Re: [PATCH] mm: fix pgdat->kswap accessed concurrently

[Andrew Morton <akpm@linux-foundation.org>]

On Sat, 20 Aug 2022 15:33:04 +0800 Muchun Song <muchun.song@linux.dev> wrote:

> 
> 
> > +	if (IS_ERR(t)) {
> > 		/* failure at boot is fatal */
> > 		BUG_ON(system_state < SYSTEM_RUNNING);
> > 		pr_err("Failed to start kswapd on node %d\n", nid);
> > -		pgdat->kswapd = NULL;
> > +		WRITE_ONCE(pgdat->kswapd, NULL);
> > +	} else {
> > +		WRITE_ONCE(pgdat->kswapd, t);
> > 	}
> > }
> 
> IIUC, the race is like the followings:
> 
> CPU 0:					CPU 1:
> 
> kswapd_run()
> 	pgdat->kswapd = kthread_run()
> 	if (IS_ERR(pgdat->kswapd))
> 					kswapd_is_running
> 						// load pgdat->kswapd and it is NOT NULL.
> 		pgdat->kswapd = NULL
> 						task_is_running(pgdat->kswapd); // NULL pointer dereference
> 

But don't we still have a bug?  Sure, kswapd_is_running() will no
longer deref a null pointer.  But it now runs kswapd_is_running()
against a task which has exited - a use-after-free?