[PATCH] KVM: x86: Guard against division by zero in adjust_lapic_timer_advance()

[Peng Hao <flyingpenghao@gmail.com>]

When TSC calibration fails and userspace never programs a tsc khz,
vcpu->arch.virtual_tsc_khz stays zero, but the lapic timer's
software-mode expiry path still calls adjust_lapic_timer_advance()
unconditionally on every timer expiration (unlike
__wait_lapic_expire(), this path is not gated on tsc_scaling_ratio).
Both the "too early" and "too late" branches do_div() by
virtual_tsc_khz, so a zero khz crashes the host with #DE as soon as
the timer advance drifts outside the adjust window.

Bail out before either division.

Signed-off-by: Peng Hao <flyingpeng@tencent.com>
---
 arch/x86/kvm/lapic.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
index 13e32c4915e2..4dcbc66c8e07 100644
--- a/arch/x86/kvm/lapic.c
+++ b/arch/x86/kvm/lapic.c
@@ -1991,6 +1991,9 @@ static inline void adjust_lapic_timer_advance(struct kvm_vcpu *vcpu,
 	    abs(advance_expire_delta) < LAPIC_TIMER_ADVANCE_ADJUST_MIN)
 		return;
 
+	if (unlikely(!vcpu->arch.virtual_tsc_khz))
+		return;
+
 	/* too early */
 	if (advance_expire_delta < 0) {
 		ns = -advance_expire_delta * 1000000ULL;
-- 
2.43.7