[PATCH wireless 2/2] wifi: mac80211: fix fils_discovery double free on alloc failure

[Xiang Mei]

ieee80211_set_fils_discovery() calls kfree_rcu() on the old template
before allocating the replacement. If the kzalloc() then fails, it
returns -ENOMEM while link->u.ap.fils_discovery still points at the
object already queued for freeing. A later update or AP teardown
(ieee80211_stop_ap()) re-queues that same rcu_head; the second free is
caught by KASAN when the RCU sheaf is processed in softirq:

  BUG: KASAN: double-free in rcu_free_sheaf (mm/slub.c:5850)
  Free of addr ffff88800c065280 by task swapper/0/0
   ...
   __rcu_free_sheaf_prepare (mm/slub.c:2634 mm/slub.c:2940)
   rcu_free_sheaf (mm/slub.c:5850)
   rcu_core (kernel/rcu/tree.c:2617 kernel/rcu/tree.c:2869)
   handle_softirqs (kernel/softirq.c:622)
  The buggy address belongs to the cache kmalloc-96 of size 96

Queue the old object for kfree_rcu() only after the new one is published,
matching ieee80211_set_probe_resp() and ieee80211_set_s1g_short_beacon().

Fixes: 3b1c256eb4ae ("wifi: mac80211: fixes in FILS discovery updates")
Reported-by: Weiming Shi <bestswngs@xxxxxxxxx>
Assisted-by: Claude:claude-opus-4-8
Signed-off-by: Xiang Mei <xmei5@xxxxxxx>
---
 net/mac80211/cfg.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/net/mac80211/cfg.c b/net/mac80211/cfg.c
index e926bced6..0bc10136b 100644
--- a/net/mac80211/cfg.c
+++ b/net/mac80211/cfg.c
@@ -1145,9 +1145,6 @@ static int ieee80211_set_fils_discovery(struct ieee80211_sub_if_data *sdata,
 	fd->max_interval = params->max_interval;
 
 	old = sdata_dereference(link->u.ap.fils_discovery, sdata);
-	if (old)
-		kfree_rcu(old, rcu_head);
-
 	if (params->tmpl && params->tmpl_len) {
 		new = kzalloc(sizeof(*new) + params->tmpl_len, GFP_KERNEL);
 		if (!new)
@@ -1159,6 +1156,9 @@ static int ieee80211_set_fils_discovery(struct ieee80211_sub_if_data *sdata,
 		RCU_INIT_POINTER(link->u.ap.fils_discovery, NULL);
 	}
 
+	if (old)
+		kfree_rcu(old, rcu_head);
+
 	*changed |= BSS_CHANGED_FILS_DISCOVERY;
 	return 0;
 }
-- 
2.43.0