Re: powerpc/vdso64: Fix CLOCK_MONOTONIC inconsistencies across Y2038

[Michael Ellerman]

On Wed, 2019-03-13 at 13:14:38 UTC, Michael Ellerman wrote:
> Jakub Drnec reported:
>   Setting the realtime clock can sometimes make the monotonic clock go
>   back by over a hundred years. Decreasing the realtime clock across
>   the y2k38 threshold is one reliable way to reproduce. Allegedly this
>   can also happen just by running ntpd, I have not managed to
>   reproduce that other than booting with rtc at >2038 and then running
>   ntp. When this happens, anything with timers (e.g. openjdk) breaks
>   rather badly.
> 
> And included a test case (slightly edited for brevity):
>   #define _POSIX_C_SOURCE 199309L
>   #include <stdio.h>
>   #include <time.h>
>   #include <stdlib.h>
>   #include <unistd.h>
> 
>   long get_time(void) {
>     struct timespec tp;
>     clock_gettime(CLOCK_MONOTONIC, &tp);
>     return tp.tv_sec + tp.tv_nsec / 1000000000;
>   }
> 
>   int main(void) {
>     long last = get_time();
>     while(1) {
>       long now = get_time();
>       if (now < last) {
>         printf("clock went backwards by %ld seconds!\n", last - now);
>       }
>       last = now;
>       sleep(1);
>     }
>     return 0;
>   }
> 
> Which when run concurrently with:
>  # date -s 2040-1-1
>  # date -s 2037-1-1
> 
> Will detect the clock going backward.
> 
> The root cause is that wtom_clock_sec in struct vdso_data is only a
> 32-bit signed value, even though we set its value to be equal to
> tk->wall_to_monotonic.tv_sec which is 64-bits.
> 
> Because the monotonic clock starts at zero when the system boots the
> wall_to_montonic.tv_sec offset is negative for current and future
> dates. Currently on a freshly booted system the offset will be in the
> vicinity of negative 1.5 billion seconds.
> 
> However if the wall clock is set past the Y2038 boundary, the offset
> from wall to monotonic becomes less than negative 2^31, and no longer
> fits in 32-bits. When that value is assigned to wtom_clock_sec it is
> truncated and becomes positive, causing the VDSO assembly code to
> calculate CLOCK_MONOTONIC incorrectly.
> 
> That causes CLOCK_MONOTONIC to jump ahead by ~4 billion seconds which
> it is not meant to do. Worse, if the time is then set back before the
> Y2038 boundary CLOCK_MONOTONIC will jump backward.
> 
> We can fix it simply by storing the full 64-bit offset in the
> vdso_data, and using that in the VDSO assembly code. We also shuffle
> some of the fields in vdso_data to avoid creating a hole.
> 
> The original commit that added the CLOCK_MONOTONIC support to the VDSO
> did actually use a 64-bit value for wtom_clock_sec, see commit
> a7f290dad32e ("[PATCH] powerpc: Merge vdso's and add vdso support to
> 32 bits kernel") (Nov 2005). However just 3 days later it was
> converted to 32-bits in commit 0c37ec2aa88b ("[PATCH] powerpc: vdso
> fixes (take #2)"), and the bug has existed since then AFAICS.
> 
> Fixes: 0c37ec2aa88b ("[PATCH] powerpc: vdso fixes (take #2)")
> Cc: stable@xxxxxxxxxxxxxxx # v2.6.15+
> Link: http://lkml.kernel.org/r/HaC.ZfES.62bwlnvAvMP.1STMMj@xxxxxxxxx
> Reported-by: Jakub Drnec <jaydee@xxxxxxxx>
> Signed-off-by: Michael Ellerman <mpe@xxxxxxxxxxxxxx>

Applied to powerpc fixes.

https://git.kernel.org/powerpc/c/b5b4453e7912f056da1ca7572574cada

cheers