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

From: Rafał Miłecki <[email-masked]>
Date: Wed, 14 Mar 2018 16:39:04 +0100

On 2018-03-14 13:58, Arend van Spriel wrote:
> On 3/14/2018 12:01 PM, Rafał Miłecki wrote:
>> From: Rafał Miłecki <[email-masked]>
>> 
>> Testing brcmfmac with more recent firmwares resulted in AP interfaces
>> not working in some specific setups. Debugging resulted in discovering
>> support for IAPP in Broadcom's firmwares. This is an obsoleted 
>> standard
>> and its implementation is something that:
>> 1) Most people don't need / want to use
>> 2) Can allow local DoS attacks
>> 3) Breaks AP interfaces in some specific bridge setups
>> 
>> To solve issues it can cause this commit modifies brcmfmac to drop 
>> IAPP
>> packets. If affects:
>> 1) Rx path: driver won't be sending these unwanted packets up.
>> 2) Tx path: driver will reject packets that would trigger STA
>>     disassociation perfromed by a firmware (possible local DoS 
>> attack).
>> 
>> It appears there are some Broadcom's clients/users who care about this
>> feature despite the drawbacks. They can switch it on by a newly added
>> Kconfig option.
> 
> Thanks for taking this approach. Looks fine except for .... (see below)
> 
> Reviewed-by: Arend van Spriel <arend.vanspriel-dY08KVG/lbpWk0Htik3J/[email-masked]>
>> Signed-off-by: Rafał Miłecki <[email-masked]>
>> ---
>>   drivers/net/wireless/broadcom/brcm80211/Kconfig    | 20 +++++++++++
>>   .../wireless/broadcom/brcm80211/brcmfmac/core.c    | 39 
>> ++++++++++++++++++++++
>>   2 files changed, 59 insertions(+)
>> 
>> diff --git a/drivers/net/wireless/broadcom/brcm80211/Kconfig 
>> b/drivers/net/wireless/broadcom/brcm80211/Kconfig
>> index 9d99eb42d917..876787ef991a 100644
>> --- a/drivers/net/wireless/broadcom/brcm80211/Kconfig
>> +++ b/drivers/net/wireless/broadcom/brcm80211/Kconfig
>> @@ -68,6 +68,26 @@ config BRCMFMAC_PCIE
>>   	  IEEE802.11ac embedded FullMAC WLAN driver. Say Y if you want to
>>   	  use the driver for an PCIE wireless card.
>> 
>> +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.
> 
> I do not see any evidence that this would occur only for recent
> firmware. That stuff is old and not touched recently.

My evidence is comparing firmwares for 4366b1: 10.10.69.3309 (r610991)
vs. 10.10 (TOB) (r663589).

The first one is from linux-firmware.git and it doesn't implement IAPP
in the TX path. The later one is what I got from you privately and it
implements it.

Also a firmware for 4366c0: 10.10.122.20 (r683106) which is relatively
new implements IAPP in the TX path.

>> diff --git a/drivers/net/wireless/broadcom/brcm80211/brcmfmac/core.c 
>> b/drivers/net/wireless/broadcom/brcm80211/brcmfmac/core.c
>> index 19048526b4af..db6987015fb1 100644
>> --- a/drivers/net/wireless/broadcom/brcm80211/brcmfmac/core.c
>> +++ b/drivers/net/wireless/broadcom/brcm80211/brcmfmac/core.c
> 
> [...]
> 
>>   static netdev_tx_t brcmf_netdev_start_xmit(struct sk_buff *skb,
>>   					   struct net_device *ndev)
>>   {
>> @@ -250,6 +278,12 @@ static netdev_tx_t brcmf_netdev_start_xmit(struct 
>> sk_buff *skb,
>>   		goto done;
>>   	}
>> 
>> +	if (!IS_ENABLED(CONFIG_BRCMFMAC_IAPP) && brcmf_skb_is_iapp(skb)) {
>> +		dev_kfree_skb(skb);
>> +		ret = -EINVAL;
>> +		goto done;
>> +	}
> 
> This is not right. The function must return netdev_tx_t type. Here is
> kerneldoc of .start_xmit():
> 
>  * netdev_tx_t (*ndo_start_xmit)(struct sk_buff *skb,
>  *                               struct net_device *dev);
>  *	Called when a packet needs to be transmitted.
>  *	Returns NETDEV_TX_OK.  Can return NETDEV_TX_BUSY, but you should 
> stop
>  *	the queue before that can happen; it's for obsolete devices and 
> weird
>  *	corner cases, but the stack really does a non-trivial amount
>  *	of useless work if you return NETDEV_TX_BUSY.
>  *	Required; cannot be NULL.

Please take a closer look at how brcmf_netdev_start_xmit() works. Above
  code *will* return netdev_tx_t.

> You may want to increase dropped netstat or add driver internal
> statistic counter so there is visibility of IAPP packets being
> dropped.

OK, I'll try to find a stat to increase.