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

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Date:   Sat, 20 Aug 2022 13:59:55 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Muchun Song <muchun.song@...ux.dev>
Cc:     Kefeng Wang <wangkefeng.wang@...wei.com>,
        Linux MM <linux-mm@...ck.org>, Qian Cai <cai@....pw>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: fix pgdat->kswap accessed concurrently

On Sat, 20 Aug 2022 15:33:04 +0800 Muchun Song <muchun.song@...ux.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?