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>

1 files changed, 3 insertions, 4 deletions

diff --git a/arch/arm/kernel/vdso.c b/arch/arm/kernel/vdso.c
index efe17dd9b921..54a5aeab988d 100644
--- a/arch/arm/kernel/vdso.c
+++ b/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;