Re: [PATCH] Fix: membarrier: racy access to p->mm in membarrier_global_expedited()

["Paul E. McKenney" <paulmck@linux.ibm.com>]

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/CAG48ez2G8ctF8dHS42TF37pThfr3y0RNOOYTmxvACm4u8Yu3cw@mail.gmail.com
> Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
> Tested-by: Jann Horn <jannh@google.com>
> CC: Jann Horn <jannh@google.com>
> CC: Thomas Gleixner <tglx@linutronix.de>
> CC: Peter Zijlstra (Intel) <peterz@infradead.org>
> CC: Ingo Molnar <mingo@kernel.org>
> CC: Andrea Parri <parri.andrea@gmail.com>
> CC: Andy Lutomirski <luto@kernel.org>
> CC: Avi Kivity <avi@scylladb.com>
> CC: Benjamin Herrenschmidt <benh@kernel.crashing.org>
> CC: Boqun Feng <boqun.feng@gmail.com>
> CC: Dave Watson <davejwatson@fb.com>
> CC: David Sehr <sehr@google.com>
> CC: H. Peter Anvin <hpa@zytor.com>
> CC: Linus Torvalds <torvalds@linux-foundation.org>
> CC: Maged Michael <maged.michael@gmail.com>
> CC: Michael Ellerman <mpe@ellerman.id.au>
> CC: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> CC: Paul Mackerras <paulus@samba.org>
> CC: Russell King <linux@armlinux.org.uk>
> CC: Will Deacon <will.deacon@arm.com>
> CC: stable@vger.kernel.org # v4.16+
> CC: linux-api@vger.kernel.org
> ---
>  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
>