Re: [PATCH 3/6] ipc/mqueue.c: Update/document memory barriers

[Peter Zijlstra]

On Mon, Oct 14, 2019 at 02:59:11PM +0200, Peter Zijlstra wrote:
> On Sat, Oct 12, 2019 at 07:49:55AM +0200, Manfred Spraul wrote:
> 
> >  	for (;;) {
> > +		/* memory barrier not required, we hold info->lock */
> >  		__set_current_state(TASK_INTERRUPTIBLE);
> >  
> >  		spin_unlock(&info->lock);
> >  		time = schedule_hrtimeout_range_clock(timeout, 0,
> >  			HRTIMER_MODE_ABS, CLOCK_REALTIME);
> >  
> > +		if (READ_ONCE(ewp->state) == STATE_READY) {
> > +			/*
> > +			 * Pairs, together with READ_ONCE(), with
> > +			 * the barrier in __pipelined_op().
> > +			 */
> > +			smp_acquire__after_ctrl_dep();
> >  			retval = 0;
> >  			goto out;
> >  		}
> >  		spin_lock(&info->lock);
> > +
> > +		/* we hold info->lock, so no memory barrier required */
> > +		if (READ_ONCE(ewp->state) == STATE_READY) {
> >  			retval = 0;
> >  			goto out_unlock;
> >  		}
> > @@ -925,14 +933,12 @@ static inline void __pipelined_op(struct wake_q_head *wake_q,
> >  	list_del(&this->list);
> >  	wake_q_add(wake_q, this->task);
> >  	/*
> > +	 * The barrier is required to ensure that the refcount increase
> > +	 * inside wake_q_add() is completed before the state is updated.
> 
> fails to explain *why* this is important.
> 
> > +	 *
> > +	 * The barrier pairs with READ_ONCE()+smp_mb__after_ctrl_dep().
> >  	 */
> > +        smp_store_release(&this->state, STATE_READY);
> 
> You retained the whitespace damage.
> 
> And I'm terribly confused by this code, probably due to the lack of
> 'why' as per the above. What is this trying to do?
> 
> Are we worried about something like:
> 
> 	A			B				C
> 
> 
> 				wq_sleep()
> 				  schedule_...();
> 
> 								/* spuriuos wakeup */
> 								wake_up_process(B)
> 
> 	wake_q_add(A)
> 	  if (cmpxchg()) // success
> 
> 	->state = STATE_READY (reordered)
> 
> 				  if (READ_ONCE() == STATE_READY)
> 				    goto out;
> 
> 				exit();
> 
> 
> 	    get_task_struct() // UaF
> 
> 
> Can we put the exact and full race in the comment please?

Like Davidlohr already suggested, elsewhere we write it like so:


--- a/ipc/mqueue.c
+++ b/ipc/mqueue.c
@@ -930,15 +930,10 @@ static inline void __pipelined_op(struct
 				  struct mqueue_inode_info *info,
 				  struct ext_wait_queue *this)
 {
+	get_task_struct(this->task);
 	list_del(&this->list);
-	wake_q_add(wake_q, this->task);
-	/*
-	 * The barrier is required to ensure that the refcount increase
-	 * inside wake_q_add() is completed before the state is updated.
-	 *
-	 * The barrier pairs with READ_ONCE()+smp_mb__after_ctrl_dep().
-	 */
-        smp_store_release(&this->state, STATE_READY);
+	smp_store_release(&this->state, STATE_READY);
+	wake_q_add_safe(wake_q, this->task);
 }
 
 /* pipelined_send() - send a message directly to the task waiting in