[PATCH 2/2] KVM: arm64: nv: Fix null ptr deref in kvm_nested_s2_unmap() on S2 teardown

["Lorenzo Stoakes (ARM)" <ljs@kernel.org>]

Commit 7270cc9157f4 ("KVM: arm64: nv: Handle VNCR_EL2 invalidation from MMU
notifiers") introduced VNCR_EL2 invalidation in kvm_nested_s2_unmap().

However at the point of this being performed concurrent stage 2 teardown of
a nested guest can cause kvm->arch.mmu.pgt to be set to NULL.

This happens in kvm_flush_shadow_all() -> kvm_arch_flush_shadow_all() ->
kvm_free_stage2_pgd() and is performed under the kvm->mmu_lock.

Commit ec14c272408a ("KVM: arm64: nv: Unmap/flush shadow stage 2 page
tables") introduced the teardown of the entire nested MMU range, which then
invokes stage2_apply_range() with resched=true:

mmu_notifier_invalidate_range_start()
  -> ... -> kvm_mmu_notifier_invalidate_range_start()
    -> kvm_mmu_unmap_gfn_range()
      -> kvm_unmap_gfn_range()
        -> kvm_nested_s2_unmap()
          -> kvm_stage2_unmap_range()
            -> __unmap_stage2_range()
                -> stage2_apply_range()

This means that stage2_apply_range() can drop the kvm->mmu_lock and thus
concurrent progress can be made in lockstep with
kvm_arch_flush_shadow_all().

If kvm_arch_flush_shadow_all() advances ahead of stage2_apply_range() and
completes its operation it guarantees a NULL pointer deref.

Since kvm_free_stage2_pgd() is performed under the kvm->mmu_lock this will
either be observed NULL or not and serialised against
kvm_free_stage2_pgd().

Resolve the issue by explicitly checking for a NULL value. Since this
shouldn't be possible if blocking is not allowed, raise a warning if it is
ever NULL in this case.

Fixes: 7270cc9157f4 ("KVM: arm64: nv: Handle VNCR_EL2 invalidation from MMU notifiers")
Cc: stable@vger.kernel.org
Signed-off-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>
---
 arch/arm64/kvm/nested.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/arch/arm64/kvm/nested.c b/arch/arm64/kvm/nested.c
index 17123f0b6dab..9fc0b1696dcb 100644
--- a/arch/arm64/kvm/nested.c
+++ b/arch/arm64/kvm/nested.c
@@ -1295,7 +1295,11 @@ void kvm_nested_s2_unmap(struct kvm *kvm, bool may_block)
 			kvm_stage2_unmap_range(mmu, 0, kvm_phys_size(mmu), may_block);
 	}
 
-	kvm_invalidate_vncr_ipa(kvm, 0, BIT(kvm->arch.mmu.pgt->ia_bits));
+	/* NULL pgt should only be possible if raced when mmu_lock dropped. */
+	if (kvm->arch.mmu.pgt)
+		kvm_invalidate_vncr_ipa(kvm, 0, BIT(kvm->arch.mmu.pgt->ia_bits));
+	else
+		WARN_ON(!may_block);
 }
 
 void kvm_nested_s2_flush(struct kvm *kvm)

-- 
2.55.0