KVM: x86/mmu: Don't freak out if pml5_root is NULL on 4-level host

Include pml5_root in the set of special roots if and only if the host,
and thus NPT, is using 5-level paging.  mmu_alloc_special_roots() expects
special roots to be allocated as a bundle, i.e. they're either all valid
or all NULL.  But for pml5_root, that expectation only holds true if the
host uses 5-level paging, which causes KVM to WARN about pml5_root being
NULL when the other special roots are valid.

The silver lining of 4-level vs. 5-level NPT being tied to the host
kernel's paging level is that KVM's shadow root level is constant; unlike
VMX's EPT, KVM can't choose 4-level NPT based on guest.MAXPHYADDR.  That
means KVM can still expect pml5_root to be bundled with the other special
roots, it just needs to be conditioned on the shadow root level.

Fixes: cb0f722a ("KVM: x86/mmu: Support shadowing NPT when 5-level paging is enabled in host")
Reported-by: Maxim Levitsky <mlevitsk@redhat.com>
Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20210824005824.205536-1-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/mmu/mmu.c

@@ -3588,6 +3588,7 @@ static int mmu_alloc_shadow_roots(struct kvm_vcpu *vcpu)
 static int mmu_alloc_special_roots(struct kvm_vcpu *vcpu)
 {
 	struct kvm_mmu *mmu = vcpu->arch.mmu;
+	bool need_pml5 = mmu->shadow_root_level > PT64_ROOT_4LEVEL;
 	u64 *pml5_root = NULL;
 	u64 *pml4_root = NULL;
 	u64 *pae_root;
@@ -3602,7 +3603,14 @@ static int mmu_alloc_special_roots(struct kvm_vcpu *vcpu)
 	    mmu->shadow_root_level < PT64_ROOT_4LEVEL)
 		return 0;
 
-	if (mmu->pae_root && mmu->pml4_root && mmu->pml5_root)
+	/*
+	 * NPT, the only paging mode that uses this horror, uses a fixed number
+	 * of levels for the shadow page tables, e.g. all MMUs are 4-level or
+	 * all MMus are 5-level.  Thus, this can safely require that pml5_root
+	 * is allocated if the other roots are valid and pml5 is needed, as any
+	 * prior MMU would also have required pml5.
+	 */
+	if (mmu->pae_root && mmu->pml4_root && (!need_pml5 || mmu->pml5_root))
 		return 0;
 
 	/*
@@ -3610,7 +3618,7 @@ static int mmu_alloc_special_roots(struct kvm_vcpu *vcpu)
 	 * bail if KVM ends up in a state where only one of the roots is valid.
 	 */
 	if (WARN_ON_ONCE(!tdp_enabled || mmu->pae_root || mmu->pml4_root ||
-			 mmu->pml5_root))
+			 (need_pml5 && mmu->pml5_root)))
 		return -EIO;
 
 	/*
@@ -3626,7 +3634,7 @@ static int mmu_alloc_special_roots(struct kvm_vcpu *vcpu)
 	if (!pml4_root)
 		goto err_pml4;
 
-	if (mmu->shadow_root_level > PT64_ROOT_4LEVEL) {
+	if (need_pml5) {
 		pml5_root = (void *)get_zeroed_page(GFP_KERNEL_ACCOUNT);
 		if (!pml5_root)
 			goto err_pml5;