netdev - Re: [PATCH] brcmfmac: drop Inter-Access Point Protocol packets by default

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Message-ID: <d9bd640972faf36138ec6579083a0276@milecki.pl>
Date:   Wed, 14 Mar 2018 16:28:30 +0100
From:   Rafał Miłecki <rafal@...ecki.pl>
To:     Kalle Valo <kvalo@...eaurora.org>
Cc:     Arend van Spriel <arend.vanspriel@...adcom.com>,
        Rafał Miłecki 
        <zajec5@...il.com>, Franky Lin <franky.lin@...adcom.com>,
        Hante Meuleman <hante.meuleman@...adcom.com>,
        Chi-Hsien Lin <chi-hsien.lin@...ress.com>,
        Wright Feng <wright.feng@...ress.com>,
        Pieter-Paul Giesberts <pieter-paul.giesberts@...adcom.com>,
        James Hughes <james.hughes@...pberrypi.org>,
        linux-wireless@...r.kernel.org,
        brcm80211-dev-list.pdl@...adcom.com,
        brcm80211-dev-list@...ress.com, netdev@...r.kernel.org,
        Linus Lüssing <linus.luessing@...3.blue>,
        Felix Fietkau <nbd@....name>, bridge@...ts.linux-foundation.org
Subject: Re: [PATCH] brcmfmac: drop Inter-Access Point Protocol packets by
 default

On 2018-03-14 16:08, Kalle Valo wrote:
> Arend van Spriel <arend.vanspriel@...adcom.com> writes:
> 
>> On 3/14/2018 3:24 PM, Kalle Valo wrote:
>>>> +config BRCMFMAC_IAPP
>>>> >+	bool "Partial support for obsoleted Inter-Access Point Protocol"
>>>> >+	depends on BRCMFMAC
>>>> >+	---help---
>>>> >+	  Most of Broadcom's firmwares can send 802.11f ADD frame every
>>>> >+	  time new STA connects to the AP interface. Some recent ones
>>>> >+	  can also disassociate STA when they receive such a frame.
>>>> >+
>>>> >+	  It's important to understand this behavior can lead to a local
>>>> >+	  DoS security issue. Attacker may trigger disassociation of any
>>>> >+	  STA by sending a proper Ethernet frame to the wireless
>>>> >+	  interface.
>>>> >+
>>>> >+	  Moreover this feature may break AP interfaces in some specific
>>>> >+	  setups. This applies e.g. to the bridge with hairpin mode
>>>> >+	  enabled and IFLA_BRPORT_MCAST_TO_UCAST set. IAPP packet
>>>> >+	  generated by a firmware will get passed back to the wireless
>>>> >+	  interface and cause immediate disassociation of just-connected
>>>> >+	  STA.
>>> Sorry for jumping late, but does it really make sense to have a 
>>> Kconfig
>>> option for this? I don't think we should add a Kconfig option for 
>>> every
>>> strange feature, there should be stronger reasons (size savings etc)
>>> before adding a Kconfig option.
>>> 
>>> And in this case the size savings can't be much. Wouldn't a module
>>> parameter be simpler for a functionality change like this?
>> 
>> Hi Kalle,
>> 
>> Good to be wary about Kconfig option.
> 
> I think Linus doesn't like pointless Kconfig options, me neither for
> that matter, so I try to make sure the justifications are really there
> before adding anything new.
> 
>> So my reason for asking a Kconfig option is that this is directly in
>> the datapaths (tx and rx) so I prefer to disable/enable it compile
>> time rather then runtime.
> 
> I'm no cpu profile expert but is really one (or two?) if checks of a
> cached variable in the datapath really measurable? My guess is that 
> it's
> just noise in the results.
> 
> But I'm not going to argue about it, if you think it's still needed I'm
> fine with that. Just mention in the commit log the justification the 
> new
> Kconfig option.

I think you should be right and that's also why I put
skb->len - skb->mac_len != 6
as the first check in that function. That simple (quick?) check should
reject 99.9% of packets.

I could move skb_mac_header() call a bit further which should optimize
this function even more and maybe then we could switch to the module
parameter?