CVE-2026-98736 in Linux
Summary
by MITRE • 10/18/2026
In the Linux kernel, the following vulnerability has been resolved:
Bluetooth: hci_conn: Lock parent access during enhanced SCO setup
Bluetooth: hci_conn: Lock parent access during enhanced SCO setup
hci_enhanced_setup_sync() runs on the request workqueue without the hci_dev_lock held by its caller when setup was queued. Its CVSD capability check and find_next_esco_param() dereference conn->parent while the receive workqueue can unlink and release that parent.
The following interleaving can cause a use-after-free:
hci_enhanced_setup_sync() hci_disconn_complete_evt() load conn->parent hci_dev_lock() hci_conn_del(ACL parent) unlink SCO child drop link's parent reference clear child->parent release ACL parent bt_link_release() kfree(parent) hci_dev_unlock() read parent->features[0][3]
The reference held for the queued SCO child does not keep its ACL parent alive after unlinking. Commit 42de40abe25d ("Bluetooth: hci_conn: fix the SCO setup context lifetime") protects the child stored in the queued context, but leaves these parent accesses unprotected.
With a 40 ms diagnostic delay after loading conn->parent, an instrumented kernel based on fd179f8a05be, which already contains 42de40abe25d,
BUG: KASAN: slab-use-after-free in hci_enhanced_setup_sync+0xda5/0xdf0 Read of size 1 at addr ffff888102204047 by task kworker/u17:0/93 Call Trace: hci_enhanced_setup_sync+0xda5/0xdf0 hci_cmd_sync_work+0x13c/0x290 process_one_work+0x6b4/0x10e0
Allocated by task 92: __hci_conn_add+0x304/0x1df0 hci_connect_acl+0x349/0x3e0 hci_connect_sco+0x3b/0x9a0 sco_sock_connect+0x475/0xca0
Freed by task 94: kfree+0x121/0x3c0 bt_link_release+0x79/0xa0 device_release+0xc8/0x240 kobject_put+0x14d/0x280 hci_conn_del+0x524/0xe30 hci_disconn_complete_evt+0x403/0x8c0 hci_event_packet+0x71b/0xb20 hci_rx_work+0x293/0x730
The accessed address is 71 bytes into the freed ACL parent, at its features[0][3] byte; the queued SCO child is a different object. The
diagnostic preserves the loaded parent across the delay, matching the unmodified compiled capability check, and does not change the parent references or teardown path.
Hold hci_dev_lock() across the codec switch, including every call to find_next_esco_param(), and release it on all selection errors. Keep configure_datapath_sync() outside the critical section because it waits for HCI events. Preserve parameter selection and existing return values.
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Analysis
by VulDB Data Team • 10/18/2026
The Linux kernel Bluetooth subsystem contains a concurrency vulnerability within the Enhanced Synchronous Connection-Oriented link management logic, specifically in the hci_enhanced_setup_sync function. This flaw arises from an improper handling of reference counts and locking mechanisms when managing parent-child relationships between ACL links and SCO connections. The core issue is that hci_ennaced_setup_sync executes on a request workqueue without holding the global device lock, known as hci_dev_lock, which its caller held before queuing the setup operation. During this execution window, the function performs capability checks and invokes find_next_esco_param to determine codec parameters. Both of these operations dereference the conn->parent pointer, assuming it remains valid throughout the process. However, because the lock is not held during this specific phase, another thread operating on the receive workqueue can trigger a disconnection event that unlinks and releases the parent ACL link concurrently.
This race condition leads to a classic use-after-free vulnerability where memory belonging to the freed ACL parent object is accessed by the SCO setup process. The operational sequence begins when hci_enhanced_setup_sync loads the conn->parent pointer while still holding or having just released certain locks, but before acquiring the necessary protection for subsequent operations. Simultaneously, a disconnection complete event arrives via the receive workqueue, triggering hci_disconn_complete_evt. This function acquires the device lock and proceeds to delete the ACL parent connection through hci_conn_del. The deletion process unlinks the SCO child from its parent, drops the reference count on the link's parent, clears the child->parent pointer, and ultimately releases the ACL parent object via bt_link_release, which calls kfree to deallocate the memory. If the original workqueue task proceeds after this point but before it can safely access or copy data, it will read from the now-freed memory region.
The technical impact of this vulnerability is significant as it allows for potential kernel crashes and arbitrary code execution depending on how the freed memory is reused by subsequent allocations. In practical scenarios observed during testing with diagnostic delays inserted to widen the race window, the system exhibits a KASAN slab-use-after-free error when attempting to read from address ffff888102204047, which corresponds to offset 71 within the freed ACL parent structure, specifically accessing the features[0][3] byte. This confirms that the queued SCO child object is distinct from the parent but relies on its memory being valid during parameter selection. The vulnerability persists despite previous fixes like commit 42de40abe25d which protected the child stored in the queued context; however, those protections did not extend to protecting access against concurrent teardown of the parent structure.
From a classification perspective, this flaw aligns with CWE-416 Use After Free and falls under MITRE ATT&CK technique T1203 Exploitation for Defense Evasion or potentially T1059 Command and Scripting Interpreter if leveraged in specific contexts involving kernel module loading via memory corruption. The root cause is a synchronization failure where the critical section protecting shared state was not extended to cover all accesses of that state, particularly during asynchronous workqueue processing.
To mitigate this vulnerability, developers must ensure that hci_dev_lock is held across the entire codec switch operation within hci_enhanced_setup_sync. This includes every invocation of find_next_esco_param and any other access to conn->parent or related structures derived from it. The lock should be released only on selection errors where no further parent-dependent operations are required. It is crucial to preserve parameter selection logic and existing return values while adjusting the locking scope. Additionally, configure_datapath_sync must remain outside this critical section because it waits for HCI events that could deadlock if held under the device lock. By strictly enforcing lock coverage during all dereferences of potentially transient parent pointers, the race condition preventing use-after-free scenarios is eliminated, ensuring memory safety and system stability in concurrent Bluetooth connection management operations.