KVM: arm64: nv: Avoid NV stage-2 code when NV is not supported

The NV stage-2 manipulation functions kvm_nested_s2_unmap(),
kvm_nested_s2_wp(), and others, are being called for any stage-2
manipulation regardless of whether nested virtualization is supported or
enabled for the VM.

For protected KVM (pKVM), `struct kvm_pgtable` uses the
`pkvm_mappings` member of the union. This member aliases `ia_bits`,
which is used by the non-protected NV code paths. Attempting to
read `pgt->ia_bits` in these functions results in treating
protected mapping pointers or state values as bit-shift amounts.
This triggers a UBSAN shift-out-of-bounds error:

    UBSAN: shift-out-of-bounds in arch/arm64/kvm/nested.c:1127:34
    shift exponent 174565952 is too large for 64-bit type 'unsigned long'
    Call trace:
     __ubsan_handle_shift_out_of_bounds+0x28c/0x2c0
     kvm_nested_s2_unmap+0x228/0x248
     kvm_arch_flush_shadow_memslot+0x98/0xc0
     kvm_set_memslot+0x248/0xce0

Since pKVM and NV are mutually exclusive, prevent entry into these
NV handling functions if the VM has not allocated any nested MMUs
(i.e., `kvm->arch.nested_mmus_size` is 0).

Fixes: 7270cc9 ("KVM: arm64: nv: Handle VNCR_EL2 invalidation from MMU notifiers")
Suggested-by: Marc Zyngier <maz@kernel.org>
Signed-off-by: Fuad Tabba <tabba@google.com>
Link: https://patch.msgid.link/20260202152310.113467-1-tabba@google.com
Signed-off-by: Marc Zyngier <maz@kernel.org>

Author: Fuad Tabba

arch/arm64/kvm/nested.c

@@ -1101,6 +1101,9 @@ void kvm_nested_s2_wp(struct kvm *kvm)
 
 	lockdep_assert_held_write(&kvm->mmu_lock);
 
+	if (!kvm->arch.nested_mmus_size)
+		return;
+
 	for (i = 0; i < kvm->arch.nested_mmus_size; i++) {
 		struct kvm_s2_mmu *mmu = &kvm->arch.nested_mmus[i];
 
@@ -1117,6 +1120,9 @@ void kvm_nested_s2_unmap(struct kvm *kvm, bool may_block)
 
 	lockdep_assert_held_write(&kvm->mmu_lock);
 
+	if (!kvm->arch.nested_mmus_size)
+		return;
+
 	for (i = 0; i < kvm->arch.nested_mmus_size; i++) {
 		struct kvm_s2_mmu *mmu = &kvm->arch.nested_mmus[i];
 
@@ -1133,6 +1139,9 @@ void kvm_nested_s2_flush(struct kvm *kvm)
 
 	lockdep_assert_held_write(&kvm->mmu_lock);
 
+	if (!kvm->arch.nested_mmus_size)
+		return;
+
 	for (i = 0; i < kvm->arch.nested_mmus_size; i++) {
 		struct kvm_s2_mmu *mmu = &kvm->arch.nested_mmus[i];
 
@@ -1145,6 +1154,9 @@ void kvm_arch_flush_shadow_all(struct kvm *kvm)
 {
 	int i;
 
+	if (!kvm->arch.nested_mmus_size)
+		return;
+
 	for (i = 0; i < kvm->arch.nested_mmus_size; i++) {
 		struct kvm_s2_mmu *mmu = &kvm->arch.nested_mmus[i];