[PATCH v2 RESEND] Bluetooth: L2CAP: Fix use-after-free in l2cap_sock_new_connection_cb()

[Siwei Zhang <oss@fourdim.xyz>]

l2cap_sock_new_connection_cb() accesses l2cap_pi(sk)->chan after
release_sock(parent). Once the parent lock is released, the child
socket sk can be freed by another task.

Save the channel pointer into a local variable while the parent lock
is still held to prevent this.

Fixes: 8ffb929098a5 ("Bluetooth: Remove parent socket usage from l2cap_core.c")
Cc: stable@kernel.org
Signed-off-by: Siwei Zhang <oss@fourdim.xyz>
---
 net/bluetooth/l2cap_sock.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/net/bluetooth/l2cap_sock.c b/net/bluetooth/l2cap_sock.c
index 71e8c1b45bce..1625e4fc38d6 100644
--- a/net/bluetooth/l2cap_sock.c
+++ b/net/bluetooth/l2cap_sock.c
@@ -1497,6 +1497,7 @@ static void l2cap_sock_cleanup_listen(struct sock *parent)
 static struct l2cap_chan *l2cap_sock_new_connection_cb(struct l2cap_chan *chan)
 {
 	struct sock *sk, *parent = chan->data;
+	struct l2cap_chan *child_chan;
 
 	lock_sock(parent);
 
@@ -1520,9 +1521,11 @@ static struct l2cap_chan *l2cap_sock_new_connection_cb(struct l2cap_chan *chan)
 
 	bt_accept_enqueue(parent, sk, false);
 
+	child_chan = l2cap_pi(sk)->chan;
+
 	release_sock(parent);
 
-	return l2cap_pi(sk)->chan;
+	return child_chan;
 }
 
 static int l2cap_sock_recv_cb(struct l2cap_chan *chan, struct sk_buff *skb)
-- 
2.53.0