[PATCH RFC] wifi: mac80211: reject station association if AP is not started

["syzbot" <syzbot@kernel.org>]

If an interface is changed to AP mode but not started, its channel context
configuration (chanctx_conf) remains NULL. If a station is then added to
this interface, the kernel may automatically set the
NL80211_STA_FLAG_ASSOCIATED flag for compatibility with older userspace
applications.

When this flag is set, sta_apply_auth_flags() attempts to initialize rate
control for the station by calling rate_control_rate_init_all_links(). This
eventually leads to rate_control_rate_init(), which dereferences the NULL
chanctx_conf, triggering a WARN_ON:

WARNING: net/mac80211/rate.c:51 at rate_control_rate_init+0x5a6/0x630
...
Call Trace:
 rate_control_rate_init_all_links+0xf4/0x190 net/mac80211/rate.c:84
 sta_apply_auth_flags+0x1bc/0x430 net/mac80211/cfg.c:2152
 sta_apply_parameters+0x126d/0x1b10 net/mac80211/cfg.c:2618
 ieee80211_add_station+0x3de/0x700 net/mac80211/cfg.c:2684
 rdev_add_station+0xfc/0x290 net/wireless/rdev-ops.h:201
 nl80211_new_station+0x1b4e/0x1fd0 net/wireless/nl80211.c:9505

Fix this by rejecting the addition or modification of a station to the
associated state if the AP has not been started (chanctx_conf is NULL).
Exempt Multi-Link Operation (MLO) interfaces from this check, as they
handle chanctx_conf per-link rather than globally on the VIF, and
rate_control_rate_init() already handles them correctly.

Fixes: 55de908ab292 ("mac80211: use channel contexts")
Assisted-by: Gemini:gemini-3.1-pro-preview best-expensive syzbot
Reported-by: syzbot+9bdc0c5998ab45b05030@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=9bdc0c5998ab45b05030
Link: https://syzkaller.appspot.com/ai_job?id=86851195-2aa0-410f-9fc9-952d95ae035d
To: "Johannes Berg" <johannes@sipsolutions.net>
To: <linux-wireless@vger.kernel.org>
Cc: <linux-kernel@vger.kernel.org>

---
diff --git a/net/mac80211/cfg.c b/net/mac80211/cfg.c
index 3b58af59f..b45d7923c 100644
--- a/net/mac80211/cfg.c
+++ b/net/mac80211/cfg.c
@@ -2143,6 +2143,10 @@ static int sta_apply_auth_flags(struct ieee80211_local *local,
 	if (mask & BIT(NL80211_STA_FLAG_ASSOCIATED) &&
 	    set & BIT(NL80211_STA_FLAG_ASSOCIATED) &&
 	    !test_sta_flag(sta, WLAN_STA_ASSOC)) {
+		if (!ieee80211_vif_is_mld(&sta->sdata->vif) &&
+		    !rcu_access_pointer(sta->sdata->vif.bss_conf.chanctx_conf))
+			return -EINVAL;
+
 		/*
 		 * When peer becomes associated, init rate control as
 		 * well. Some drivers require rate control initialized


base-commit: dc59e4fea9d83f03bad6bddf3fa2e52491777482
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