llc: avoid blocking in llc_sap_close()

BugLink: https://bugs.launchpad.net/bugs/1853881

[ Upstream commit 9708d2b5 ]

llc_sap_close() is called by llc_sap_put() which
could be called in BH context in llc_rcv(). We can't
block in BH.

There is no reason to block it here, kfree_rcu() should
be sufficient.
Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Connor Kuehl <connor.kuehl@canonical.com>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>

include/net/llc.h

@@ -66,6 +66,7 @@ struct llc_sap {
 	int sk_count;
 	struct hlist_nulls_head sk_laddr_hash[LLC_SK_LADDR_HASH_ENTRIES];
 	struct hlist_head sk_dev_hash[LLC_SK_DEV_HASH_ENTRIES];
+	struct rcu_head rcu;
 };
 
 static inline

net/llc/llc_core.c

@@ -127,9 +127,7 @@ void llc_sap_close(struct llc_sap *sap)
 	list_del_rcu(&sap->node);
 	spin_unlock_bh(&llc_sap_list_lock);
 
-	synchronize_rcu();
-
-	kfree(sap);
+	kfree_rcu(sap, rcu);
 }
 
 static struct packet_type llc_packet_type __read_mostly = {