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

On Mon, Jan 28, 2019 at 11:39 PM Paul E. McKenney [off-list ref] wrote:

On Mon, Jan 28, 2019 at 05:07:07PM -0500, Mathieu Desnoyers wrote:

quoted

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 (local)
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>

[...]

quoted

--- 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?

No; the way I read it, that's just an optimization and has no effect
on correctness.

Either way, wouldn't it be better to READ_ONCE() it a
single time and use the same value everywhere?

No; the first READ_ONCE() returns a pointer that you can't access
because it wasn't read under a lock. You can only use it for a NULL
check.