timekeeping/vsyscall: Prevent math overflow in BOOTTIME update

The VDSO update for CLOCK_BOOTTIME has a overflow issue as it shifts the
nanoseconds based boot time offset left by the clocksource shift. That
overflows once the boot time offset becomes large enough. As a consequence
CLOCK_BOOTTIME in the VDSO becomes a random number causing applications to
misbehave.

Fix it by storing a timespec64 representation of the offset when boot time
is adjusted and add that to the MONOTONIC base time value in the vdso data
page. Using the timespec64 representation avoids a 64bit division in the
update code.

Fixes: 44f57d78 ("timekeeping: Provide a generic update_vsyscall() implementation")
Reported-by: Chris Clayton <chris2553@googlemail.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Chris Clayton <chris2553@googlemail.com>
Tested-by: Vincenzo Frascino <vincenzo.frascino@arm.com>
Link: https://lkml.kernel.org/r/alpine.DEB.2.21.1908221257580.1983@nanos.tec.linutronix.de

include/linux/timekeeper_internal.h

@@ -57,6 +57,7 @@ struct tk_read_base {
  * @cs_was_changed_seq:	The sequence number of clocksource change events
  * @next_leap_ktime:	CLOCK_MONOTONIC time value of a pending leap-second
  * @raw_sec:		CLOCK_MONOTONIC_RAW  time in seconds
+ * @monotonic_to_boot:	CLOCK_MONOTONIC to CLOCK_BOOTTIME offset
  * @cycle_interval:	Number of clock cycles in one NTP interval
  * @xtime_interval:	Number of clock shifted nano seconds in one NTP
  *			interval.
@@ -84,6 +85,9 @@ struct tk_read_base {
  *
  * wall_to_monotonic is no longer the boot time, getboottime must be
  * used instead.
+ *
+ * @monotonic_to_boottime is a timespec64 representation of @offs_boot to
+ * accelerate the VDSO update for CLOCK_BOOTTIME.
  */
 struct timekeeper {
 	struct tk_read_base	tkr_mono;
@@ -99,6 +103,7 @@ struct timekeeper {
 	u8			cs_was_changed_seq;
 	ktime_t			next_leap_ktime;
 	u64			raw_sec;
+	struct timespec64	monotonic_to_boot;
 
 	/* The following members are for timekeeping internal use */
 	u64			cycle_interval;

kernel/time/timekeeping.c

@@ -146,6 +146,11 @@ static void tk_set_wall_to_mono(struct timekeeper *tk, struct timespec64 wtm)
 static inline void tk_update_sleep_time(struct timekeeper *tk, ktime_t delta)
 {
 	tk->offs_boot = ktime_add(tk->offs_boot, delta);
+	/*
+	 * Timespec representation for VDSO update to avoid 64bit division
+	 * on every update.
+	 */
+	tk->monotonic_to_boot = ktime_to_timespec64(tk->offs_boot);
 }
 
 /*

kernel/time/vsyscall.c

@@ -17,7 +17,7 @@ static inline void update_vdso_data(struct vdso_data *vdata,
 				    struct timekeeper *tk)
 {
 	struct vdso_timestamp *vdso_ts;
-	u64 nsec;
+	u64 nsec, sec;
 
 	vdata[CS_HRES_COARSE].cycle_last	= tk->tkr_mono.cycle_last;
 	vdata[CS_HRES_COARSE].mask		= tk->tkr_mono.mask;
@@ -45,23 +45,27 @@ static inline void update_vdso_data(struct vdso_data *vdata,
 	}
 	vdso_ts->nsec	= nsec;
 
-	/* CLOCK_MONOTONIC_RAW */
-	vdso_ts		= &vdata[CS_RAW].basetime[CLOCK_MONOTONIC_RAW];
-	vdso_ts->sec	= tk->raw_sec;
-	vdso_ts->nsec	= tk->tkr_raw.xtime_nsec;
+	/* Copy MONOTONIC time for BOOTTIME */
+	sec	= vdso_ts->sec;
+	/* Add the boot offset */
+	sec	+= tk->monotonic_to_boot.tv_sec;
+	nsec	+= (u64)tk->monotonic_to_boot.tv_nsec << tk->tkr_mono.shift;
 
 	/* CLOCK_BOOTTIME */
 	vdso_ts		= &vdata[CS_HRES_COARSE].basetime[CLOCK_BOOTTIME];
-	vdso_ts->sec	= tk->xtime_sec + tk->wall_to_monotonic.tv_sec;
-	nsec = tk->tkr_mono.xtime_nsec;
-	nsec += ((u64)(tk->wall_to_monotonic.tv_nsec +
-		       ktime_to_ns(tk->offs_boot)) << tk->tkr_mono.shift);
+	vdso_ts->sec	= sec;
+
 	while (nsec >= (((u64)NSEC_PER_SEC) << tk->tkr_mono.shift)) {
 		nsec -= (((u64)NSEC_PER_SEC) << tk->tkr_mono.shift);
 		vdso_ts->sec++;
 	}
 	vdso_ts->nsec	= nsec;
 
+	/* CLOCK_MONOTONIC_RAW */
+	vdso_ts		= &vdata[CS_RAW].basetime[CLOCK_MONOTONIC_RAW];
+	vdso_ts->sec	= tk->raw_sec;
+	vdso_ts->nsec	= tk->tkr_raw.xtime_nsec;
+
 	/* CLOCK_TAI */
 	vdso_ts		= &vdata[CS_HRES_COARSE].basetime[CLOCK_TAI];
 	vdso_ts->sec	= tk->xtime_sec + (s64)tk->tai_offset;