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

[Wanpeng Li]

From: Wanpeng Li <wanpeng.li@xxxxxxxxxxx>

 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
 RSP: 002b:00007f9d0f6768f8 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
 RAX: ffffffffffffffda RBX: ffffffffa64d53c3 RCX: 00007f9d164ed357
 RDX: 0000000000000000 RSI: 000000000000ae80 RDI: 000000000000000d
 RBP: ffffbb260856bf88 R08: 0000556b2a13eeb0 R09: 0000000000000000
 R10: 00007f9d080000c8 R11: 0000000000000246 R12: 0000000000000000
 R13: 00007f9d1853d000 R14: 0000000000000000 R15: 000000000000ae80
  ? __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 fix it by holding pvclock_gtod_sync_lock lock when calculates 
pvclock's time scale in order to disable preemption for host per-CPU tsc 
rate read.

Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx>
Cc: Radim KrÄmÃÅ <rkrcmar@xxxxxxxxxx>
Signed-off-by: Wanpeng Li <wanpeng.li@xxxxxxxxxxx>
---
 arch/x86/kvm/x86.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index b54125b..8008d56 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -1772,11 +1772,11 @@ u64 get_kvmclock_ns(struct kvm *kvm)
 
 	hv_clock.tsc_timestamp = ka->master_cycle_now;
 	hv_clock.system_time = ka->master_kernel_ns + ka->kvmclock_offset;
-	spin_unlock(&ka->pvclock_gtod_sync_lock);
 
 	kvm_get_time_scale(NSEC_PER_SEC, __this_cpu_read(cpu_tsc_khz) * 1000LL,
 			   &hv_clock.tsc_shift,
 			   &hv_clock.tsc_to_system_mul);
+	spin_unlock(&ka->pvclock_gtod_sync_lock);
 	return __pvclock_read_cycles(&hv_clock, rdtsc());
 }
 
-- 
2.7.4