mptcp: fix lockless access in subflow ULP diag

Since the introduction of the subflow ULP diag interface, the
dump callback accessed all the subflow data with lockless.

We need either to annotate all the read and write operation accordingly,
or acquire the subflow socket lock. Let's do latter, even if slower, to
avoid a diffstat havoc.

Fixes: 5147dfb50832 ("mptcp: allow dumping subflow context to userspace")
Cc: [e-mail]
Signed-off-by: Paolo Abeni <[e-mail]>
Reviewed-by: Mat Martineau <[e-mail]>
Signed-off-by: Matthieu Baerts (NGI0) <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

include/net/tcp.h

···
 	/* cleanup ulp */
 	void (*release)(struct sock *sk);
 	/* diagnostic */
-	int (*get_info)(const struct sock *sk, struct sk_buff *skb);
+	int (*get_info)(struct sock *sk, struct sk_buff *skb);
 	size_t (*get_info_size)(const struct sock *sk);
 	/* clone ulp */
 	void (*clone)(const struct request_sock *req, struct sock *newsk,

net/mptcp/diag.c

···
 #include <uapi/linux/mptcp.h>
 #include "protocol.h"
 
-static int subflow_get_info(const struct sock *sk, struct sk_buff *skb)
+static int subflow_get_info(struct sock *sk, struct sk_buff *skb)
 {
 	struct mptcp_subflow_context *sf;
 	struct nlattr *start;
 	u32 flags = 0;
+	bool slow;
 	int err;
 
 	start = nla_nest_start_noflag(skb, INET_ULP_INFO_MPTCP);
 	if (!start)
 		return -EMSGSIZE;
 
+	slow = lock_sock_fast(sk);
 	rcu_read_lock();
 	sf = rcu_dereference(inet_csk(sk)->icsk_ulp_data);
 	if (!sf) {
···
 	}
 
 	rcu_read_unlock();
+	unlock_sock_fast(sk, slow);
 	nla_nest_end(skb, start);
 	return 0;
 
 nla_failure:
 	rcu_read_unlock();
+	unlock_sock_fast(sk, slow);
 	nla_nest_cancel(skb, start);
 	return err;
 }

net/tls/tls_main.c

···
 	return 0;
 }
 
-static int tls_get_info(const struct sock *sk, struct sk_buff *skb)
+static int tls_get_info(struct sock *sk, struct sk_buff *skb)
 {
 	u16 version, cipher_type;
 	struct tls_context *ctx;