Re: [PATCH MANUALSEL 5.17 5/7] KVM: x86/mmu: avoid NULL-pointer dereference on page freeing bugs

Paolo Bonzini
2022-04-27T16:18:29Z

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>