KVM: x86/mmu: WARN on NULL pae_root or lm_root, or bad shadow root level

WARN if KVM is about to dereference a NULL pae_root or lm_root when
loading an MMU, and convert the BUG() on a bad shadow_root_level into a
WARN (now that errors are handled cleanly).  With nested NPT, botching
the level and sending KVM down the wrong path is all too easy, and the
on-demand allocation of pae_root and lm_root means bugs crash the host.
Obviously, KVM could unconditionally allocate the roots, but that's
arguably a worse failure mode as it would potentially corrupt the guest
instead of crashing it.
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20210305011101.3597423-18-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/mmu/mmu.c

@@ -3255,6 +3255,9 @@ static int mmu_alloc_direct_roots(struct kvm_vcpu *vcpu)
 		root = mmu_alloc_root(vcpu, 0, 0, shadow_root_level, true);
 		mmu->root_hpa = root;
 	} else if (shadow_root_level == PT32E_ROOT_LEVEL) {
+		if (WARN_ON_ONCE(!mmu->pae_root))
+			return -EIO;
+
 		for (i = 0; i < 4; ++i) {
 			WARN_ON_ONCE(mmu->pae_root[i] &&
 				     VALID_PAGE(mmu->pae_root[i]));
@@ -3265,8 +3268,10 @@ static int mmu_alloc_direct_roots(struct kvm_vcpu *vcpu)
 					   shadow_me_mask;
 		}
 		mmu->root_hpa = __pa(mmu->pae_root);
-	} else
-		BUG();
+	} else {
+		WARN_ONCE(1, "Bad TDP root level = %d\n", shadow_root_level);
+		return -EIO;
+	}
 
 	/* root_pgd is ignored for direct MMUs. */
 	mmu->root_pgd = 0;
@@ -3310,6 +3315,9 @@ static int mmu_alloc_shadow_roots(struct kvm_vcpu *vcpu)
 		goto set_root_pgd;
 	}
 
+	if (WARN_ON_ONCE(!mmu->pae_root))
+		return -EIO;
+
 	/*
 	 * We shadow a 32 bit page table. This may be a legacy 2-level
 	 * or a PAE 3-level page table. In either case we need to be aware that
@@ -3319,6 +3327,9 @@ static int mmu_alloc_shadow_roots(struct kvm_vcpu *vcpu)
 	if (mmu->shadow_root_level == PT64_ROOT_4LEVEL) {
 		pm_mask |= PT_ACCESSED_MASK | PT_WRITABLE_MASK | PT_USER_MASK;
 
+		if (WARN_ON_ONCE(!mmu->lm_root))
+			return -EIO;
+
 		mmu->lm_root[0] = __pa(mmu->pae_root) | pm_mask;
 	}