Bluetooth: L2CAP: Fix UAF in channel timeout by holding conn ref

l2cap_chan_timeout() runs asynchronously and accesses chan->conn. If
the connection is torn down while the timer is running or pending,
chan->conn can be freed, leading to a use-after-free when the timer
worker attempts to lock 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
|  do_syscall_64+0x263/0x3d0 arch/x86/entry/syscall_64.c:100
|  entry_SYSCALL_64_after_hwframe+0x77/0x7f
|
| 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 having chan->conn hold a reference to l2cap_conn (via
l2cap_conn_get) when the channel is added to the connection, and
releasing it in the channel destructor. This ensures the l2cap_conn
remains alive as long as the channel exists.

A new FLAG_DEL channel flag is introduced to indicate that the channel
has been deleted from its connection. l2cap_chan_del() atomically sets
this flag using test_and_set_bit() instead of setting chan->conn to
NULL. All asynchronous workers (l2cap_chan_timeout, l2cap_ack_timeout,
l2cap_monitor_timeout, l2cap_retrans_timeout) and l2cap_chan_send()
check FLAG_DEL to determine whether the channel has been torn down,
rather than testing chan->conn for NULL.

Fixes: 8c8e620467a7 ("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>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>

2 files changed, 21 insertions, 14 deletions

diff --git a/include/net/bluetooth/l2cap.h b/include/net/bluetooth/l2cap.h
index 790935950a0c..1640cc9bf83a 100644
--- a/include/net/bluetooth/l2cap.h
+++ b/include/net/bluetooth/l2cap.h
@@ -745,6 +745,7 @@ enum {
 	FLAG_ECRED_CONN_REQ_SENT,
 	FLAG_PENDING_SECURITY,
 	FLAG_HOLD_HCI_CONN,
+	FLAG_DEL,
 };
 
 /* Lock nesting levels for L2CAP channels. We need these because lockdep
diff --git a/net/bluetooth/l2cap_core.c b/net/bluetooth/l2cap_core.c
index 863fc4b8a55e..a97d492473e2 100644
--- a/net/bluetooth/l2cap_core.c
+++ b/net/bluetooth/l2cap_core.c
@@ -408,7 +408,7 @@ static void l2cap_chan_timeout(struct work_struct *work)
 
 	BT_DBG("chan %p state %s", chan, state_to_string(chan->state));
 
-	if (!conn) {
+	if (test_bit(FLAG_DEL, &chan->flags)) {
 		l2cap_chan_put(chan);
 		return;
 	}
@@ -419,6 +419,9 @@ static void l2cap_chan_timeout(struct work_struct *work)
 	 */
 	l2cap_chan_lock(chan);
 
+	if (test_bit(FLAG_DEL, &chan->flags))
+		goto unlock;
+
 	if (chan->state == BT_CONNECTED || chan->state == BT_CONFIG)
 		reason = ECONNREFUSED;
 	else if (chan->state == BT_CONNECT &&
@@ -431,10 +434,10 @@ static void l2cap_chan_timeout(struct work_struct *work)
 
 	chan->ops->close(chan);
 
+unlock:
 	l2cap_chan_unlock(chan);
-	l2cap_chan_put(chan);
-
 	mutex_unlock(&conn->lock);
+	l2cap_chan_put(chan);
 }
 
 struct l2cap_chan *l2cap_chan_create(void)
@@ -487,6 +490,9 @@ static void l2cap_chan_destroy(struct kref *kref)
 	list_del(&chan->global_l);
 	write_unlock(&chan_list_lock);
 
+	if (chan->conn)
+		l2cap_conn_put(chan->conn);
+
 	kfree(chan);
 }
 
@@ -590,7 +596,7 @@ void __l2cap_chan_add(struct l2cap_conn *conn, struct l2cap_chan *chan)
 
 	conn->disc_reason = HCI_ERROR_REMOTE_USER_TERM;
 
-	chan->conn = conn;
+	chan->conn = l2cap_conn_get(conn);
 
 	switch (chan->chan_type) {
 	case L2CAP_CHAN_CONN_ORIENTED:
@@ -645,30 +651,26 @@ void l2cap_chan_add(struct l2cap_conn *conn, struct l2cap_chan *chan)
 
 void l2cap_chan_del(struct l2cap_chan *chan, int err)
 {
-	struct l2cap_conn *conn = chan->conn;
-
 	__clear_chan_timer(chan);
 
-	BT_DBG("chan %p, conn %p, err %d, state %s", chan, conn, err,
+	BT_DBG("chan %p, err %d, state %s", chan, err,
 	       state_to_string(chan->state));
 
 	chan->ops->teardown(chan, err);
 
-	if (conn) {
+	if (!test_and_set_bit(FLAG_DEL, &chan->flags)) {
 		/* Delete from channel list */
 		list_del(&chan->list);
 
 		l2cap_chan_put(chan);
 
-		chan->conn = NULL;
-
 		/* Reference was only held for non-fixed channels or
 		 * fixed channels that explicitly requested it using the
 		 * FLAG_HOLD_HCI_CONN flag.
 		 */
 		if (chan->chan_type != L2CAP_CHAN_FIXED ||
 		    test_bit(FLAG_HOLD_HCI_CONN, &chan->flags))
-			hci_conn_drop(conn->hcon);
+			hci_conn_drop(chan->conn->hcon);
 	}
 
 	if (test_bit(CONF_NOT_COMPLETE, &chan->conf_state))
@@ -1900,7 +1902,7 @@ static void l2cap_monitor_timeout(struct work_struct *work)
 
 	l2cap_chan_lock(chan);
 
-	if (!chan->conn) {
+	if (test_bit(FLAG_DEL, &chan->flags)) {
 		l2cap_chan_unlock(chan);
 		l2cap_chan_put(chan);
 		return;
@@ -1921,7 +1923,7 @@ static void l2cap_retrans_timeout(struct work_struct *work)
 
 	l2cap_chan_lock(chan);
 
-	if (!chan->conn) {
+	if (test_bit(FLAG_DEL, &chan->flags)) {
 		l2cap_chan_unlock(chan);
 		l2cap_chan_put(chan);
 		return;
@@ -2562,7 +2564,7 @@ int l2cap_chan_send(struct l2cap_chan *chan, struct msghdr *msg, size_t len,
 	int err;
 	struct sk_buff_head seg_queue;
 
-	if (!chan->conn)
+	if (test_bit(FLAG_DEL, &chan->flags))
 		return -ENOTCONN;
 
 	/* Connectionless channel */
@@ -3157,12 +3159,16 @@ static void l2cap_ack_timeout(struct work_struct *work)
 
 	l2cap_chan_lock(chan);
 
+	if (test_bit(FLAG_DEL, &chan->flags))
+		goto unlock;
+
 	frames_to_ack = __seq_offset(chan, chan->buffer_seq,
 				     chan->last_acked_seq);
 
 	if (frames_to_ack)
 		l2cap_send_rr_or_rnr(chan, 0);
 
+unlock:
 	l2cap_chan_unlock(chan);
 	l2cap_chan_put(chan);
 }