Commit a939e817 authored by John Stultz's avatar John Stultz
Browse files

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: default avatarJohn Stultz <john.stultz@linaro.org>
parent a078c6d0

arch/x86/vdso/vclock_gettime.c

+46 −26
Original line number Diff line number Diff line
@@ -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);
@@ -171,20 +190,22 @@ notrace int __vdso_clock_gettime(clockid_t clock, struct timespec *ts) 
		return do_monotonic_coarse(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)) {

	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));

			do_realtime((struct timespec *)tv);

		ret = do_realtime((struct timespec *)tv);

		tv->tv_usec /= 1000;

	}

	if (unlikely(tz != NULL)) {
@@ -192,12 +213,11 @@ notrace int __vdso_gettimeofday(struct timeval *tv, struct timezone *tz) 
		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;

}

	asm("syscall" : "=a" (ret) :

	    "0" (__NR_gettimeofday), "D" (tv), "S" (tz) : "memory");

	return ret;

}

int gettimeofday(struct timeval *, struct timezone *)

	__attribute__((weak, alias("__vdso_gettimeofday")));

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