KVM: x86: Fix potential preemption when get the current kvmclock timestamp

 BUG: using __this_cpu_read() in preemptible [00000000] code: qemu-system-x86/2809
 caller is __this_cpu_preempt_check+0x13/0x20
 CPU: 2 PID: 2809 Comm: qemu-system-x86 Not tainted 4.11.0+ #13
 Call Trace:
  dump_stack+0x99/0xce
  check_preemption_disabled+0xf5/0x100
  __this_cpu_preempt_check+0x13/0x20
  get_kvmclock_ns+0x6f/0x110 [kvm]
  get_time_ref_counter+0x5d/0x80 [kvm]
  kvm_hv_process_stimers+0x2a1/0x8a0 [kvm]
  ? kvm_hv_process_stimers+0x2a1/0x8a0 [kvm]
  ? kvm_arch_vcpu_ioctl_run+0xac9/0x1ce0 [kvm]
  kvm_arch_vcpu_ioctl_run+0x5bf/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
  entry_SYSCALL_64_fastpath+0x23/0xc2
 RIP: 0033:0x7f9d164ed357
  ? __this_cpu_preempt_check+0x13/0x20

This can be reproduced by run kvm-unit-tests/hyperv_stimer.flat w/
CONFIG_PREEMPT and CONFIG_DEBUG_PREEMPT enabled.

Safe access to per-CPU data requires a couple of constraints, though: the
thread working with the data cannot be preempted and it cannot be migrated
while it manipulates per-CPU variables. If the thread is preempted, the
thread that replaces it could try to work with the same variables; migration
to another CPU could also cause confusion. However there is no preemption
disable when reads host per-CPU tsc rate to calculate the current kvmclock
timestamp.

This patch fixes it by utilizing get_cpu/put_cpu pair to guarantee both
__this_cpu_read() and rdtsc() are not preempted.

Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Radim Krčmář <rkrcmar@redhat.com>
Signed-off-by: Wanpeng Li <wanpeng.li@hotmail.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Cc: stable@vger.kernel.org
Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>

arch/x86/kvm/x86.c

@@ -1763,6 +1763,7 @@ u64 get_kvmclock_ns(struct kvm *kvm)
 {
 	struct kvm_arch *ka = &kvm->arch;
 	struct pvclock_vcpu_time_info hv_clock;
+	u64 ret;
 
 	spin_lock(&ka->pvclock_gtod_sync_lock);
 	if (!ka->use_master_clock) {
@@ -1774,10 +1775,17 @@ u64 get_kvmclock_ns(struct kvm *kvm)
 	hv_clock.system_time = ka->master_kernel_ns + ka->kvmclock_offset;
 	spin_unlock(&ka->pvclock_gtod_sync_lock);
 
+	/* both __this_cpu_read() and rdtsc() should be on the same cpu */
+	get_cpu();
+
 	kvm_get_time_scale(NSEC_PER_SEC, __this_cpu_read(cpu_tsc_khz) * 1000LL,
 			   &hv_clock.tsc_shift,
 			   &hv_clock.tsc_to_system_mul);
-	return __pvclock_read_cycles(&hv_clock, rdtsc());
+	ret = __pvclock_read_cycles(&hv_clock, rdtsc());
+
+	put_cpu();
+
+	return ret;
 }
 
 static void kvm_setup_pvclock_page(struct kvm_vcpu *v)