[PATCH v2 net-next 7/7] mptcp: Use __sk_dst_get() and dst_dev_rcu() in mptcp_active_enable().

[Kuniyuki Iwashima <kuniyu@google.com>]

mptcp_active_enable() is called from subflow_finish_connect(),
which is icsk->icsk_af_ops->sk_rx_dst_set() and it's not always
under RCU.

Using sk_dst_get(sk)->dev could trigger UAF.

Let's use __sk_dst_get() and dst_dev_rcu().

Fixes: 27069e7cb3d1 ("mptcp: disable active MPTCP in case of blackhole")
Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>
---
Cc: Matthieu Baerts <matttbe@kernel.org>
Cc: Mat Martineau <martineau@kernel.org>
Cc: Geliang Tang <geliang@kernel.org>
---
 net/mptcp/ctrl.c | 11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

diff --git a/net/mptcp/ctrl.c b/net/mptcp/ctrl.c
index c0e516872b4b..e8ffa62ec183 100644
--- a/net/mptcp/ctrl.c
+++ b/net/mptcp/ctrl.c
@@ -501,12 +501,15 @@ void mptcp_active_enable(struct sock *sk)
 	struct mptcp_pernet *pernet = mptcp_get_pernet(sock_net(sk));
 
 	if (atomic_read(&pernet->active_disable_times)) {
-		struct dst_entry *dst = sk_dst_get(sk);
+		struct net_device *dev;
+		struct dst_entry *dst;
 
-		if (dst && dst->dev && (dst->dev->flags & IFF_LOOPBACK))
+		rcu_read_lock();
+		dst = __sk_dst_get(sk);
+		dev = dst ? dst_dev_rcu(dst) : NULL;
+		if (dev && (dev->flags & IFF_LOOPBACK))
 			atomic_set(&pernet->active_disable_times, 0);
-
-		dst_release(dst);
+		rcu_read_unlock();
 	}
 }
 
-- 
2.51.0.384.g4c02a37b29-goog