Commit 43fb96ae authored by Sean Christopherson's avatar Sean Christopherson Committed by Paolo Bonzini
Browse files

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: 931656b9 ("kvm: defer huge page recovery vhost task to later")
Cc: stable@vger.kernel.org
Cc: Keith Busch <kbusch@kernel.org>
Signed-off-by: default avatarSean Christopherson <seanjc@google.com>
Message-ID: <20250124234623.3609069-1-seanjc@google.com>
Signed-off-by: default avatarPaolo Bonzini <pbonzini@redhat.com>
parent 6f612694

arch/x86/kvm/mmu/mmu.c

+26 −7
Original line number Diff line number Diff line
@@ -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,

	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)