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

Paolo Bonzini
2022-02-17T21:04:16Z

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@redhat.com>
---
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);
--
2.31.1