timekeeping: Make delta calculation overflow safe

Kernel timekeeping is designed to keep the change in cycles (since the last
timer interrupt) below max_cycles, which prevents multiplication overflow
when converting cycles to nanoseconds. However, if timer interrupts stop,
the calculation will eventually overflow.

Add protection against that. In timekeeping_cycles_to_ns() calculation,
check against max_cycles, falling back to a slower higher precision
calculation. In timekeeping_forward_now(), process delta in chunks of at
most max_cycles.
Suggested-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20240325064023.2997-18-adrian.hunter@intel.com

kernel/time/timekeeping.c

@@ -364,19 +364,32 @@ static void tk_setup_internals(struct timekeeper *tk, struct clocksource *clock)
 }
 
 /* Timekeeper helper functions. */
+static noinline u64 delta_to_ns_safe(const struct tk_read_base *tkr, u64 delta)
+{
+	return mul_u64_u32_add_u64_shr(delta, tkr->mult, tkr->xtime_nsec, tkr->shift);
+}
+
 static inline u64 timekeeping_cycles_to_ns(const struct tk_read_base *tkr, u64 cycles)
 {
 	/* Calculate the delta since the last update_wall_time() */
 	u64 mask = tkr->mask, delta = (cycles - tkr->cycle_last) & mask;
 
-	if (IS_ENABLED(CONFIG_CLOCKSOURCE_VALIDATE_LAST_CYCLE)) {
-		/*
-		 * Handle clocksource inconsistency between CPUs to prevent
-		 * time from going backwards by checking for the MSB of the
-		 * mask being set in the delta.
-		 */
-		if (unlikely(delta & ~(mask >> 1)))
-			return tkr->xtime_nsec >> tkr->shift;
+	/*
+	 * This detects the case where the delta overflows the multiplication
+	 * with tkr->mult.
+	 */
+	if (unlikely(delta > tkr->clock->max_cycles)) {
+		if (IS_ENABLED(CONFIG_CLOCKSOURCE_VALIDATE_LAST_CYCLE)) {
+			/*
+			 * Handle clocksource inconsistency between CPUs to prevent
+			 * time from going backwards by checking for the MSB of the
+			 * mask being set in the delta.
+			 */
+			if (unlikely(delta & ~(mask >> 1)))
+				return tkr->xtime_nsec >> tkr->shift;
+		}
+
+		return delta_to_ns_safe(tkr, delta);
 	}
 
 	return ((delta * tkr->mult) + tkr->xtime_nsec) >> tkr->shift;
@@ -789,10 +802,15 @@ static void timekeeping_forward_now(struct timekeeper *tk)
 	tk->tkr_mono.cycle_last = cycle_now;
 	tk->tkr_raw.cycle_last  = cycle_now;
 
-	tk->tkr_mono.xtime_nsec += delta * tk->tkr_mono.mult;
-	tk->tkr_raw.xtime_nsec += delta * tk->tkr_raw.mult;
+	while (delta > 0) {
+		u64 max = tk->tkr_mono.clock->max_cycles;
+		u64 incr = delta < max ? delta : max;
 
-	tk_normalize_xtime(tk);
+		tk->tkr_mono.xtime_nsec += incr * tk->tkr_mono.mult;
+		tk->tkr_raw.xtime_nsec += incr * tk->tkr_raw.mult;
+		tk_normalize_xtime(tk);
+		delta -= incr;
+	}
 }
 
 /**