brcmfmac: avoid potential stack overflow in brcmf_cfg80211_start_ap()

commit ded89912 upstream.

User-space can choose to omit NL80211_ATTR_SSID and only provide raw
IE TLV data. When doing so it can provide SSID IE with length exceeding
the allowed size. The driver further processes this IE copying it
into a local variable without checking the length. Hence stack can be
corrupted and used as exploit.
Reported-by: Daxing Guo <freener.gdx@gmail.com>
Reviewed-by: Hante Meuleman <hante.meuleman@broadcom.com>
Reviewed-by: Pieter-Paul Giesberts <pieter-paul.giesberts@broadcom.com>
Reviewed-by: Franky Lin <franky.lin@broadcom.com>
Signed-off-by: Arend van Spriel <arend.vanspriel@broadcom.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
[bwh: Backported to 3.16: adjust filename]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

drivers/net/wireless/brcm80211/brcmfmac/wl_cfg80211.c

@@ -3822,7 +3822,7 @@ brcmf_cfg80211_start_ap(struct wiphy *wiphy, struct net_device *ndev,
 				(u8 *)&settings->beacon.head[ie_offset],
 				settings->beacon.head_len - ie_offset,
 				WLAN_EID_SSID);
-		if (!ssid_ie)
+		if (!ssid_ie || ssid_ie->len > IEEE80211_MAX_SSID_LEN)
 			return -EINVAL;
 
 		memcpy(ssid_le.SSID, ssid_ie->data, ssid_ie->len);