Patch "KVM: X86: Fix preempt the preemption timer cancel" has been added to the 4.9-stable tree

This is a note to let you know that I've just added the patch titled

KVM: X86: Fix preempt the preemption timer cancel

to the 4.9-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
kvm-x86-fix-preempt-the-preemption-timer-cancel.patch
and it can be found in the queue-4.9 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Wanpeng Li <[email protected]>
Date: Sat, 20 May 2017 20:32:32 -0700
Subject: KVM: X86: Fix preempt the preemption timer cancel

From: Wanpeng Li <[email protected]>

[ Upstream commit 5acc1ca4fb15f00bfa3d4046e35ca381bc25d580 ]

Preemption can occur during cancel preemption timer, and there will be inconsistent status in lapic, vmx and vmcs field.

CPU0 CPU1

  preemption timer vmexit
  handle_preemption_timer(vCPU0)
    kvm_lapic_expired_hv_timer
      vmx_cancel_hv_timer
        vmx->hv_deadline_tsc = -1
        vmcs_clear_bits
        /* hv_timer_in_use still true */
  sched_out
                           sched_in
                           kvm_arch_vcpu_load
                             vmx_set_hv_timer
                               write vmx->hv_deadline_tsc
                               vmcs_set_bits
                           /* back in kvm_lapic_expired_hv_timer */
                           hv_timer_in_use = false
                           ...
                           vmx_vcpu_run
                             vmx_arm_hv_run
                               write preemption timer deadline
                             spurious preemption timer vmexit
                               handle_preemption_timer(vCPU0)
                                 kvm_lapic_expired_hv_timer
                                   WARN_ON(!apic->lapic_timer.hv_timer_in_use);

This can be reproduced sporadically during boot of L2 on a preemptible L1, causing a splat on L1.

 WARNING: CPU: 3 PID: 1952 at arch/x86/kvm/lapic.c:1529 kvm_lapic_expired_hv_timer+0xb5/0xd0 [kvm]
 CPU: 3 PID: 1952 Comm: qemu-system-x86 Not tainted 4.12.0-rc1+ #24 RIP: 0010:kvm_lapic_expired_hv_timer+0xb5/0xd0 [kvm]
  Call Trace:
  handle_preemption_timer+0xe/0x20 [kvm_intel]
  vmx_handle_exit+0xc9/0x15f0 [kvm_intel]
  ? lock_acquire+0xdb/0x250
  ? lock_acquire+0xdb/0x250
  ? kvm_arch_vcpu_ioctl_run+0xdf3/0x1ce0 [kvm]
  kvm_arch_vcpu_ioctl_run+0xe55/0x1ce0 [kvm]
  kvm_vcpu_ioctl+0x384/0x7b0 [kvm]
  ? kvm_vcpu_ioctl+0x384/0x7b0 [kvm]
  ? __fget+0xf3/0x210
  do_vfs_ioctl+0xa4/0x700
  ? __fget+0x114/0x210
  SyS_ioctl+0x79/0x90
  do_syscall_64+0x8f/0x750
  ? trace_hardirqs_on_thunk+0x1a/0x1c
  entry_SYSCALL64_slow_path+0x25/0x25

This patch fixes it by disabling preemption while cancelling preemption timer. This way cancel_hv_timer is atomic with respect to kvm_arch_vcpu_load.

---
 arch/x86/kvm/lapic.c |    2 ++
 1 file changed, 2 insertions(+)

@@ -1363,8 +1363,10 @@ EXPORT_SYMBOL_GPL(kvm_lapic_hv_timer_in_
 
 static void cancel_hv_tscdeadline(struct kvm_lapic *apic)
 {
+	preempt_disable();
 	kvm_x86_ops->cancel_hv_timer(apic->vcpu);
 	apic->lapic_timer.hv_timer_in_use = false;
+	preempt_enable();
 }
 
 void kvm_lapic_expired_hv_timer(struct kvm_vcpu *vcpu)

Patches currently in stable-queue which might be from [email protected] are

queue-4.9/kvm-x86-fix-preempt-the-preemption-timer-cancel.patch queue-4.9/kvm-nvmx-fix-handling-of-lmsw-instruction.patch