timekeeping: Really make sure wall_to_monotonic isn't positive

commit 4e8c11b6b3f0b6a283e898344f154641eda94266 upstream.

Even after commit e1d7ba873555 ("time: Always make sure wall_to_monotonic
isn't positive") it is still possible to make wall_to_monotonic positive
by running the following code:

    int main(void)
    {
        struct timespec time;

        clock_gettime(CLOCK_MONOTONIC, &time);
        time.tv_nsec = 0;
        clock_settime(CLOCK_REALTIME, &time);
        return 0;
    }

The reason is that the second parameter of timespec64_compare(), ts_delta,
may be unnormalized because the delta is calculated with an open coded
substraction which causes the comparison of tv_sec to yield the wrong
result:

  wall_to_monotonic = { .tv_sec = -10, .tv_nsec =  900000000 }
  ts_delta 	    = { .tv_sec =  -9, .tv_nsec = -900000000 }

That makes timespec64_compare() claim that wall_to_monotonic < ts_delta,
but actually the result should be wall_to_monotonic > ts_delta.

After normalization, the result of timespec64_compare() is correct because
the tv_sec comparison is not longer misleading:

  wall_to_monotonic = { .tv_sec = -10, .tv_nsec =  900000000 }
  ts_delta 	    = { .tv_sec = -10, .tv_nsec =  100000000 }

Use timespec64_sub() to ensure that ts_delta is normalized, which fixes the
issue.

Fixes: e1d7ba873555 ("time: Always make sure wall_to_monotonic isn't positive")
Signed-off-by: Yu Liao <liaoyu15@huawei.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/20211213135727.1656662-1-liaoyu15@huawei.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

1 files changed, 1 insertions, 2 deletions

diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
index 5b6f815a74ee..602b476627dd 100644
--- a/kernel/time/timekeeping.c
+++ b/kernel/time/timekeeping.c
@@ -1235,8 +1235,7 @@ int do_settimeofday64(const struct timespec64 *ts)
 	timekeeping_forward_now(tk);
 
 	xt = tk_xtime(tk);
-	ts_delta.tv_sec = ts->tv_sec - xt.tv_sec;
-	ts_delta.tv_nsec = ts->tv_nsec - xt.tv_nsec;
+	ts_delta = timespec64_sub(*ts, xt);
 
 	if (timespec64_compare(&tk->wall_to_monotonic, &ts_delta) > 0) {
 		ret = -EINVAL;