Re: [PATCH] Bluetooth: L2CAP: Fix UAF in l2cap_chan_timeout

[Marco Elver <elver@google.com>]

On Wed, Jun 03, 2026 at 01:31PM -0400, Luiz Augusto von Dentz wrote:
> Hi Marco,
> 
> On Wed, Jun 3, 2026 at 9:16 AM Marco Elver <elver@google.com> wrote:
> >
> > On Wed, 3 Jun 2026 at 14:31, Marco Elver <elver@google.com> wrote:
> > >
> > > l2cap_chan_timeout() accesses chan->conn without holding a reference to
> > > the connection object. If l2cap_conn_del() races and tears down the
> > > connection while the timer is waiting for locks, it can result in a
> > > use-after-free when the timer wakes up and attempts to acquire
> > > conn->lock:
> > >
> > > | BUG: KASAN: slab-use-after-free in instrument_atomic_read_write include/linux/instrumented.h:112 [inline]
> > > | BUG: KASAN: slab-use-after-free in atomic_long_try_cmpxchg_acquire include/linux/atomic/atomic-instrumented.h:4456 [inline]
> > > | BUG: KASAN: slab-use-after-free in __mutex_trylock_fast kernel/locking/mutex.c:161 [inline]
> > > | BUG: KASAN: slab-use-after-free in mutex_lock+0x4f/0xa0 kernel/locking/mutex.c:318
> > > | Write of size 8 at addr ffff8881298d9550 by task kworker/2:1/83
> > > |
> > > | CPU: 2 UID: 0 PID: 83 Comm: kworker/2:1 Not tainted 7.1.0-rc6-next-20260601-dirty #6 PREEMPT(full)
> > > | Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.17.0-debian-1.17.0-1 04/01/2014
> > > | Workqueue: events l2cap_chan_timeout
> > > | Call Trace:
> > > |  <TASK>
> > > |  instrument_atomic_read_write include/linux/instrumented.h:112 [inline]
> > > |  atomic_long_try_cmpxchg_acquire include/linux/atomic/atomic-instrumented.h:4456 [inline]
> > > |  __mutex_trylock_fast kernel/locking/mutex.c:161 [inline]
> > > |  mutex_lock+0x4f/0xa0 kernel/locking/mutex.c:318
> > > |  l2cap_chan_timeout+0x5d/0x1b0 net/bluetooth/l2cap_core.c:422
> > > |  process_one_work kernel/workqueue.c:3326 [inline]
> > > |  process_scheduled_works+0x7c8/0xfb0 kernel/workqueue.c:3409
> > > |  worker_thread+0x8a9/0xcf0 kernel/workqueue.c:3490
> > > |  kthread+0x346/0x430 kernel/kthread.c:436
> > > |  ret_from_fork+0x1a3/0x470 arch/x86/kernel/process.c:158
> > > |  ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245
> > > |  </TASK>
> > > |
> > > | Allocated by task 320:
> > > |  l2cap_conn_add+0xa7/0x820 net/bluetooth/l2cap_core.c:7075
> > > |  l2cap_connect_cfm+0xdb/0xd70 net/bluetooth/l2cap_core.c:7452
> > > |  hci_connect_cfm include/net/bluetooth/hci_core.h:2139 [inline]
> > > |  hci_remote_features_evt+0x52f/0x9f0 net/bluetooth/hci_event.c:3760
> > > |  hci_event_func net/bluetooth/hci_event.c:7796 [inline]
> > > |  hci_event_packet+0x561/0xa70 net/bluetooth/hci_event.c:7847
> > > |  hci_rx_work+0x370/0x890 net/bluetooth/hci_core.c:4040
> > > |  process_one_work kernel/workqueue.c:3326 [inline]
> > > |  process_scheduled_works+0x7c8/0xfb0 kernel/workqueue.c:3409
> > > |  worker_thread+0x8a9/0xcf0 kernel/workqueue.c:3490
> > > |  kthread+0x346/0x430 kernel/kthread.c:436
> > > |  ret_from_fork+0x1a3/0x470 arch/x86/kernel/process.c:158
> > > |  ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245
> > > |
> > > | Freed by task 322:
> > > |  hci_disconn_cfm include/net/bluetooth/hci_core.h:2154 [inline]
> > > |  hci_conn_hash_flush+0x101/0x1f0 net/bluetooth/hci_conn.c:2736
> > > |  hci_dev_close_sync+0x889/0xde0 net/bluetooth/hci_sync.c:5405
> > > |  hci_dev_do_close net/bluetooth/hci_core.c:502 [inline]
> > > |  hci_unregister_dev+0x1f7/0x370 net/bluetooth/hci_core.c:2679
> > > |  vhci_release+0x12a/0x180 drivers/bluetooth/hci_vhci.c:690
> > > |  __fput+0x369/0x890 fs/file_table.c:510
> > > |  task_work_run+0x160/0x1d0 kernel/task_work.c:233
> > > |  get_signal+0xf5b/0x1120 kernel/signal.c:2810
> > > |  arch_do_signal_or_restart+0x4d/0x600 arch/x86/kernel/signal.c:337
> > > |  __exit_to_user_mode_loop kernel/entry/common.c:64 [inline]
> > > |  exit_to_user_mode_loop+0x85/0x510 kernel/entry/common.c:98
> > > |  __exit_to_user_mode_prepare include/linux/irq-entry-common.h:207 [inline]
> > > |  syscall_exit_to_user_mode_prepare include/linux/irq-entry-common.h:230 [inline]
> > > |  syscall_exit_to_user_mode include/linux/entry-common.h:318 [inline]
> > > |  do_syscall_64+0x263/0x3d0 arch/x86/entry/syscall_64.c:100
> > > |  entry_SYSCALL_64_after_hwframe+0x77/0x7f
> > > |
> > > | Last potentially related work creation:
> > > |  hci_connect_cfm include/net/bluetooth/hci_core.h:2139 [inline]
> > > |  hci_remote_features_evt+0x52f/0x9f0 net/bluetooth/hci_event.c:3760
> > > |  hci_event_func net/bluetooth/hci_event.c:7796 [inline]
> > > |  hci_event_packet+0x561/0xa70 net/bluetooth/hci_event.c:7847
> > > |  hci_rx_work+0x370/0x890 net/bluetooth/hci_core.c:4040
> > > |  process_one_work kernel/workqueue.c:3326 [inline]
> > > |  process_scheduled_works+0x7c8/0xfb0 kernel/workqueue.c:3409
> > > |  worker_thread+0x8a9/0xcf0 kernel/workqueue.c:3490
> > > |  kthread+0x346/0x430 kernel/kthread.c:436
> > > |  ret_from_fork+0x1a3/0x470 arch/x86/kernel/process.c:158
> > > |  ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245
> > > |
> > > | The buggy address belongs to the object at ffff8881298d9400
> > > |  which belongs to the cache kmalloc-512 of size 512
> > > | The buggy address is located 336 bytes inside of
> > > |  freed 512-byte region [ffff8881298d9400, ffff8881298d9600)
> > >
> > > Fix it by holding a reference to the connection when the channel timer
> > > is scheduled, and releasing it when the timer is either canceled or
> > > executes to completion.
> > >
> > > Since l2cap_chan_del() nullifies chan->conn to disassociate the channel
> > > during teardown, the timer handler might read NULL from chan->conn even
> > > if it held a reference. To address this, introduce a `timer_conn` field
> > > to `struct l2cap_chan` to store the connection pointer associated with
> > > the active timer. The timer handler uses this field to acquire locks and
> > > release the connection reference, and skips channel closing operations
> > > if chan->conn has already been nullified by teardown.
> > >
> > > Fixes: 75780ca4c6a8 ("Bluetooth: L2CAP: use chan timer to close channels in cleanup_listen()")
> > > Cc: <stable@vger.kernel.org>
> > > Cc: Siwei Zhang <oss@fourdim.xyz>
> > > Cc: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>
> > > Assisted-by: Gemini:gemini-3.1-pro-preview
> > > Reported-by: https://sashiko.dev/#/patchset/20260521021249.3258069-1-oss%40fourdim.xyz
> > > Signed-off-by: Marco Elver <elver@google.com>
> >
> > Sigh, Sashiko points out more problems here:
> > https://sashiko.dev/#/patchset/20260603123111.2334409-1-elver%40google.com
> >
> > > Can this lockless read of chan->timer_conn cause a use-after-free or double
> > > free if another thread re-arms the timer concurrently?
> >
> > I haven't analyzed this further yet, so consider this patch a
> > bug-report-only. If anyone finds a better fix sooner, please go ahead.
> 
> I was thinking or something like the following:

I tested that and my repro didn't trigger the UAF here, but I still
think it has the same fundamental issue:

If the timer worker is preempted immediately after reading chan->conn
but before entering l2cap_conn_hold_unless_zero(), l2cap_conn_del() can
complete concurrently.

When the timer worker resumes, l2cap_conn_hold_unless_zero(conn) will
attempt to read conn->ref that has already been freed, resulting in
another UAF.