netdev - Re: use-after-free in usbnet

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Date:	Sat, 21 Apr 2012 15:23:45 -0400 (EDT)
From:	David Miller <davem@...emloft.net>
To:	tom.leiming@...il.com
Cc:	huajun.li.lee@...il.com, oneukum@...e.de,
	stern@...land.harvard.edu, davej@...hat.com,
	netdev@...r.kernel.org, linux-usb@...r.kernel.org,
	kernel-team@...oraproject.org
Subject: Re: use-after-free in usbnet

From: Ming Lei <tom.leiming@...il.com>
Date: Sat, 21 Apr 2012 09:49:51 +0800

> I see the problem, so looks skb_queue_walk_safe is not safe.
> I don' know why the 2nd ' tmp = skb->next' in  skb_queue_walk_safe
> is needed and it may become unsafe if skb is freed during current loop.

I can't see what the problem is, skb_queue_walk_safe() is perfect
and does exactly what it advertises to do.

If 'skb' is unlinked inside of an skb_queue_walk_safe() loop, that's
fine, because we won't touch 'skb' in the loop iteration tail code.

Instead, before the loop contents, we pre-fetch skb->next into 'tmp'
and then at the end we move 'skb' forward by simply assigning 'tmp'.
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