wifi: nl80211: reject oversized EMA RNR lists

wifi: nl80211: reject oversized EMA RNR lists

nl80211_parse_rnr_elems() stores the parsed element count in a
u8-backed cfg80211_rnr_elems::cnt field and uses that count to size
the flexible array allocation.

Reject nested NL80211_ATTR_EMA_RNR_ELEMS input once the count reaches
255, before incrementing it again. This keeps the parser aligned with
the data structure it fills and matches the existing bound check used
by nl80211_parse_mbssid_elems().

Fixes: dbbb27e183b1 ("cfg80211: support RNR for EMA AP")
Cc: stable@kernel.org
Reported-by: Yuan Tan <yuantan098@gmail.com>
Reported-by: Zhengchuan Liang <zcliangcn@gmail.com>
Reported-by: Xin Liu <bird@lzu.edu.cn>
Assisted-by: Codex:gpt-5.4
Signed-off-by: Yuqi Xu <xuyuqiabc@gmail.com>
Signed-off-by: Ren Wei <n05ec@lzu.edu.cn>
Link: https://patch.msgid.link/20260529152542.1412734-1-n05ec@lzu.edu.cn
Signed-off-by: Johannes Berg <johannes.berg@intel.com>