Use-after-free in Linux kernel Bluetooth L2CAP l2cap_unregister_user
CVE-2026-23461 is a Linux kernel Bluetooth L2CAP vulnerability caused by inconsistent locking around the l2cap_conn structure. After commit ab4eedb790ca ("Bluetooth: L2CAP: Fix corrupted list in hci_chan_del"), l2cap_conn_del() began using conn->lock to protect access to conn->users. However, l2cap_register_user() and l2cap_unregister_user() continued to use different locking semantics and did not take conn->lock. This created a race condition in which l2cap_register_user() or l2cap_unregister_user() could access conn->users and conn->hchan concurrently with l2cap_conn_del(). The resulting concurrency bug can trigger use-after-free and list corruption in the Bluetooth L2CAP subsystem. The issue was reported by syzbot. The fix changes l2cap_register_user() and l2cap_unregister_user() to use conn->lock instead of hci_dev_lock(), restoring consistent synchronization for l2cap_conn state.
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