[PATCH net] mptcp: Ensure RCU read lock is held when calling mptcp_sched_find()

[Breno Leitao]

The mptcp_sched_find() function must be called with the RCU read lock
held, as it accesses RCU-protected data structures. This requirement was
not properly enforced in the mptcp_init_sock() function, leading to a
RCU list traversal in a non-reader section error when
CONFIG_PROVE_RCU_LIST is enabled.

	net/mptcp/sched.c:44 RCU-list traversed in non-reader section!!

Fix it by acquiring the RCU read lock before calling the
mptcp_sched_find() function. This ensures that the function is invoked
with the necessary RCU protection in place, as it accesses RCU-protected
data structures.

Additionally, the patch breaks down the mptcp_init_sched() call into
smaller parts, with the RCU read lock only covering the specific call to
mptcp_sched_find(). This helps minimize the critical section, reducing
the time during which RCU grace periods are blocked.

The mptcp_sched_list_lock is not held in this case, and it is not clear
if it is necessary.

Signed-off-by: Breno Leitao <leitao@xxxxxxxxxx>
Fixes: 1730b2b2c5a5 ("mptcp: add sched in mptcp_sock")
Cc: stable@xxxxxxxxxxxxxxx
---
 net/mptcp/protocol.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/net/mptcp/protocol.c b/net/mptcp/protocol.c
index 6d0e201c3eb2..8ece630f80d4 100644
--- a/net/mptcp/protocol.c
+++ b/net/mptcp/protocol.c
@@ -2854,6 +2854,7 @@ static void mptcp_ca_reset(struct sock *sk)
 static int mptcp_init_sock(struct sock *sk)
 {
 	struct net *net = sock_net(sk);
+	struct mptcp_sched_ops *sched;
 	int ret;
 
 	__mptcp_init_sock(sk);
@@ -2864,8 +2865,10 @@ static int mptcp_init_sock(struct sock *sk)
 	if (unlikely(!net->mib.mptcp_statistics) && !mptcp_mib_alloc(net))
 		return -ENOMEM;
 
-	ret = mptcp_init_sched(mptcp_sk(sk),
-			       mptcp_sched_find(mptcp_get_scheduler(net)));
+	rcu_read_lock();
+	sched = mptcp_sched_find(mptcp_get_scheduler(net));
+	rcu_read_unlock();
+	ret = mptcp_init_sched(mptcp_sk(sk), sched);
 	if (ret)
 		return ret;
 
-- 
2.43.5