ntp: Limit TAI-UTC offset

Don't allow the TAI-UTC offset of the system clock to be set by adjtimex()
to a value larger than 100000 seconds.

This prevents an overflow in the conversion to int, prevents the CLOCK_TAI
clock from getting too far ahead of the CLOCK_REALTIME clock, and it is
still large enough to allow leap seconds to be inserted at the maximum rate
currently supported by the kernel (once per day) for the next ~270 years,
however unlikely it is that someone can survive a catastrophic event which
slowed down the rotation of the Earth so much.

Reported-by: Weikang shi <swkhack@gmail.com>
Signed-off-by: Miroslav Lichvar <mlichvar@redhat.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: John Stultz <john.stultz@linaro.org>
Cc: Prarit Bhargava <prarit@redhat.com>
Cc: Richard Cochran <richardcochran@gmail.com>
Cc: Stephen Boyd <sboyd@kernel.org>
Link: https://lkml.kernel.org/r/20190618154713.20929-1-mlichvar@redhat.com

kernel/time/ntp.c

@@ -43,6 +43,7 @@ static u64			tick_length_base;
 #define MAX_TICKADJ		500LL		/* usecs */
 #define MAX_TICKADJ_SCALED \
 	(((MAX_TICKADJ * NSEC_PER_USEC) << NTP_SCALE_SHIFT) / NTP_INTERVAL_FREQ)
+#define MAX_TAI_OFFSET		100000
 
 /*
  * phase-lock loop variables
@@ -691,7 +692,8 @@ static inline void process_adjtimex_modes(const struct __kernel_timex *txc,
 		time_constant = max(time_constant, 0l);
 	}
 
-	if (txc->modes & ADJ_TAI && txc->constant >= 0)
+	if (txc->modes & ADJ_TAI &&
+			txc->constant >= 0 && txc->constant <= MAX_TAI_OFFSET)
 		*time_tai = txc->constant;
 
 	if (txc->modes & ADJ_OFFSET)