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

["Rafał Miłecki" <zajec5@gmail.com>]

From: Rafał Miłecki <rafal@milecki.pl>

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 <rafal@milecki.pl>
---
 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.
+
+	  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.
+
 config BRCM_TRACING
 	bool "Broadcom device tracing"
 	depends on BRCMSMAC || BRCMFMAC
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
@@ -230,6 +230,34 @@ 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)
+{
+	const u8 iapp_l2_update_packet[6] __aligned(2) = {
+		0x00, 0x01, 0xaf, 0x81, 0x01, 0x00,
+	};
+	unsigned char *eth_data = skb_mac_header(skb) + ETH_HLEN;
+#if !defined(CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS)
+	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)
+	return !(((*(const u32 *)eth_data) ^ (*(const u32 *)iapp_l2_update_packet)) |
+		 ((*(const u16 *)(eth_data + 4)) ^ (*(const u16 *)(iapp_l2_update_packet + 4))));
+#else
+	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 +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;
+	}
+
 	/* 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 +359,11 @@ void brcmf_txflowblock_if(struct brcmf_if *ifp,
 
 void brcmf_netif_rx(struct brcmf_if *ifp, struct sk_buff *skb)
 {
+	if (!IS_ENABLED(CONFIG_BRCMFMAC_IAPP) && brcmf_skb_is_iapp(skb)) {
+		brcmu_pkt_buf_free_skb(skb);
+		return;
+	}
+
 	if (skb->pkt_type == PACKET_MULTICAST)
 		ifp->ndev->stats.multicast++;
 
-- 
2.11.0