[Patch net] kcm: fix a race condition in kcm_recvmsg()

[Cong Wang <xiyou.wangcong@gmail.com>]

From: Cong Wang <cong.wang@bytedance.com>

sk->sk_receive_queue is protected by skb queue lock, but for KCM
sockets its RX path takes mux->rx_lock to protect more than just
skb queue, so grabbing skb queue lock is not necessary when
mux->rx_lock is already held. But kcm_recvmsg() still only grabs
the skb queue lock, so race conditions still exist.

Close this race condition by taking mux->rx_lock in kcm_recvmsg()
too. This way is much simpler than enforcing skb queue lock
everywhere.

Fixes: ab7ac4eb9832 ("kcm: Kernel Connection Multiplexor module")
Tested-by: shaozhengchao <shaozhengchao@huawei.com>
Cc: Paolo Abeni <pabeni@redhat.com>
Cc: Tom Herbert <tom@herbertland.com>
Signed-off-by: Cong Wang <cong.wang@bytedance.com>
---
 net/kcm/kcmsock.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/net/kcm/kcmsock.c b/net/kcm/kcmsock.c
index 27725464ec08..8b4e5d0ab2b6 100644
--- a/net/kcm/kcmsock.c
+++ b/net/kcm/kcmsock.c
@@ -1116,6 +1116,7 @@ static int kcm_recvmsg(struct socket *sock, struct msghdr *msg,
 {
 	struct sock *sk = sock->sk;
 	struct kcm_sock *kcm = kcm_sk(sk);
+	struct kcm_mux *mux = kcm->mux;
 	int err = 0;
 	long timeo;
 	struct strp_msg *stm;
@@ -1156,8 +1157,10 @@ static int kcm_recvmsg(struct socket *sock, struct msghdr *msg,
 msg_finished:
 			/* Finished with message */
 			msg->msg_flags |= MSG_EOR;
+			spin_lock_bh(&mux->rx_lock);
 			KCM_STATS_INCR(kcm->stats.rx_msgs);
 			skb_unlink(skb, &sk->sk_receive_queue);
+			spin_unlock_bh(&mux->rx_lock);
 			kfree_skb(skb);
 		}
 	}
-- 
2.34.1