Commit 78fd8c72 authored Jan 04, 2016 by Andy Lutomirski Committed by Ingo Molnar Jan 13, 2016

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>

parent 4eaffdd5

arch/x86/entry/vdso/vclock_gettime.c

+6 −6

Original line number	Diff line number	Diff line
		@@ -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;