mptcp: fix sk_forward_memory corruption on retransmission

MPTCP sk_forward_memory handling is a bit special, as such field
is protected by the msk socket spin_lock, instead of the plain
socket lock.

Currently we have a code path updating such field without handling
the relevant lock:

__mptcp_retrans() -> __mptcp_clean_una_wakeup()

Several helpers in __mptcp_clean_una_wakeup() will update
sk_forward_alloc, possibly causing such field corruption, as reported
by Matthieu.

Address the issue providing and using a new variant of blamed function
which explicitly acquires the msk spin lock.

Fixes: 64b9cea ("mptcp: fix spurious retransmissions")
Closes: multipath-tcp/mptcp_net-next#172
Reported-by: Matthieu Baerts <[email protected]>
Tested-by: Matthieu Baerts <[email protected]>
Signed-off-by: Paolo Abeni <[email protected]>
Signed-off-by: Mat Martineau <[email protected]>

net/mptcp/protocol.c

@@ -947,6 +947,10 @@ static void __mptcp_update_wmem(struct sock *sk)
 {
 	struct mptcp_sock *msk = mptcp_sk(sk);
 
+#ifdef CONFIG_LOCKDEP
+	WARN_ON_ONCE(!lockdep_is_held(&sk->sk_lock.slock));
+#endif
+
 	if (!msk->wmem_reserved)
 		return;
 
@@ -1085,10 +1089,20 @@ static void __mptcp_clean_una(struct sock *sk)
 
 static void __mptcp_clean_una_wakeup(struct sock *sk)
 {
+#ifdef CONFIG_LOCKDEP
+	WARN_ON_ONCE(!lockdep_is_held(&sk->sk_lock.slock));
+#endif
 	__mptcp_clean_una(sk);
 	mptcp_write_space(sk);
 }
 
+static void mptcp_clean_una_wakeup(struct sock *sk)
+{
+	mptcp_data_lock(sk);
+	__mptcp_clean_una_wakeup(sk);
+	mptcp_data_unlock(sk);
+}
+
 static void mptcp_enter_memory_pressure(struct sock *sk)
 {
 	struct mptcp_subflow_context *subflow;
@@ -2299,7 +2313,7 @@ static void __mptcp_retrans(struct sock *sk)
 	struct sock *ssk;
 	int ret;
 
-	__mptcp_clean_una_wakeup(sk);
+	mptcp_clean_una_wakeup(sk);
 	dfrag = mptcp_rtx_head(sk);
 	if (!dfrag) {
 		if (mptcp_data_fin_enabled(msk)) {