Re: [PATCH v2 04/18] KVM: x86/mmu: avoid NULL-pointer dereference on page freeing bugs

[Maxim Levitsky]

On Thu, 2022-02-17 at 16:03 -0500, Paolo Bonzini wrote:
> 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.
> 
> Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> ---
>  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 a67071ac80f3..6ea423b00824 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -3222,6 +3222,8 @@ 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);



Reviewed-by: Maxim Levitsky <mlevitsk@xxxxxxxxxx>
Best regards,
	Maxim Levitsky