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

From: Zefir Kurtisi <[email-masked]>
Date: Mon, 14 Mar 2016 16:28:27 +0100

On 03/12/2016 11:14 AM, 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.
> 
> 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
> 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 <[email-masked]>
> Cc: Zefir Kurtisi <[email-masked]>
> Signed-off-by: Florian Westphal <[email-masked]>
> ---
>  
Looks good: applying the same fix-pattern to OpenWRT private patches solved the
oops previously observed.

Thanks for the quick resolution.

Tested-by: Zefir Kurtisi <[email-masked]>