mac80211: fix racy usage of chanctx->refcount

Channel context refcount is protected by
chanctx_mtx. Accessing the value without holding
the mutex is racy. RCU section didn't guarantee
anything here.

Theoretically ieee80211_channel_switch() could
fail to see refcount change and read "1" instead
of, e.g. "2". This means mac80211 could accept CSA
even though it shouldn't have.

Signed-off-by: Michal Kazior <michal.kazior@tieto.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

1 files changed, 9 insertions, 8 deletions

diff --git a/net/mac80211/cfg.c b/net/mac80211/cfg.c
index aa39381ca46d..9620d4fba0d1 100644
--- a/net/mac80211/cfg.c
+++ b/net/mac80211/cfg.c
@@ -3225,7 +3225,7 @@ int ieee80211_channel_switch(struct wiphy *wiphy, struct net_device *dev,
 {
         struct ieee80211_sub_if_data *sdata = IEEE80211_DEV_TO_SUB_IF(dev);
         struct ieee80211_local *local = sdata->local;
-        struct ieee80211_chanctx_conf *chanctx_conf;
+        struct ieee80211_chanctx_conf *conf;
         struct ieee80211_chanctx *chanctx;
         int err, num_chanctx, changed = 0;
 
@@ -3241,23 +3241,24 @@ int ieee80211_channel_switch(struct wiphy *wiphy, struct net_device *dev,
                                        &sdata->vif.bss_conf.chandef))
                 return -EINVAL;
 
-        rcu_read_lock();
-        chanctx_conf = rcu_dereference(sdata->vif.chanctx_conf);
-        if (!chanctx_conf) {
-                rcu_read_unlock();
+        mutex_lock(&local->chanctx_mtx);
+        conf = rcu_dereference_protected(sdata->vif.chanctx_conf,
+                                         lockdep_is_held(&local->chanctx_mtx));
+        if (!conf) {
+                mutex_unlock(&local->chanctx_mtx);
                 return -EBUSY;
         }
 
         /* don't handle for multi-VIF cases */
-        chanctx = container_of(chanctx_conf, struct ieee80211_chanctx, conf);
+        chanctx = container_of(conf, struct ieee80211_chanctx, conf);
         if (chanctx->refcount > 1) {
-                rcu_read_unlock();
+                mutex_unlock(&local->chanctx_mtx);
                 return -EBUSY;
         }
         num_chanctx = 0;
         list_for_each_entry_rcu(chanctx, &local->chanctx_list, list)
                 num_chanctx++;
-        rcu_read_unlock();
+        mutex_unlock(&local->chanctx_mtx);
 
         if (num_chanctx > 1)
                 return -EBUSY;