x86/vdso/pvclock: Protect STABLE check with the seqcount

If the clock becomes unstable while we're reading it, we need to
bail.  We can do this by simply moving the check into the
seqcount loop.

Reported-by: Marcelo Tosatti <mtosatti@redhat.com>
Signed-off-by: Andy Lutomirski <luto@kernel.org>
Cc: Alexander Graf <agraf@suse.de>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Radim Krcmar <rkrcmar@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/755dcedb17269e1d7ce12a9a713dea303835137e.1451949191.git.luto@kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>

diff --git a/arch/x86/entry/vdso/vclock_gettime.c b/arch/x86/entry/vdso/vclock_gettime.c
index 8602f06c759f7eec2786eaa17df10b6a6720e8ea..1a50e09c945bd41564a0ef94e43e3f19d8d05b88 100644 (file)
--- a/arch/x86/entry/vdso/vclock_gettime.c
+++ b/arch/x86/entry/vdso/vclock_gettime.c
@@ -126,23 +126,23 @@ static notrace cycle_t vread_pvclock(int *mode)
         *
         * On Xen, we don't appear to have that guarantee, but Xen still
         * supplies a valid seqlock using the version field.
-
+        *
         * We only do pvclock vdso timing at all if
         * PVCLOCK_TSC_STABLE_BIT is set, and we interpret that bit to
         * mean that all vCPUs have matching pvti and that the TSC is
         * synced, so we can just look at vCPU 0's pvti.
         */
 
-       if (unlikely(!(pvti->flags & PVCLOCK_TSC_STABLE_BIT))) {
-               *mode = VCLOCK_NONE;
-               return 0;
-       }
-
        do {
                version = pvti->version;
 
                smp_rmb();
 
+               if (unlikely(!(pvti->flags & PVCLOCK_TSC_STABLE_BIT))) {
+                       *mode = VCLOCK_NONE;
+                       return 0;
+               }
+
                tsc = rdtsc_ordered();
                pvti_tsc_to_system_mul = pvti->tsc_to_system_mul;
                pvti_tsc_shift = pvti->tsc_shift;