @@ 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>