bt_sk(sk)->parent is a raw pointer with no refcount backing it, so a
child sitting in the listening socket's accept_q can outlive it. When
the child's channel is torn down independently (e.g. hci_error_reset()
-> l2cap_conn_del() -> l2cap_sock_teardown_cb()), bt_accept_unlink()
dereferences the freed parent, corrupting its accept_q_lock and list.

Additionally, if multiple threads concurrently tear down the child
socket, bt_accept_unlink() can be called multiple times. Without
checking if the socket is still in the list, the second thread will
perform a double list_del_init(), causing list_del corruption (kernel BUG).

Fix this by:
1. Taking a reference on parent in bt_accept_enqueue() and dropping it
   only when actually unlinked in bt_accept_unlink().
2. Checking !list_empty(&bt_sk(sk)->accept_q) inside the accept_q_lock
   to prevent double unlinking.
3. Moving sock_put() outside of the spinlock to avoid sleeping in an
   atomic context.

Reported-by: syzbot+534002670dd34a114fdc@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=534002670dd34a114fdc
Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")

Signed-off-by: Nguyen Ngoc Thang <ngocthang2710.1999@gmail.com>
---
v2:
 - Add !list_empty() check inside accept_q_lock to prevent double list_del.
 - Move sock_put() outside of the spinlock to avoid sleeping in atomic context (suggested by Paavo Helde).

 net/bluetooth/af_bluetooth.c | 23 ++++++++++++++++++-----
 1 file changed, 18 insertions(+), 5 deletions(-)

diff --git a/net/bluetooth/af_bluetooth.c b/net/bluetooth/af_bluetooth.c
index 411d66f24393..7797b1052808 100644
--- a/net/bluetooth/af_bluetooth.c
+++ b/net/bluetooth/af_bluetooth.c
@@ -224,6 +224,10 @@ void bt_accept_enqueue(struct sock *parent, struct sock *sk, bool bh)
 	else
 		lock_sock_nested(sk, SINGLE_DEPTH_NESTING);
 
+	/* Hold a reference on parent so it cannot be freed while sk keeps
+	 * a raw pointer to it via bt_sk(sk)->parent.
+	 */
+	sock_hold(parent);
 	bt_sk(sk)->parent = parent;
 
 	spin_lock_bh(&par->accept_q_lock);
@@ -257,15 +261,24 @@ EXPORT_SYMBOL(bt_accept_enqueue);
 void bt_accept_unlink(struct sock *sk)
 {
 	struct sock *parent = bt_sk(sk)->parent;
+	bool unlinked = false;
 
-	BT_DBG("sk %p state %d", sk, sk->sk_state);
+	if (!parent)
+		return;
 
 	spin_lock_bh(&bt_sk(parent)->accept_q_lock);
-	list_del_init(&bt_sk(sk)->accept_q);
-	sk_acceptq_removed(parent);
+	if (!list_empty(&bt_sk(sk)->accept_q)) {
+		list_del_init(&bt_sk(sk)->accept_q);
+		sk_acceptq_removed(parent);
+		unlinked = true;
+	}
 	spin_unlock_bh(&bt_sk(parent)->accept_q_lock);
-	bt_sk(sk)->parent = NULL;
-	sock_put(sk);
+
+	if (unlinked) {
+		bt_sk(sk)->parent = NULL;
+		sock_put(parent);
+		sock_put(sk);
+	}
 }
 EXPORT_SYMBOL(bt_accept_unlink);
 
-- 
2.43.0