netdev - Re: [PATCH net] rxrpc: fix a race in rxrpc_exit

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Date:   Fri, 08 Apr 2022 07:41:30 +0100
From:   David Howells <dhowells@...hat.com>
To:     Eric Dumazet <eric.dumazet@...il.com>
Cc:     dhowells@...hat.com, "David S . Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        netdev <netdev@...r.kernel.org>,
        Eric Dumazet <edumazet@...gle.com>,
        Marc Dionne <marc.dionne@...istor.com>,
        linux-afs@...ts.infradead.org, syzbot <syzkaller@...glegroups.com>
Subject: Re: [PATCH net] rxrpc: fix a race in rxrpc_exit_net()

Hi Eric,

[Note that your patch is appl/octet-stream for some reason.]

> 	rxnet->live = false;
> -	del_timer_sync(&rxnet->peer_keepalive_timer);
>  	cancel_work_sync(&rxnet->peer_keepalive_work);
> +	del_timer_sync(&rxnet->peer_keepalive_timer);

That fixes that problem, but introduces another.  The timer could be in the
throes of queueing the worker:

	CPU 1			CPU 2
	====================	=====================
				if (rxnet->live)
				<INTERRUPT>
	rxnet->live = false;
 	cancel_work_sync(&rxnet->peer_keepalive_work);
				rxrpc_queue_work(&rxnet->peer_keepalive_work);
	del_timer_sync(&rxnet->peer_keepalive_timer);

I think keeping the first del_timer_sync() that you removed and the one after
the cancel would be sufficient.

David