On Mon, Jan 28, 2019 at 05:07:07PM -0500, Mathieu Desnoyers wrote:
> Jann Horn identified a racy access to p->mm in the global expedited
> command of the membarrier system call.
>
> The suggested fix is to hold the task_lock() around the accesses to
> p->mm and to the mm_struct membarrier_state field to guarantee the
> existence of the mm_struct.
>
> Link:
> https://lore.kernel.org/lkml/cag48ez2g8ctf8dhs42tf37pthfr3y0rnooytmxvacm4u8yu...@mail.gmail.com
> Signed-off-by: Mathieu Desnoyers <[email protected]>
> Tested-by: Jann Horn <[email protected]>
> CC: Jann Horn <[email protected]>
> CC: Thomas Gleixner <[email protected]>
> CC: Peter Zijlstra (Intel) <[email protected]>
> CC: Ingo Molnar <[email protected]>
> CC: Andrea Parri <[email protected]>
> CC: Andy Lutomirski <[email protected]>
> CC: Avi Kivity <[email protected]>
> CC: Benjamin Herrenschmidt <[email protected]>
> CC: Boqun Feng <[email protected]>
> CC: Dave Watson <[email protected]>
> CC: David Sehr <[email protected]>
> CC: H. Peter Anvin <[email protected]>
> CC: Linus Torvalds <[email protected]>
> CC: Maged Michael <[email protected]>
> CC: Michael Ellerman <[email protected]>
> CC: Paul E. McKenney <[email protected]>
> CC: Paul Mackerras <[email protected]>
> CC: Russell King <[email protected]>
> CC: Will Deacon <[email protected]>
> CC: [email protected] # v4.16+
> CC: [email protected]
> ---
> kernel/sched/membarrier.c | 27 +++++++++++++++++++++------
> 1 file changed, 21 insertions(+), 6 deletions(-)
>
> diff --git a/kernel/sched/membarrier.c b/kernel/sched/membarrier.c
> index 76e0eaf4654e..305fdcc4c5f7 100644
> --- a/kernel/sched/membarrier.c
> +++ b/kernel/sched/membarrier.c
> @@ -81,12 +81,27 @@ static int membarrier_global_expedited(void)
>
> rcu_read_lock();
> p = task_rcu_dereference(&cpu_rq(cpu)->curr);
> - if (p && p->mm && (atomic_read(&p->mm->membarrier_state) &
> - MEMBARRIER_STATE_GLOBAL_EXPEDITED)) {
> - if (!fallback)
> - __cpumask_set_cpu(cpu, tmpmask);
> - else
> - smp_call_function_single(cpu, ipi_mb, NULL, 1);
> + /*
> + * Skip this CPU if the runqueue's current task is NULL or if
> + * it is a kernel thread.
> + */
> + if (p && READ_ONCE(p->mm)) {
> + bool mm_match;
> +
> + /*
> + * Read p->mm and access membarrier_state while holding
> + * the task lock to ensure existence of mm.
> + */
> + task_lock(p);
> + mm_match = p->mm &&
> (atomic_read(&p->mm->membarrier_state) &
Are we guaranteed that this p->mm will be the same as the one loaded via
READ_ONCE() above? Either way, wouldn't it be better to READ_ONCE() it a
single time and use the same value everywhere?
Thanx, Paul
> + MEMBARRIER_STATE_GLOBAL_EXPEDITED);
> + task_unlock(p);
> + if (mm_match) {
> + if (!fallback)
> + __cpumask_set_cpu(cpu, tmpmask);
> + else
> + smp_call_function_single(cpu, ipi_mb,
> NULL, 1);
> + }
> }
> rcu_read_unlock();
> }
> --
> 2.17.1
>
