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>

1 files 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(&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")));