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/CAG48ez2G8ctF8dHS42TF37pThfr3y0RNOOYTmxvACm4u8Y... 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) & + 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(); }