brcmfmac: drop Inter-Access Point Protocol packets by default

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.

Older firmwares were only generating 802.11f frames. Newer ones like:
1) 10.10 (TOB) (r663589)
2) 10.10.122.20 (r683106)
for 4366b1 and 4366c0 respectively seem to also /respect/ 802.11f frames
in the Tx path by performing a STA disassociation.

This 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 using a new module
param.

This change results in only two more comparisons (check for module param
and check for Ethernet packet length) for 99.9% of packets. Its overhead
should be very minimal.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Acked-by: Arend van Spriel <arend.vanspriel@broadcom.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>

drivers/net/wireless/broadcom/brcm80211/brcmfmac/common.c

@@ -75,6 +75,10 @@ static int brcmf_roamoff;
 module_param_named(roamoff, brcmf_roamoff, int, S_IRUSR);
 MODULE_PARM_DESC(roamoff, "Do not use internal roaming engine");
 
+static int brcmf_iapp_enable;
+module_param_named(iapp, brcmf_iapp_enable, int, 0);
+MODULE_PARM_DESC(iapp, "Enable partial support for the obsoleted Inter-Access Point Protocol");
+
 #ifdef DEBUG
 /* always succeed brcmf_bus_started() */
 static int brcmf_ignore_probe_fail;
@@ -441,6 +445,7 @@ struct brcmf_mp_device *brcmf_get_module_param(struct device *dev,
 	settings->feature_disable = brcmf_feature_disable;
 	settings->fcmode = brcmf_fcmode;
 	settings->roamoff = !!brcmf_roamoff;
+	settings->iapp = !!brcmf_iapp_enable;
 #ifdef DEBUG
 	settings->ignore_probe_fail = !!brcmf_ignore_probe_fail;
 #endif

drivers/net/wireless/broadcom/brcm80211/brcmfmac/common.h

@@ -58,6 +58,7 @@ struct brcmf_mp_device {
 	unsigned int	feature_disable;
 	int		fcmode;
 	bool		roamoff;
+	bool		iapp;
 	bool		ignore_probe_fail;
 	struct brcmfmac_pd_cc *country_codes;
 	union {

drivers/net/wireless/broadcom/brcm80211/brcmfmac/core.c

@@ -230,6 +230,37 @@ static void brcmf_netdev_set_multicast_list(struct net_device *ndev)
 	schedule_work(&ifp->multicast_work);
 }
 
+/**
+ * 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)
+{
+	static const u8 iapp_l2_update_packet[6] __aligned(2) = {
+		0x00, 0x01, 0xaf, 0x81, 0x01, 0x00,
+	};
+	unsigned char *eth_data;
+#if !defined(CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS)
+	const u16 *a, *b;
+#endif
+
+	if (skb->len - skb->mac_len != 6 ||
+	    !is_multicast_ether_addr(eth_hdr(skb)->h_dest))
+		return false;
+
+	eth_data = skb_mac_header(skb) + ETH_HLEN;
+#if defined(CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS)
+	return !(((*(const u32 *)eth_data) ^ (*(const u32 *)iapp_l2_update_packet)) |
+		 ((*(const u16 *)(eth_data + 4)) ^ (*(const u16 *)(iapp_l2_update_packet + 4))));
+#else
+	a = (const u16 *)eth_data;
+	b = (const u16 *)iapp_l2_update_packet;
+
+	return !((a[0] ^ b[0]) | (a[1] ^ b[1]) | (a[2] ^ b[2]));
+#endif
+}
+
 static netdev_tx_t brcmf_netdev_start_xmit(struct sk_buff *skb,
 					   struct net_device *ndev)
 {
@@ -250,6 +281,23 @@ static netdev_tx_t brcmf_netdev_start_xmit(struct sk_buff *skb,
 		goto done;
 	}
 
+	/* Some recent Broadcom's firmwares disassociate STA when they receive
+	 * an 802.11f ADD frame. 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 a just-connected STA.
+	 */
+	if (!drvr->settings->iapp && brcmf_skb_is_iapp(skb)) {
+		dev_kfree_skb(skb);
+		ret = -EINVAL;
+		goto done;
+	}
+
 	/* Make sure there's enough writeable headroom */
 	if (skb_headroom(skb) < drvr->hdrlen || skb_header_cloned(skb)) {
 		head_delta = max_t(int, drvr->hdrlen - skb_headroom(skb), 0);
@@ -325,6 +373,15 @@ void brcmf_txflowblock_if(struct brcmf_if *ifp,
 
 void brcmf_netif_rx(struct brcmf_if *ifp, struct sk_buff *skb)
 {
+	/* Most of Broadcom's firmwares send 802.11f ADD frame every time a new
+	 * STA connects to the AP interface. This is an obsoleted standard most
+	 * users don't use, so don't pass these frames up unless requested.
+	 */
+	if (!ifp->drvr->settings->iapp && brcmf_skb_is_iapp(skb)) {
+		brcmu_pkt_buf_free_skb(skb);
+		return;
+	}
+
 	if (skb->pkt_type == PACKET_MULTICAST)
 		ifp->ndev->stats.multicast++;