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) & + 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
