KVM: x86/mmu: Ensure NX huge page recovery thread is alive before waking

When waking a VM's NX huge page recovery thread, ensure the thread is
actually alive before trying to wake it.  Now that the thread is spawned
on-demand during KVM_RUN, a VM without a recovery thread is reachable via
the related module params.

  BUG: kernel NULL pointer dereference, address: 0000000000000040
  #PF: supervisor read access in kernel mode
  #PF: error_code(0x0000) - not-present page
  Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 0.0.0 02/06/2015
  RIP: 0010:vhost_task_wake+0x5/0x10
  Call Trace:
   <TASK>
   set_nx_huge_pages+0xcc/0x1e0 [kvm]
   param_attr_store+0x8a/0xd0
   module_attr_store+0x1a/0x30
   kernfs_fop_write_iter+0x12f/0x1e0
   vfs_write+0x233/0x3e0
   ksys_write+0x60/0xd0
   do_syscall_64+0x5b/0x160
   entry_SYSCALL_64_after_hwframe+0x4b/0x53
  RIP: 0033:0x7f3b52710104
   </TASK>
  Modules linked in: kvm_intel kvm
  CR2: 0000000000000040

Fixes: 931656b ("kvm: defer huge page recovery vhost task to later")
Cc: stable@vger.kernel.org
Cc: Keith Busch <kbusch@kernel.org>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-ID: <20250124234623.3609069-1-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/mmu/mmu.c

@@ -7120,6 +7120,19 @@ static void mmu_destroy_caches(void)
 	kmem_cache_destroy(mmu_page_header_cache);
 }
 
+static void kvm_wake_nx_recovery_thread(struct kvm *kvm)
+{
+	/*
+	 * The NX recovery thread is spawned on-demand at the first KVM_RUN and
+	 * may not be valid even though the VM is globally visible.  Do nothing,
+	 * as such a VM can't have any possible NX huge pages.
+	 */
+	struct vhost_task *nx_thread = READ_ONCE(kvm->arch.nx_huge_page_recovery_thread);
+
+	if (nx_thread)
+		vhost_task_wake(nx_thread);
+}
+
 static int get_nx_huge_pages(char *buffer, const struct kernel_param *kp)
 {
 	if (nx_hugepage_mitigation_hard_disabled)
@@ -7180,7 +7193,7 @@ static int set_nx_huge_pages(const char *val, const struct kernel_param *kp)
 			kvm_mmu_zap_all_fast(kvm);
 			mutex_unlock(&kvm->slots_lock);
 
-			vhost_task_wake(kvm->arch.nx_huge_page_recovery_thread);
+			kvm_wake_nx_recovery_thread(kvm);
 		}
 		mutex_unlock(&kvm_lock);
 	}
@@ -7315,7 +7328,7 @@ static int set_nx_huge_pages_recovery_param(const char *val, const struct kernel
 		mutex_lock(&kvm_lock);
 
 		list_for_each_entry(kvm, &vm_list, vm_list)
-			vhost_task_wake(kvm->arch.nx_huge_page_recovery_thread);
+			kvm_wake_nx_recovery_thread(kvm);
 
 		mutex_unlock(&kvm_lock);
 	}
@@ -7451,14 +7464,20 @@ static void kvm_mmu_start_lpage_recovery(struct once *once)
 {
 	struct kvm_arch *ka = container_of(once, struct kvm_arch, nx_once);
 	struct kvm *kvm = container_of(ka, struct kvm, arch);
+	struct vhost_task *nx_thread;
 
 	kvm->arch.nx_huge_page_last = get_jiffies_64();
-	kvm->arch.nx_huge_page_recovery_thread = vhost_task_create(
-		kvm_nx_huge_page_recovery_worker, kvm_nx_huge_page_recovery_worker_kill,
-		kvm, "kvm-nx-lpage-recovery");
+	nx_thread = vhost_task_create(kvm_nx_huge_page_recovery_worker,
+				      kvm_nx_huge_page_recovery_worker_kill,
+				      kvm, "kvm-nx-lpage-recovery");
 
-	if (kvm->arch.nx_huge_page_recovery_thread)
-		vhost_task_start(kvm->arch.nx_huge_page_recovery_thread);
+	if (!nx_thread)
+		return;
+
+	vhost_task_start(nx_thread);
+
+	/* Make the task visible only once it is fully started. */
+	WRITE_ONCE(kvm->arch.nx_huge_page_recovery_thread, nx_thread);
 }
 
 int kvm_mmu_post_init_vm(struct kvm *kvm)