ARM: 8410/1: VDSO: fix coarse clock monotonicity regression

Since 906c55579a63 ("timekeeping: Copy the shadow-timekeeper over the
real timekeeper last") it has become possible on ARM to:

- Obtain a CLOCK_MONOTONIC_COARSE or CLOCK_REALTIME_COARSE timestamp
  via syscall.
- Subsequently obtain a timestamp for the same clock ID via VDSO which
  predates the first timestamp (by one jiffy).

This is because ARM's update_vsyscall is deriving the coarse time
using the __current_kernel_time interface, when it should really be
using the timekeeper object provided to it by the timekeeping core.
It happened to work before only because __current_kernel_time would
access the same timekeeper object which had been passed to
update_vsyscall.  This is no longer the case.

Cc: stable@vger.kernel.org
Fixes: 906c55579a63 ("timekeeping: Copy the shadow-timekeeper over the real timekeeper last")
Signed-off-by: Nathan Lynch <nathan_lynch@mentor.com>
Acked-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>

arch/arm/kernel/vdso.c

@@ -296,7 +296,6 @@ static bool tk_is_cntvct(const struct timekeeper *tk)
  */
 void update_vsyscall(struct timekeeper *tk)
 {
-	struct timespec xtime_coarse;
 	struct timespec64 *wtm = &tk->wall_to_monotonic;
 
 	if (!cntvct_ok) {
@@ -308,10 +307,10 @@ void update_vsyscall(struct timekeeper *tk)
 
 	vdso_write_begin(vdso_data);
 
-	xtime_coarse = __current_kernel_time();
 	vdso_data->tk_is_cntvct			= tk_is_cntvct(tk);
-	vdso_data->xtime_coarse_sec		= xtime_coarse.tv_sec;
-	vdso_data->xtime_coarse_nsec		= xtime_coarse.tv_nsec;
+	vdso_data->xtime_coarse_sec		= tk->xtime_sec;
+	vdso_data->xtime_coarse_nsec		= (u32)(tk->tkr_mono.xtime_nsec >>
+							tk->tkr_mono.shift);
 	vdso_data->wtm_clock_sec		= wtm->tv_sec;
 	vdso_data->wtm_clock_nsec		= wtm->tv_nsec;