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

[Muchun Song <muchun.song@linux.dev>]

> On Aug 21, 2022, at 04:59, Andrew Morton <akpm@linux-foundation.org> wrote:
> 
> 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?
> 

Agree. I missed that.

Thanks,
Muchun