On Mon, Jan 28, 2019 at 05:07:07PM -0500, Mathieu Desnoyers wrote: > 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) & Are we guaranteed that this p->mm will be the same as the one loaded via READ_ONCE() above? Either way, wouldn't it be better to READ_ONCE() it a single time and use the same value everywhere? Thanx, Paul > + 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 >
