wifi: cfg80211: bound element ID read when checking non-inheritance

wifi: cfg80211: bound element ID read when checking non-inheritance

cfg80211_is_element_inherited() reads the first data octet of the
candidate element (id = elem->data[0]) to look it up in an extension
non-inheritance list. It does so after testing elem->id, but without
verifying that the element actually has a data octet. A zero-length
extension element (WLAN_EID_EXTENSION with length 0) therefore makes it
read one octet past the end of the element.

_ieee802_11_parse_elems_full() runs this check for every element of a
frame once a non-inheritance context exists -- e.g. while parsing a
per-STA profile of a Multi-Link element in a (re)association response,
or a non-transmitted BSS profile -- so a crafted frame from an AP can
trigger a one-octet slab-out-of-bounds read during element parsing:

  BUG: KASAN: slab-out-of-bounds in cfg80211_is_element_inherited
  Read of size 1 ... in net/wireless/scan.c

Return early (treat the element as inherited) when an extension element
carries no data, mirroring the existing handling of empty ID lists.

The bug was found by fuzzing ieee802_11_parse_elems_full() under KASAN.

Fixes: f7dacfb11475 ("cfg80211: support non-inheritance element")
Signed-off-by: HE WEI (ギカク) <skyexpoc@gmail.com>
Link: https://patch.msgid.link/20260707094828.16465-1-skyexpoc@gmail.com
Signed-off-by: Johannes Berg <johannes.berg@intel.com>