time: x86: Fix race switching from vsyscall to non-vsyscall clock

When switching from a vsyscall capable to a non-vsyscall capable
clocksource, there was a small race, where the last vsyscall
gettimeofday before the switch might return a invalid time value
using the new non-vsyscall enabled clocksource values after the
switch is complete.

This is due to the vsyscall code checking the vclock_mode once
outside of the seqcount protected section. After it reads the
vclock mode, it doesn't re-check that the sampled clock data
that is obtained in the seqcount critical section still matches.

The fix is to sample vclock_mode inside the protected section,
and as long as it isn't VCLOCK_NONE, return the calculated
value. If it has changed and is now VCLOCK_NONE, fall back
to the syscall gettime calculation.

v2:
  * Cleanup checks as suggested by tglx
  * Also fix same issue present in gettimeofday path

CC: Andy Lutomirski <luto@amacapital.net>
CC: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: John Stultz <john.stultz@linaro.org>

diff --git a/arch/x86/vdso/vclock_gettime.c b/arch/x86/vdso/vclock_gettime.c
index 6bc0e72..7eeb1f6 100644 (file)
--- a/arch/x86/vdso/vclock_gettime.c
+++ b/arch/x86/vdso/vclock_gettime.c
@@ -70,14 +70,26 @@ notrace static long vdso_fallback_gettime(long clock, struct timespec *ts)
        return ret;
 }
 
+notrace static long vdso_fallback_gtod(struct timeval *tv, struct timezone *tz)
+{
+       long ret;
+
+       asm("syscall" : "=a" (ret) :
+           "0" (__NR_gettimeofday), "D" (tv), "S" (tz) : "memory");
+       return ret;
+}
+
+
 notrace static inline long vgetns(void)
 {
        long v;
        cycles_t cycles;
        if (gtod->clock.vclock_mode == VCLOCK_TSC)
                cycles = vread_tsc();
-       else
+       else if (gtod->clock.vclock_mode == VCLOCK_HPET)
                cycles = vread_hpet();
+       else
+               return 0;
        v = (cycles - gtod->clock.cycle_last) & gtod->clock.mask;
        return (v * gtod->clock.mult) >> gtod->clock.shift;
 }
@@ -85,21 +97,28 @@ notrace static inline long vgetns(void)
 notrace static noinline int do_realtime(struct timespec *ts)
 {
        unsigned long seq, ns;
+       int mode;
+
        do {
                seq = read_seqbegin(&gtod->lock);
+               mode = gtod->clock.vclock_mode;
                ts->tv_sec = gtod->wall_time_sec;
                ts->tv_nsec = gtod->wall_time_nsec;
                ns = vgetns();
        } while (unlikely(read_seqretry(&gtod->lock, seq)));
+
        timespec_add_ns(ts, ns);
-       return 0;
+       return mode;
 }
 
 notrace static noinline int do_monotonic(struct timespec *ts)
 {
        unsigned long seq, ns, secs;
+       int mode;
+
        do {
                seq = read_seqbegin(&gtod->lock);
+               mode = gtod->clock.vclock_mode;
                secs = gtod->wall_time_sec;
                ns = gtod->wall_time_nsec + vgetns();
                secs += gtod->wall_to_monotonic.tv_sec;
@@ -116,7 +135,7 @@ notrace static noinline int do_monotonic(struct timespec *ts)
        ts->tv_sec = secs;
        ts->tv_nsec = ns;
 
-       return 0;
+       return mode;
 }
 
 notrace static noinline int do_realtime_coarse(struct timespec *ts)
@@ -156,14 +175,14 @@ notrace static noinline int do_monotonic_coarse(struct timespec *ts)
 
 notrace int __vdso_clock_gettime(clockid_t clock, struct timespec *ts)
 {
+       int ret = VCLOCK_NONE;
+
        switch (clock) {
        case CLOCK_REALTIME:
-               if (likely(gtod->clock.vclock_mode != VCLOCK_NONE))
-                       return do_realtime(ts);
+               ret = do_realtime(ts);
                break;
        case CLOCK_MONOTONIC:
-               if (likely(gtod->clock.vclock_mode != VCLOCK_NONE))
-                       return do_monotonic(ts);
+               ret = do_monotonic(ts);
                break;
        case CLOCK_REALTIME_COARSE:
                return do_realtime_coarse(ts);
@@ -171,32 +190,33 @@ notrace int __vdso_clock_gettime(clockid_t clock, struct timespec *ts)
                return do_monotonic_coarse(ts);
        }
 
-       return vdso_fallback_gettime(clock, ts);
+       if (ret == VCLOCK_NONE)
+               return vdso_fallback_gettime(clock, ts);
+       return 0;
 }
 int clock_gettime(clockid_t, struct timespec *)
        __attribute__((weak, alias("__vdso_clock_gettime")));
 
 notrace int __vdso_gettimeofday(struct timeval *tv, struct timezone *tz)
 {
-       long ret;
-       if (likely(gtod->clock.vclock_mode != VCLOCK_NONE)) {
-               if (likely(tv != NULL)) {
-                       BUILD_BUG_ON(offsetof(struct timeval, tv_usec) !=
-                                    offsetof(struct timespec, tv_nsec) ||
-                                    sizeof(*tv) != sizeof(struct timespec));
-                       do_realtime((struct timespec *)tv);
-                       tv->tv_usec /= 1000;
-               }
-               if (unlikely(tz != NULL)) {
-                       /* Avoid memcpy. Some old compilers fail to inline it */
-                       tz->tz_minuteswest = gtod->sys_tz.tz_minuteswest;
-                       tz->tz_dsttime = gtod->sys_tz.tz_dsttime;
-               }
-               return 0;
+       long ret = VCLOCK_NONE;
+
+       if (likely(tv != NULL)) {
+               BUILD_BUG_ON(offsetof(struct timeval, tv_usec) !=
+                            offsetof(struct timespec, tv_nsec) ||
+                            sizeof(*tv) != sizeof(struct timespec));
+               ret = do_realtime((struct timespec *)tv);
+               tv->tv_usec /= 1000;
        }
-       asm("syscall" : "=a" (ret) :
-           "0" (__NR_gettimeofday), "D" (tv), "S" (tz) : "memory");
-       return ret;
+       if (unlikely(tz != NULL)) {
+               /* Avoid memcpy. Some old compilers fail to inline it */
+               tz->tz_minuteswest = gtod->sys_tz.tz_minuteswest;
+               tz->tz_dsttime = gtod->sys_tz.tz_dsttime;
+       }
+
+       if (ret == VCLOCK_NONE)
+               return vdso_fallback_gtod(tv, tz);
+       return 0;
 }
 int gettimeofday(struct timeval *, struct timezone *)
        __attribute__((weak, alias("__vdso_gettimeofday")));