[PATCH] Bluetooth: L2CAP: Fix use-after-free in l2cap_unregister_user

ssrane_b23
2025-11-05T14:23:23Z

From: Shaurya Rane <ssrane_b23@ee.vjti.ac.in>

Syzbot reported a use-after-free in l2cap_unregister_user(), caused by
missing reference counting on the associated hci_dev. If the device is
unregistered while L2CAP users are still active, l2cap_unregister_user()
may access a freed hci_dev when taking its lock.

Fix this by taking a device reference in l2cap_register_user() using
hci_dev_hold(), and releasing it in l2cap_unregister_user() via
hci_dev_put(). This ensures the hci_dev remains valid for the lifetime
of registered L2CAP users.

Reported-by: syzbot+14b6d57fb728e27ce23c@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=14b6d57fb728e27ce23c
Fixes: c8992cffbe74 ("Bluetooth: hci_event: Use of a function table to handle Command Complete")
Signed-off-by: Shaurya Rane <ssrane_b23@ee.vjti.ac.in>
---
net/bluetooth/l2cap_core.c | 12 ++++++++++++
1 file changed, 12 insertions(+)

diff --git a/net/bluetooth/l2cap_core.c b/net/bluetooth/l2cap_core.c
index 805c752ac0a9..6a880f8ab6c2 100644
--- a/net/bluetooth/l2cap_core.c
+++ b/net/bluetooth/l2cap_core.c
@@ -1688,6 +1688,11 @@ int l2cap_register_user(struct l2cap_conn *conn, struct l2cap_user *user)
struct hci_dev *hdev = conn->hcon->hdev;
int ret;

+ /* Hold a reference to hdev to prevent it from being freed while
+ * we have registered users.
+ */
+ hci_dev_hold(hdev);
+
/* We need to check whether l2cap_conn is registered. If it is not, we
* must not register the l2cap_user. l2cap_conn_del() is unregisters
* l2cap_conn objects, but doesn't provide its own locking. Instead, it
@@ -1717,6 +1722,10 @@ int l2cap_register_user(struct l2cap_conn *conn, struct l2cap_user *user)

out_unlock:
hci_dev_unlock(hdev);
+
+ if (ret)
+ hci_dev_put(hdev);
+
return ret;
}
EXPORT_SYMBOL(l2cap_register_user);
@@ -1735,6 +1744,9 @@ void l2cap_unregister_user(struct l2cap_conn *conn, struct l2cap_user *user)

out_unlock:
hci_dev_unlock(hdev);
+
+ /* Release the reference we took in l2cap_register_user */
+ hci_dev_put(hdev);
}
EXPORT_SYMBOL(l2cap_unregister_user);

--
2.34.1