[PATCH v2] KVM: x86: Protect virtualization-enable VM walks with RCU

[Chengfeng Ye <nicoyip.dev@gmail.com>]

Protect the vm_list walks in kvm_arch_enable_virtualization_cpu() with
RCU, publish and remove VMs with RCU list operations, and wait for
pre-existing readers before tearing down a VM.

The change that introduced kvm_usage_lock replaced kvm_lock with a
dedicated usage-count mutex when enabling virtualization.  Because
vm_list mutators do not take the usage-count mutex, this left the x86
walks without lifetime protection.

During CPU online, the hotplug thread can load a VM from vm_list.  A
concurrent close of the last VM file can then remove that VM and destroy
its vCPUs before the hotplug thread dereferences the stale VM or vCPU.
Reacquiring kvm_lock from the CPU-hotplug path would restore the lock cycle
that the same change fixed.

KASAN reported:

  BUG: KASAN: wild-memory-access in kvm_arch_enable_virtualization_cpu+0x25f/0x870
  Read of size 4 at addr dead0000000000f0 by task cpuhp/3/31
  Call Trace:
   kvm_arch_enable_virtualization_cpu+0x25f/0x870
   kvm_online_cpu+0x1a/0x50
   cpuhp_invoke_callback+0x291/0xfd0
   cpuhp_thread_fun+0x256/0x4a0
   smpboot_thread_fn+0x287/0x6c0

Fixes: 44d174596260 ("KVM: Use dedicated mutex to protect kvm_usage_count to avoid deadlock")
Cc: stable@vger.kernel.org
Assisted-by: GPT-6-Astra
Signed-off-by: Chengfeng Ye <nicoyip.dev@gmail.com>
Signed-off-by: Qi Zhang <marsy12010123@gmail.com>
---
Changes in v2:
- Rebase onto current kvm-x86/next; keep the same RCU lifetime fix.
- Clarify the scope of the Sashiko finding: the hotplug update of
  tsc_offset_adjustment and the vCPU-load read/reset are unchanged by
  this patch. RCU protects VM/vCPU lifetime; it does not synchronize
  those field accesses. A separate synchronization change would need
  its own analysis and review.

Link: https://lore.kernel.org/r/20260804123243.2213854-1-marsy12010123@gmail.com/ [v1]
Link: https://lore.kernel.org/r/20260804125109.B20491F000E9@smtp.kernel.org/ [review]

 arch/x86/kvm/x86.c  | 6 ++++--
 virt/kvm/kvm_main.c | 5 +++--
 2 files changed, 7 insertions(+), 4 deletions(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 1705e7be46ec..6f57374dbb16 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -9664,7 +9664,8 @@ int kvm_arch_enable_virtualization_cpu(void)
 
 	local_tsc = rdtsc();
 	stable = !kvm_check_tsc_unstable();
-	list_for_each_entry(kvm, &vm_list, vm_list) {
+	rcu_read_lock();
+	list_for_each_entry_rcu(kvm, &vm_list, vm_list) {
 		kvm_for_each_vcpu(i, vcpu, kvm) {
 			if (!stable && vcpu->cpu == smp_processor_id())
 				kvm_make_request(KVM_REQ_CLOCK_UPDATE, vcpu);
@@ -9716,7 +9717,7 @@ int kvm_arch_enable_virtualization_cpu(void)
 	 */
 	if (backwards_tsc) {
 		u64 delta_cyc = max_tsc - local_tsc;
-		list_for_each_entry(kvm, &vm_list, vm_list) {
+		list_for_each_entry_rcu(kvm, &vm_list, vm_list) {
 			kvm->arch.backwards_tsc_observed = true;
 			kvm_for_each_vcpu(i, vcpu, kvm) {
 				vcpu->arch.tsc_offset_adjustment += delta_cyc;
@@ -9735,6 +9736,7 @@ int kvm_arch_enable_virtualization_cpu(void)
 		}
 
 	}
+	rcu_read_unlock();
 	return 0;
 }
 
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 9fff2f4bf2f1..750ee2f8c720 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -1217,7 +1217,7 @@ static struct kvm *kvm_create_vm(unsigned long type, const char *fdname)
 		goto out_err_no_debugfs;
 
 	mutex_lock(&kvm_lock);
-	list_add(&kvm->vm_list, &vm_list);
+	list_add_rcu(&kvm->vm_list, &vm_list);
 	mutex_unlock(&kvm_lock);
 
 	preempt_notifier_inc();
@@ -1279,8 +1279,9 @@ static void kvm_destroy_vm(struct kvm *kvm)
 	kvm_uevent_notify_change(KVM_EVENT_DESTROY_VM, kvm);
 	kvm_destroy_vm_debugfs(kvm);
 	mutex_lock(&kvm_lock);
-	list_del(&kvm->vm_list);
+	list_del_rcu(&kvm->vm_list);
 	mutex_unlock(&kvm_lock);
+	synchronize_rcu();
 	kvm_arch_pre_destroy_vm(kvm);
 
 	kvm_free_irq_routing(kvm);
-- 
2.43.0