linux-kernel - Re: [PATCH] Advertise PPPoE MTU / avoid memory leak.

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Date:	Sun, 24 Sep 2006 07:29:25 -0500
From:	Michal Ostrowski <mostrows@...thlink.net>
To:	David Miller <davem@...emloft.net>
Cc:	linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
	ppp-bugs@...samba.org
Subject: Re: [PATCH] Advertise PPPoE MTU / avoid memory leak.

On Sat, 2006-09-23 at 14:56 -0700, David Miller wrote:
> From: mostrows@...thlink.net
> Date: Sat, 23 Sep 2006 12:30:23 -0500
> 
> > __pppoe_xmit must free any skb it allocates if there is an error
> > submitting the skb downstream.
> 
> This isn't right, dev_queue_xmit() can return -ENETDOWN and still
> free the SKB, so your change will cause the SKB to be freed up
> twice in that case, from dev_queue_xmit():
> 
> 	rc = -ENETDOWN;
> 	rcu_read_unlock_bh();
> 
> out_kfree_skb:
> 	kfree_skb(skb);
> 	return rc;
> 
> dev_queue_xmit() is basically expected to consume the packet,
> error or not.
> 
> What case of calling dev_queue_xmit() did you discover that did not
> kfree the SKB on error?  We should fix that.  On a quick scan on the
> entire dev_queue_xmit() implmentation, I cannot find such a case.
> 

I think the call path via dev->hard_start_xmit, if it fails, may result
in an skb not being freed.  This appears to be the case with the e100.c
driver.  The qdisc_restart path to dev->hard_start_xmit also appears
susceptible to this.  It appears that not all devices agree as to who
should clean-up an skb on error.

-- 
Michal Ostrowski <mostrows@...thlink.net>

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