[PATCH] mac80211: mesh: fix RCU warning

[Thomas Pedersen <thomas@eero.com>]

ifmsh->csa was being dereferenced without the RCU read
lock held.

fixes the following warning:

[   74.930435] =============================
[   74.932066] WARNING: suspicious RCU usage
[   74.933671] 4.20.13 #5 Tainted: G        W
[   74.935804] -----------------------------
[   74.937427] net/mac80211/mesh.c:1218 suspicious rcu_dereference_check() usage!
[   74.940473] other info that might help us debug this:
[   74.943654] rcu_scheduler_active = 2, debug_locks = 1
[   74.946311] 5 locks held by kworker/u8:3/107:
[   74.948087]  #0: 000000007623c1f0 ((wq_completion)"%s"wiphy_name(local->hw.wiphy)){+.+.}, at: process_one_work+0x1a2/0x610
[   74.952464]  #1: 00000000077b4215 ((work_completion)(&sdata->csa_finalize_work)){+.+.}, at: process_one_work+0x1a2/0x610
[   74.957228]  #2: 00000000e02b12da (&wdev->mtx){+.+.}, at: ieee80211_csa_finalize_work+0x2f/0x90
[   74.959870]  #3: 00000000e6855095 (&local->mtx){+.+.}, at: ieee80211_csa_finalize_work+0x47/0x90
[   74.962937]  #4: 00000000bb5e3bca (&local->chanctx_mtx){+.+.}, at: ieee80211_csa_finalize_work+0x51/0x90

Signed-off-by: Thomas Pedersen <thomas@eero.com>
---
 net/mac80211/mesh.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/net/mac80211/mesh.c b/net/mac80211/mesh.c
index 766e5e5..70aeb34 100644
--- a/net/mac80211/mesh.c
+++ b/net/mac80211/mesh.c
@@ -1220,10 +1220,12 @@ int ieee80211_mesh_finish_csa(struct ieee80211_sub_if_data *sdata)
 	ifmsh->chsw_ttl = 0;
 
 	/* Remove the CSA and MCSP elements from the beacon */
+	rcu_read_lock();
 	tmp_csa_settings = rcu_dereference(ifmsh->csa);
 	RCU_INIT_POINTER(ifmsh->csa, NULL);
 	if (tmp_csa_settings)
 		kfree_rcu(tmp_csa_settings, rcu_head);
+	rcu_read_unlock();
 	ret = ieee80211_mesh_rebuild_beacon(sdata);
 	if (ret)
 		return -EINVAL;
-- 
2.7.4