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

[Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>]

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@xxxxxxxxxxxxxx
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
Tested-by: Jann Horn <jannh@xxxxxxxxxx>
CC: Jann Horn <jannh@xxxxxxxxxx>
CC: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
CC: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
CC: Ingo Molnar <mingo@xxxxxxxxxx>
CC: Andrea Parri <parri.andrea@xxxxxxxxx>
CC: Andy Lutomirski <luto@xxxxxxxxxx>
CC: Avi Kivity <avi@xxxxxxxxxxxx>
CC: Benjamin Herrenschmidt <benh@xxxxxxxxxxxxxxxxxxx>
CC: Boqun Feng <boqun.feng@xxxxxxxxx>
CC: Dave Watson <davejwatson@xxxxxx>
CC: David Sehr <sehr@xxxxxxxxxx>
CC: H. Peter Anvin <hpa@xxxxxxxxx>
CC: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
CC: Maged Michael <maged.michael@xxxxxxxxx>
CC: Michael Ellerman <mpe@xxxxxxxxxxxxxx>
CC: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
CC: Paul Mackerras <paulus@xxxxxxxxx>
CC: Russell King <linux@xxxxxxxxxxxxxxx>
CC: Will Deacon <will.deacon@xxxxxxx>
CC: stable@xxxxxxxxxxxxxxx # v4.16+
CC: linux-api@xxxxxxxxxxxxxxx
---
 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) &
+					     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