Re: [PATCH] can: j1939: fix memory leak of skbs

[Oleksij Rempel <o.rempel@pengutronix.de>]

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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