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Date:   Fri, 5 Aug 2022 11:55:15 +0200
From:   Oleksij Rempel <o.rempel@...gutronix.de>
To:     Fedor Pchelkin <pchelkin@...ras.ru>
Cc:     Robin van der Gracht <robin@...tonic.nl>,
        Oleksij Rempel <linux@...pel-privat.de>, kernel@...gutronix.de,
        Oliver Hartkopp <socketcan@...tkopp.net>,
        Marc Kleine-Budde <mkl@...gutronix.de>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>, linux-can@...r.kernel.org,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        ldv-project@...uxtesting.org,
        Alexey Khoroshilov <khoroshilov@...ras.ru>
Subject: Re: [PATCH] can: j1939: fix memory leak of skbs

On Fri, Aug 05, 2022 at 11:56:18AM +0300, Fedor Pchelkin wrote:
> Hi Oleksij,
> 
> On 29.07.2022 07:22, Oleksij Rempel wrote:
> > Initial issue can be reproduced by using real (slow) CAN with j1939cat[1]
> > tool. Both parts should be started to make sure the j1939_session_tx_dat() will
> > actually start using the queue. After pushing about 100K of data, application
> > will try to close the socket and exit. After socket is closed, all skb related
> > to this socket will be freed and j1939_session_tx_dat() will use freed skbs.
> 
> Ok, the patch I suggested was a kind of a guess, now I understand that
> it breaks important logic.
> 
> On 29.07.2022 07:22, Oleksij Rempel wrote:
> > This skb_get() is counter part of skb_unref()
> > j1939_session_skb_drop_old().
> 
> However, we have a case [1] where j1939_session_skb_queue() is called
> but the corresponding j1939_session_skb_drop_old() is not called and it
> causes a memory leak.
> 
> I tried to investigate it a little bit: the thing is that
> j1939_session_skb_drop_old() can be called only when processing
> J1939_ETP_CMD_CTS. On the contrary, as I can see,
> j1939_session_skb_queue() can be called independently from
> J1939_ETP_CMD_CTS so the two functions obviously do not correspond to
> each other.
> 
> In reproducer case there is a J1939_ETP_CMD_RTS processing, then
> we send some messages (where j1939_session_skb_queue() is called) and
> after that J1939_ETP_CMD_ABORT is processed and we lose those skbs.

Ah.. good point.
In this case it will go to:
  j1939_session_destroy()
    skb_queue_purge(&session->skb_queue)
      kfree_skb(skb);

And in the normal path we have:
  j1939_session_skb_drop_old()
    skb_unref(do_skb);
    kfree_skb(do_skb);

It means skb_queue_purge() should be replaced with something like:
	while ((skb = skb_dequeue(list)) != NULL) {
		/* drop ref taken in j1939_session_skb_queue() */
		skb_unref(skb);
		kfree_skb(skb);
	}

Can you please test it?

Regards,
Oleksij
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