CVE-2026-98734 in Linux
Summary
by MITRE • 10/18/2026
In the Linux kernel, the following vulnerability has been resolved:
Bluetooth: hci_core: Serialize SCO and ISO scheduling with teardown
hci_low_sent() selects a connection under RCU but drops the read lock before calculating its quota and returning it to the scheduler. The connection is then used without lifetime protection by hci_sched_iso() and hci_sched_sco().
After the TX worker drops the RCU read lock, hci_abort_conn_sync() on hdev->req_workqueue can remove the connection from the hash, complete synchronize_rcu(), purge its queues and release it. The TX worker on hdev->workqueue can then access the freed connection in hci_quote_sent() or while dequeuing packets and updating conn->sent.
KASAN reported:
BUG: KASAN: slab-use-after-free in hci_low_sent+0x730/0x840 Workqueue: hci0 hci_tx_work Call Trace: hci_low_sent+0x730/0x840 hci_sched_iso+0x25e/0x4d0 hci_tx_work+0x239/0xcb0
Allocated by task 93: __hci_conn_add+0x16f/0x1b40 hci_bind_bis+0x782/0x17b0 hci_connect_bis+0xa0/0x510 iso_sock_connect+0x589/0x1050
Freed by task 88: kfree+0x131/0x3c0 device_release+0xc8/0x240 kobject_put+0x14d/0x280 hci_conn_del+0x55a/0xe80 hci_disconnect_sync+0x156/0x180 hci_abort_conn_sync+0x3e7/0x940 hci_cmd_sync_work+0x13c/0x290
Hold hci_dev_lock() across connection selection and transmission in both SCO and ISO scheduling, serializing them with connection teardown. This also prevents queuing completion timestamps after the connection queues have been purged. Extending RCU across transmission would be unsafe because the transmit path can sleep. Keep the ISO timeout check outside the mutex since hci_link_tx_to() takes it itself.
The preceding channel fix already holds this mutex in the ACL and LE schedulers, which call the SCO scheduler between packets. Move the SCO body to __hci_sched_sco(), assert that its caller holds the mutex, and use it directly from these locked paths. Keep a locking hci_sched_sco() wrapper for the direct calls from hci_tx_work(). This avoids recursively acquiring the device mutex while preserving the scheduling order.
Remove the obsolete claim that connection removal disables TX.
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Analysis
by VulDB Data Team • 10/19/2026
The Linux kernel Bluetooth subsystem contained a critical use-after-free vulnerability within the HCI core, specifically affecting the handling of SCO and ISO connections during teardown operations. The root cause lies in improper synchronization between connection selection for transmission scheduling and the concurrent process of tearing down those same connections. In the function hci_low_sent(), which determines how many packets can be sent on a given link, the code selects a Bluetooth connection while holding an RCU read lock to ensure the structure remains valid during lookup. However, this read lock is dropped before the scheduler calculates the transmission quota and returns control to the main scheduling logic. This gap in protection creates a race condition where the selected connection object can be freed by another thread between its selection and its actual use for packet dequeuing or state updates.
The operational impact of this flaw manifests when hci_abort_conn_sync() executes on the device request workqueue, which is responsible for handling disconnection requests. This function removes the target connection from the internal hash table, completes any pending RCU grace periods to ensure all prior readers have finished accessing the object, purges its associated transmission queues, and ultimately frees the memory allocated for the connection structure via kfree(). If the TX worker thread on hdev->workqueue subsequently attempts to access this now-freed connection in functions like hci_low_sent() or while updating conn->sent fields, it results in a slab-use-after-free error. Such errors can lead to kernel panics, data corruption, or potentially exploitable memory safety violations depending on the state of the freed heap region and how an attacker might manipulate allocation patterns.
To resolve this issue, the fix implements strict serialization by holding hci_dev_lock() across both connection selection and transmission scheduling for SCO and ISO protocols. This mutex ensures that teardown operations cannot proceed until all active scheduling decisions have been completed, effectively eliminating the race window where a connection could be freed while still being referenced. The solution carefully avoids extending RCU usage into the transmit path because those paths may sleep, which is incompatible with RCU read-side critical sections. Instead, the locking mechanism relies on standard mutexes to provide mutual exclusion without violating sleeping-in-atomic-context constraints. Additionally, ISO timeout checks are kept outside the mutex since hci_link_tx_to() acquires it internally, preventing deadlocks while maintaining correct timing logic.
The implementation also refactors the SCO scheduling functions to align with existing patterns used in ACL and LE schedulers. The core logic is moved into a new internal function __hci_sched_sco(), which asserts that the caller already holds the necessary mutex. A wrapper hci_sched_sco() is retained for direct calls from hci_tx_work() to prevent recursive locking issues while preserving the correct scheduling order. This structural change ensures consistency across different Bluetooth link types and removes obsolete assumptions about connection removal disabling transmission automatically. By enforcing proper lock ordering, the patch prevents queuing completion timestamps after queues have been purged, thereby maintaining data integrity during high-load or rapid disconnection scenarios.
From a vulnerability classification perspective, this issue aligns with CWE-416: Use After Free, as it involves accessing memory that has already been released to the system allocator. The attack vector typically requires local access to trigger specific race conditions between connection setup and teardown processes, potentially falling under ATT&CK techniques related to exploitation of software flaws for denial-of-service or code execution if heap grooming is feasible. Mitigation strategies include applying the kernel patch immediately in affected distributions. For environments where immediate patching is not possible, limiting exposure by disabling unnecessary Bluetooth services or restricting network access can reduce the attack surface. Regular auditing of concurrent resource management patterns and enforcing strict lock ordering policies during development are essential preventive measures against similar concurrency bugs in complex subsystems like Linux networking stacks.