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

["Rafał Miłecki" <rafal-g1n6cQUeyibVItvQsEIGlw@public.gmane.org>]

On 2018-03-14 16:39, Rafał Miłecki wrote:
> On 2018-03-14 13:58, Arend van Spriel wrote:
>> On 3/14/2018 12:01 PM, Rafał Miłecki wrote:
>>> From: Rafał Miłecki <rafal-g1n6cQUeyibVItvQsEIGlw@public.gmane.org>
>>> 
>>> 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/w@public.gmane.org>
>>> Signed-off-by: Rafał Miłecki <rafal-g1n6cQUeyibVItvQsEIGlw@public.gmane.org>
>>> ---
>>>   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.

So after checking brcmf_netdev_start_xmit() again, I realized I actually
*do* that. Doing:
ret = -EINVAL;
goto done;
results in increasing tx_dropped.