CVE-2025-39860 in Linux
Summary
by MITRE • 09/19/2025
In the Linux kernel, the following vulnerability has been resolved:
Bluetooth: Fix use-after-free in l2cap_sock_cleanup_listen()
syzbot reported the splat below without a repro.
In the splat, a single thread calling bt_accept_dequeue() freed sk and touched it after that.
The root cause would be the racy l2cap_sock_cleanup_listen() call added by the cited commit.
bt_accept_dequeue() is called under lock_sock() except for l2cap_sock_release().
Two threads could see the same socket during the list iteration in bt_accept_dequeue():
CPU1 CPU2 (close()) ---- ---- sock_hold(sk) sock_hold(sk); lock_sock(sk) <-- block close() sock_put(sk) bt_accept_unlink(sk) sock_put(sk) <-- refcnt by bt_accept_enqueue() release_sock(sk) lock_sock(sk) sock_put(sk) bt_accept_unlink(sk) sock_put(sk) <-- last refcnt bt_accept_unlink(sk) <-- UAF
Depending on the timing, the other thread could show up in the "Freed by task" part.
Let's call l2cap_sock_cleanup_listen() under lock_sock() in l2cap_sock_release().
[0]:
BUG: KASAN: slab-use-after-free in debug_spin_lock_before kernel/locking/spinlock_debug.c:86 [inline]
BUG: KASAN: slab-use-after-free in do_raw_spin_lock+0x26f/0x2b0 kernel/locking/spinlock_debug.c:115 Read of size 4 at addr ffff88803b7eb1c4 by task syz.5.3276/16995 CPU: 3 UID: 0 PID: 16995 Comm: syz.5.3276 Not tainted syzkaller #0 PREEMPT(full) Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.3-debian-1.16.3-2~bpo12+1 04/01/2014 Call Trace: <TASK> __dump_stack lib/dump_stack.c:94 [inline]
dump_stack_lvl+0x116/0x1f0 lib/dump_stack.c:120 print_address_description mm/kasan/report.c:378 [inline]
print_report+0xcd/0x630 mm/kasan/report.c:482 kasan_report+0xe0/0x110 mm/kasan/report.c:595 debug_spin_lock_before kernel/locking/spinlock_debug.c:86 [inline]
do_raw_spin_lock+0x26f/0x2b0 kernel/locking/spinlock_debug.c:115 spin_lock_bh include/linux/spinlock.h:356 [inline]
release_sock+0x21/0x220 net/core/sock.c:3746 bt_accept_dequeue+0x505/0x600 net/bluetooth/af_bluetooth.c:312 l2cap_sock_cleanup_listen+0x5c/0x2a0 net/bluetooth/l2cap_sock.c:1451 l2cap_sock_release+0x5c/0x210 net/bluetooth/l2cap_sock.c:1425 __sock_release+0xb3/0x270 net/socket.c:649 sock_close+0x1c/0x30 net/socket.c:1439 __fput+0x3ff/0xb70 fs/file_table.c:468 task_work_run+0x14d/0x240 kernel/task_work.c:227 resume_user_mode_work include/linux/resume_user_mode.h:50 [inline]
exit_to_user_mode_loop+0xeb/0x110 kernel/entry/common.c:43 exit_to_user_mode_prepare include/linux/irq-entry-common.h:225 [inline]
syscall_exit_to_user_mode_work include/linux/entry-common.h:175 [inline]
syscall_exit_to_user_mode include/linux/entry-common.h:210 [inline]
do_syscall_64+0x3f6/0x4c0 arch/x86/entry/syscall_64.c:100 entry_SYSCALL_64_after_hwframe+0x77/0x7f RIP: 0033:0x7f2accf8ebe9 Code: ff ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 40 00 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 a8 ff ff ff f7 d8 64 89 01 48 RSP: 002b:00007ffdb6cb1378 EFLAGS: 00000246 ORIG_RAX: 00000000000001b4 RAX: 0000000000000000 RBX: 00000000000426fb RCX: 00007f2accf8ebe9 RDX: 0000000000000000 RSI: 000000000000001e RDI: 0000000000000003 RBP: 00007f2acd1b7da0 R08: 0000000000000001 R09: 00000012b6cb166f R10: 0000001b30e20000 R11: 0000000000000246 R12: 00007f2acd1b609c R13: 00007f2acd1b6090 R14: ffffffffffffffff R15: 00007ffdb6cb1490 </TASK>
Allocated by task 5326: kasan_save_stack+0x33/0x60 mm/kasan/common.c:47 kasan_save_track+0x14/0x30 mm/kasan/common.c:68 poison_kmalloc_redzone mm/kasan/common.c:388 [inline]
__kasan_kmalloc+0xaa/0xb0 mm/kasan/common.c:405 kasan_kmalloc include/linux/kasan.h:260 [inline]
__do_kmalloc_node mm/slub.c:4365 [inline]
__kmalloc_nopro ---truncated---
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Analysis
by VulDB Data Team • 01/20/2026
The vulnerability described in CVE-2025-39860 resides within the Linux kernel's Bluetooth subsystem, specifically in the L2CAP (Logical Link Control and Adaptation Protocol) socket implementation. This issue manifests as a use-after-free condition in the `l2cap_sock_cleanup_listen()` function, which is triggered during the cleanup of listening Bluetooth sockets. The flaw arises from a race condition that occurs when multiple threads attempt to manipulate the same socket object concurrently, leading to memory corruption and potential exploitation. The vulnerability was initially identified by syzbot, an automated fuzzer, which reported a stack trace without a reproducible test case, underscoring the complexity and subtlety of the race condition.
The root cause of this vulnerability lies in the improper synchronization during socket cleanup operations. The `bt_accept_dequeue()` function, which is responsible for removing sockets from an accept queue, is called under the protection of `lock_sock()`, except in the case of `l2cap_sock_release()`. This inconsistency creates a window where two threads can operate on the same socket object simultaneously. When one thread calls `bt_accept_dequeue()` and subsequently frees the socket, another thread might still be accessing it, resulting in a use-after-free scenario. The race condition is particularly evident during concurrent socket close operations, where one thread holds a reference to the socket while another thread is in the process of cleaning it up. The timing of these operations determines whether the freed socket is accessed, leading to memory corruption.
The operational impact of this vulnerability is significant, as it can lead to system instability, denial of service, or potentially arbitrary code execution if exploited. The use-after-free condition allows an attacker to corrupt kernel memory, which could be leveraged to escalate privileges or cause a system crash. The vulnerability affects the core Bluetooth functionality of the Linux kernel, particularly when handling concurrent socket operations in a multi-threaded environment. The risk is amplified in systems that rely heavily on Bluetooth communication, such as mobile devices, IoT systems, or embedded platforms. Given that the vulnerability is rooted in kernel-level code, successful exploitation could compromise the integrity and confidentiality of the entire system.
Mitigation efforts should focus on ensuring proper locking mechanisms are enforced during socket cleanup operations. The fix involves calling `l2cap_sock_cleanup_listen()` under `lock_sock()` in `l2cap_sock_release()`, which prevents concurrent access to the socket during cleanup. This aligns with the principle of maintaining atomicity in critical sections and adheres to best practices for concurrent programming in kernel space. Additionally, this vulnerability maps to CWE-416, which describes the use of freed memory, and can be categorized under ATT&CK technique T1068, which involves exploiting local privileges. Users should apply the latest kernel patches and ensure their systems are updated to address this vulnerability. Organizations should also consider implementing monitoring and detection mechanisms to identify potential exploitation attempts in their environments.
The vulnerability demonstrates a classic race condition in kernel networking code and highlights the importance of thorough testing and code review in concurrent environments. The use of KASAN (Kernel Address Sanitizer) in the debugging output confirms the nature of the issue, showing that the memory access occurred after the socket was freed. This type of flaw is particularly challenging to detect and fix, as it depends on the precise timing of thread execution and memory management. The fix implemented in the kernel ensures that cleanup operations are serialized, eliminating the race condition and preventing the use-after-free scenario from occurring. This aligns with industry standards and best practices for kernel security, emphasizing the need for careful synchronization in multi-threaded kernel code to prevent memory corruption vulnerabilities.