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> 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: Andrew Hunter <ahh@xxxxxxxxxx> 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: Greg Hackmann <ghackmann@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
