CVE-2026-98739 in Linux
Summary
by MITRE • 10/18/2026
In the Linux kernel, the following vulnerability has been resolved:
Bluetooth: RFCOMM: Fix initial port reference race
For RFCOMM_RELEASE_ONHUP devices, rfcomm_tty_install() and __rfcomm_release_dev() both use RFCOMM_TTY_OWNED to decide who drops the initial tty_port reference. The release path tests the bit separately from the install path setting it, so both can drop that reference.
The synchronous hangup does not prevent this race: port->tty is not assigned until tty_port_open(), after installation. The following interleaving is possible:
1. Install and release each obtain a reference with rfcomm_dev_get(). 2. Release observes RFCOMM_TTY_OWNED clear. 3. Install sets RFCOMM_TTY_OWNED and drops the initial reference. 4. Release drops the same initial reference again. 5. TTY cleanup drops its reference and frees the device. 6. Release performs its final tty_port_put() on the freed port.
KASAN reported:
BUG: KASAN: slab-use-after-free in tty_port_put+0x22/0x190 Write of size 4 at addr ffff8881001d3d5c by task poc/92
Call Trace: tty_port_put+0x22/0x190 rfcomm_dev_ioctl+0x1d4/0x1930 sock_do_ioctl+0x110/0x260 sock_ioctl+0x380/0x590 __x64_sys_ioctl+0x134/0x1c0
Allocated by task 88: rfcomm_dev_ioctl+0x8f6/0x1930 sock_do_ioctl+0x110/0x260 sock_ioctl+0x380/0x590 __x64_sys_ioctl+0x134/0x1c0
Freed by task 65: kfree+0x131/0x3c0 rfcomm_dev_destruct+0x23b/0x2f0 release_one_tty+0xc1/0x370 process_one_work+0x661/0x1090
Use test_and_set_bit() in both paths so that only the caller that changes RFCOMM_TTY_OWNED from clear to set drops the initial reference. Each path keeps its own lookup reference until release or TTY cleanup, preserving the existing callback ordering and error handling.
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Analysis
by VulDB Data Team • 10/18/2026
The Linux kernel contains a race condition vulnerability within the Bluetooth RFCOMM subsystem that results in a use-after-free flaw during device initialization and teardown sequences. This issue specifically affects devices configured with the RFCOMM_RELEASE_ONHUP flag, where the management of tty_port references is handled by two distinct code paths: rfcomm_tty_install() for installation and __rfcomm_release_dev() for release operations. Both functions rely on a shared state bit known as RFCOMM_TTY_OWNED to determine which party should decrement the initial reference count held by the tty_port structure. The fundamental technical flaw lies in the lack of atomicity when accessing this flag; the release path checks whether it owns the port separately from the install path setting that ownership, creating a window where both paths may incorrectly conclude they are responsible for dropping the same reference.
The operational impact of this vulnerability is severe, leading to kernel memory corruption and potential system instability or denial of service through use-after-free errors. The race condition manifests because synchronous hangup mechanisms do not prevent concurrent execution during the critical initialization phase. Specifically, the port->tty structure is not assigned until tty_port_open() completes after installation, leaving a gap in synchronization logic. A typical exploitation sequence involves an interleaving where both install and release operations acquire references via rfcomm_dev_get(). The release path observes that RFCOMM_TTY_OWNED is clear and proceeds to drop its reference. Subsequently, the install path sets RFCOMM_TTY_OWNED and drops its own copy of the initial reference. This results in a double-free scenario where the same memory address for the tty_port structure is freed twice. The kernel's KASAN tool has confirmed this behavior by reporting slab-use-after-free errors within tty_port_put, triggered through ioctl calls that invoke rfcomm_dev_ioctl.
From a security classification perspective, this vulnerability aligns with CWE-362: Concurrent Execution using Shared Resource with Improper Synchronization, as the race condition stems from unsynchronized access to shared state variables controlling resource lifecycle management. It also relates to CWE-416: Use After Free, given that the double decrement of reference counts leads to premature memory deallocation while pointers may still be in use or accessed by subsequent operations. In terms of MITRE ATT&CK mapping, this flaw falls under Tactic Execution and Defense Evasion, specifically relating to techniques involving kernel exploitation for privilege escalation or system compromise if an attacker can trigger the race condition remotely via Bluetooth interfaces. The vulnerability highlights risks associated with complex reference counting schemes in network protocol stacks where timing dependencies are not strictly enforced through atomic operations.
To mitigate this vulnerability, developers must ensure that all access to shared state bits governing resource ownership is performed atomically. In this specific case, the fix involves replacing standard bit testing and setting sequences with test_and_set_bit() calls in both the install and release paths. This ensures that only the caller who successfully transitions RFCOMM_TTY_OWNED from clear to set will drop the initial reference, thereby eliminating the possibility of double-freeing the tty_port structure. Each path retains its own lookup reference until either the release process or TTY cleanup occurs, which preserves existing callback ordering and error handling logic while enforcing strict mutual exclusion on the critical section managing the port's lifecycle. System administrators should apply kernel updates that include this patch to prevent potential exploitation of this race condition in environments where Bluetooth RFCOMM services are enabled.