On Mon, Jan 28, 2019 at 11:39 PM Paul E. McKenney paulmck@linux.ibm.com wrote:
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/CAG48ez2G8ctF8dHS42TF37pThfr3y0RNOOYTmxvACm4u8Y... Signed-off-by: Mathieu Desnoyers mathieu.desnoyers@efficios.com
[...]
--- 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);elsesmp_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.