mptcp: pm: use _rcu variant under rcu_read_lock

In mptcp_pm_create_subflow_or_signal_addr(), rcu_read_(un)lock() are
used as expected to iterate over the list of local addresses, but
list_for_each_entry() was used instead of list_for_each_entry_rcu() in
__lookup_addr() (and lookup_id_by_addr() before). It is important to use
this variant which adds the required READ_ONCE() (and diagnostic checks
if enabled).

Because __lookup_addr() is also used in mptcp_pm_nl_set_flags() where it
is called under the pernet->lock, the _rcu variant cannot be used in all
cases. A new helper is then created.

Note that this new helper can be reused later to reduce some duplicated
code elsewhere in this file, and some sections could be used lockless,
also using this new helper then. But all of these extra modifications
should probably be better considered as new improvements, and not as
fixes.

Fixes: 86e39e0 ("mptcp: keep track of local endpoint still available for each msk")
Signed-off-by: Matthieu Baerts (NGI0) <[email protected]>

net/mptcp/pm_netlink.c

@@ -531,6 +531,19 @@ __lookup_addr(struct pm_nl_pernet *pernet, const struct mptcp_addr_info *info)
 	return NULL;
 }
 
+static struct mptcp_pm_addr_entry *
+__lookup_addr_rcu(struct pm_nl_pernet *pernet,
+		  const struct mptcp_addr_info *info)
+{
+	struct mptcp_pm_addr_entry *entry;
+
+	list_for_each_entry_rcu(entry, &pernet->local_addr_list, list) {
+		if (mptcp_addresses_equal(&entry->addr, info, entry->addr.port))
+			return entry;
+	}
+	return NULL;
+}
+
 static void mptcp_pm_create_subflow_or_signal_addr(struct mptcp_sock *msk)
 {
 	struct sock *sk = (struct sock *)msk;
@@ -556,7 +569,7 @@ static void mptcp_pm_create_subflow_or_signal_addr(struct mptcp_sock *msk)
 
 		mptcp_local_address((struct sock_common *)msk->first, &mpc_addr);
 		rcu_read_lock();
-		entry = __lookup_addr(pernet, &mpc_addr);
+		entry = __lookup_addr_rcu(pernet, &mpc_addr);
 		if (entry) {
 			__clear_bit(entry->addr.id, msk->pm.id_avail_bitmap);
 			msk->mpc_endpoint_id = entry->addr.id;