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

From: Kalle Valo <[email-masked]>
Date: Wed, 14 Mar 2018 16:24:04 +0200

Rafał Miłecki <[email-masked]> writes:

> 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.
>
> Signed-off-by: Rafał Miłecki <[email-masked]>

[...]

> --- 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.
> +
> +	  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?

> +/**
> + * brcmf_skb_is_iapp - checks if skb is an IAPP packet
> + *
> + * @skb: skb to check
> + */
> +static bool brcmf_skb_is_iapp(struct sk_buff *skb)
> +{
> +	const u8 iapp_l2_update_packet[6] __aligned(2) = {
> +		0x00, 0x01, 0xaf, 0x81, 0x01, 0x00,
> +	};

static?

> +	unsigned char *eth_data = skb_mac_header(skb) + ETH_HLEN;
> +#if !defined(CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS)

#ifndef?

> +	const u16 *a = (const u16 *)eth_data;
> +	const u16 *b = (const u16 *)iapp_l2_update_packet;
> +#endif
> +
> +	if (skb->len - skb->mac_len != 6 ||
> +	    !is_multicast_ether_addr(eth_hdr(skb)->h_dest))
> +		return false;
> +
> +#if defined(CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS)

#ifdef?

-- 
Kalle Valo