Re: use-after-free in usbnet

[Huajun Li <huajun.li.lee-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>]

On Sun, Apr 22, 2012 at 3:23 AM, David Miller <davem-fT/PcQaiUtIeIZ0/mPfg9Q@public.gmane.org> wrote:
> From: Ming Lei <tom.leiming-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
> 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'.

In this case, the problem is, 'tmp = skb->next' can be moved out of
rxq/txq, and even be freed. Then in next loop cycle, 'skb = tmp' will
refer to a freed skb.  You know, in current code stack, unlink_urbs()
releases q->lock in each loop, this gives chance to urb complete
handler to call defer_bh() and cause the problem.
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