netdev - Re: [PATCH -next] bridge: fix potential use-after-free when hook returns QUEUE or STOLEN verdict

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Date:	Sat, 12 Mar 2016 22:20:08 +0300
From:	Sergei Shtylyov <sergei.shtylyov@...entembedded.com>
To:	Florian Westphal <fw@...len.de>, netdev@...r.kernel.org
Cc:	stephen@...workplumber.org, Felix Fietkau <nbd@...nwrt.org>,
	Zefir Kurtisi <zefir.kurtisi@...atec.com>
Subject: Re: [PATCH -next] bridge: fix potential use-after-free when hook
 returns QUEUE or STOLEN verdict

Hello.

On 03/12/2016 01:14 PM, Florian Westphal wrote:

> Zefir Kurtisi reported kernel panic with an openwrt specific patch.
> However, it turns out that mainline has a similar bug waiting to happen.
>
> Once NF_HOOK() returns the skb is in undefined state and must not be
> used.   Moreover, the okfn must consume the skb to support async
> processing (NF_QUEUE).
>
> Current okfn in this spot doesn't consume it and caller assumes that
> NF_HOOK return value tells us if skb was freed or not, but thats wrong.

    That's.

> It "works" because no in-tree user registers a NFPROTO_BRIDGE hook at
> LOCAL_IN that returns STOLEN or NF_QUEUE verdicts.
>
> Once we add NF_QUEUE support for nftables bridge this will break --
> NF_QUEUE holds the skb for async processing, caller will erronoulsy

    Erroneously.

> return RX_HANDLER_PASS and on reinject netfilter will access free'd skb.
>
> Fix this by pushing skb up the stack in the okfn instead.
>
> NB: It also seems dubious to use LOCAL_IN while bypassing PRE_ROUTING
> completely in this case but this is how its been forever so it seems
> preferable to not change this.
>
> Cc: Felix Fietkau <nbd@...nwrt.org>
> Cc: Zefir Kurtisi <zefir.kurtisi@...atec.com>
> Signed-off-by: Florian Westphal <fw@...len.de>
[...]

MBR, Sergei