From: Johannes Weiner <redacted>

When a socket is cloned, the associated sock_cgroup_data is duplicated
but not its reference on the cgroup. As a result, the cgroup reference
count will underflow when both sockets are destroyed later on.

Fixes: bd1060a1d671 ("sock, cgroup: add sock->sk_cgroup")
Cc: <redacted> # 4.5+
Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
---
 kernel/cgroup.c | 6 ++++++
 net/core/sock.c | 5 ++++-
 2 files changed, 10 insertions(+), 1 deletion(-)

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 0c4db7908264..b0d727d26fc7 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c

@@ -6297,6 +6297,12 @@ void cgroup_sk_alloc(struct sock_cgroup_data *skcd)
 	if (cgroup_sk_alloc_disabled)
 		return;
 
+	/* Socket clone path */
+	if (skcd->val) {
+		cgroup_get(sock_cgroup_ptr(skcd));
+		return;
+	}
+
 	rcu_read_lock();
 
 	while (true) {

diff --git a/net/core/sock.c b/net/core/sock.c
index 51a730485649..038e660ef844 100644
--- a/net/core/sock.c
+++ b/net/core/sock.c

@@ -1340,7 +1340,6 @@ static struct sock *sk_prot_alloc(struct proto *prot, gfp_t priority,
 		if (!try_module_get(prot->owner))
 			goto out_free_sec;
 		sk_tx_queue_clear(sk);
-		cgroup_sk_alloc(&sk->sk_cgrp_data);
 	}
 
 	return sk;

@@ -1400,6 +1399,7 @@ struct sock *sk_alloc(struct net *net, int family, gfp_t priority,
 		sock_net_set(sk, net);
 		atomic_set(&sk->sk_wmem_alloc, 1);
 
+		cgroup_sk_alloc(&sk->sk_cgrp_data);
 		sock_update_classid(&sk->sk_cgrp_data);
 		sock_update_netprioidx(&sk->sk_cgrp_data);
 	}

@@ -1544,6 +1544,9 @@ struct sock *sk_clone_lock(const struct sock *sk, const gfp_t priority)
 		newsk->sk_priority = 0;
 		newsk->sk_incoming_cpu = raw_smp_processor_id();
 		atomic64_set(&newsk->sk_cookie, 0);
+
+		cgroup_sk_alloc(&newsk->sk_cgrp_data);
+
 		/*
 		 * Before updating sk_refcnt, we must commit prior changes to memory
 		 * (Documentation/RCU/rculist_nulls.txt for details)

-- 
2.9.3