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Subject: Re: [syzbot] [kvm?] [kvm-x86?] INFO: rcu detected stall in kvm_vcpu_ioctl (4)
Author: clopez@suse.de

#syz test: upstream master

diff --git a/arch/x86/kvm/hyperv.c b/arch/x86/kvm/hyperv.c
index 4438ecac9a89..d0e250d33ff0 100644
--- a/arch/x86/kvm/hyperv.c
+++ b/arch/x86/kvm/hyperv.c
@@ -626,6 +626,17 @@ static enum hrtimer_restart stimer_timer_callback(struct hrtimer *timer)
 	return HRTIMER_NORESTART;
 }
 
+/*
+ * Translate a stimer expiry given in 100 ns Hyper-V reference ticks into
+ * an absolute deadline. Saturates on overflow.
+ */
+static ktime_t stimer_add_delta(ktime_t now, u64 delta_100ns)
+{
+	if (delta_100ns > KTIME_MAX / 100)
+		return KTIME_MAX;
+	return ktime_add_safe(now, 100 * delta_100ns);
+}
+
 /*
  * stimer_start() assumptions:
  * a) stimer->count is not equal to 0
@@ -635,6 +646,7 @@ static int stimer_start(struct kvm_vcpu_hv_stimer *stimer)
 {
 	u64 time_now;
 	ktime_t ktime_now;
+	ktime_t deadline;
 
 	time_now = get_time_ref_counter(hv_stimer_to_vcpu(stimer)->kvm);
 	ktime_now = ktime_get();
@@ -657,10 +669,8 @@ static int stimer_start(struct kvm_vcpu_hv_stimer *stimer)
 					stimer->index,
 					time_now, stimer->exp_time);
 
-		hrtimer_start(&stimer->timer,
-			      ktime_add_ns(ktime_now,
-					   100 * (stimer->exp_time - time_now)),
-			      HRTIMER_MODE_ABS);
+		deadline = stimer_add_delta(ktime_now, stimer->exp_time - time_now);
+		hrtimer_start(&stimer->timer, deadline, HRTIMER_MODE_ABS);
 		return 0;
 	}
 	stimer->exp_time = stimer->count;
@@ -679,9 +689,9 @@ static int stimer_start(struct kvm_vcpu_hv_stimer *stimer)
 					   stimer->index,
 					   time_now, stimer->count);
 
-	hrtimer_start(&stimer->timer,
-		      ktime_add_ns(ktime_now, 100 * (stimer->count - time_now)),
-		      HRTIMER_MODE_ABS);
+	deadline = stimer_add_delta(ktime_now, stimer->count - time_now);
+	hrtimer_start(&stimer->timer, deadline, HRTIMER_MODE_ABS);
+
 	return 0;
 }