From a939e817aa7e199d2fff05a67cb745be32dd5c2d Mon Sep 17 00:00:00 2001 From: John Stultz Date: Thu, 1 Mar 2012 22:11:09 -0800 Subject: [PATCH] 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 CC: Thomas Gleixner Signed-off-by: John Stultz --- arch/x86/vdso/vclock_gettime.c | 72 ++++++++++++++++++++++------------ 1 file changed, 46 insertions(+), 26 deletions(-) diff --git a/arch/x86/vdso/vclock_gettime.c b/arch/x86/vdso/vclock_gettime.c index 6bc0e723b6e8..7eeb1f6188ee 100644 --- 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(>od->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(>od->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(>od->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"))); -- 2.20.1