[PATCH v2 1/2 5.10.y, 5.15.y] Bluetooth: L2CAP: Fix UAF in channel timeout by holding conn ref

[PATCH v2 1/2 5.10.y, 5.15.y] Bluetooth: L2CAP: Fix UAF in channel timeout by holding conn ref

[Todd Kjos]

From: Marco Elver <elver@google.com>

[ Upstream commit b66774b48dd98f07254951f74ea6f513efe7ff8b ]

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
Change-Id: Ie9862976d25c3e6d386689cd198ebe5ab4cc5509
Signed-off-by: Marco Elver <elver@google.com>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Todd Kjos <tkjos@google.com>
---
v2:
- Reinstated NULL check on "conn"
- Added followon patch to the series

 include/net/bluetooth/l2cap.h |  1 +
 net/bluetooth/l2cap_core.c    | 36 +++++++++++++++++++++--------------
 2 files changed, 23 insertions(+), 14 deletions(-)

diff --git a/include/net/bluetooth/l2cap.h b/include/net/bluetooth/l2cap.h
index 6e48bd56f4c3b78e459d184140a9a3648391755a..7a8322a40554361b2e5361e7a83c624fa844f3fd 100644
--- a/include/net/bluetooth/l2cap.h
+++ b/include/net/bluetooth/l2cap.h
@@ -776,6 +776,7 @@ 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 ed2388e31c6c7fe9236dfa23376bf977541bf9fc..12ac9726381a0b927fb9df380b8015c9a28b0a84 100644
--- a/net/bluetooth/l2cap_core.c
+++ b/net/bluetooth/l2cap_core.c
@@ -435,7 +435,7 @@ static void l2cap_chan_timeout(struct work_struct *work)
 
 	BT_DBG("chan %p state %s", chan, state_to_string(chan->state));
 
-	if (!conn) {
+	if (!conn || test_bit(FLAG_DEL, &chan->flags)) {
 		l2cap_chan_put(chan);
 		return;
 	}
@@ -446,6 +446,9 @@ 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 &&
@@ -458,10 +461,11 @@ 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->chan_lock);
+	l2cap_chan_put(chan);
 }
 
 struct l2cap_chan *l2cap_chan_create(void)
@@ -511,6 +515,9 @@ 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);
 }
 
@@ -593,7 +600,7 @@ 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:
@@ -648,31 +655,28 @@ 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) {
-		struct amp_mgr *mgr = conn->hcon->amp_mgr;
+	if (!test_and_set_bit(FLAG_DEL, &chan->flags)) {
+		struct amp_mgr *mgr = chan->conn->hcon->amp_mgr;
+
 		/* 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 (mgr && mgr->bredr_chan == chan)
 			mgr->bredr_chan = NULL;
@@ -2036,7 +2040,7 @@ 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;
@@ -2057,7 +2061,7 @@ 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;
@@ -2682,7 +2686,7 @@ 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 */
@@ -3272,12 +3276,16 @@ 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);
 }
-- 
2.55.0.679.g6767b8d81c-goog

[PATCH v2 2/2 5.10.y, 5.15.y] Bluetooth: 6lowpan: Fix using chan->conn as indication to no remote netdev

[Todd Kjos]

From: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>

b66774b48dd9 ("Bluetooth: L2CAP: Fix UAF in channel timeout by holding
conn ref") don't reset the chan->conn to NULL anymore making the bt#
netdev not be remove once the last l2cap_chan_del is removed.

Instead of restoring the original behavior this remove the logic of
keeping the interface after the last channel is removed because it
never worked as intended and the l2cap_chan_del always detach its
l2cap_conn which results in always removing the channel anyway.

Fixes: b66774b48dd9 ("Bluetooth: L2CAP: Fix UAF in channel timeout by holding conn ref")
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>
---
v2:
- added this patch to the series which fixes 1/1

 net/bluetooth/6lowpan.c | 18 +++---------------
 1 file changed, 3 insertions(+), 15 deletions(-)

diff --git a/net/bluetooth/6lowpan.c b/net/bluetooth/6lowpan.c
index 9424d153f323fdfb1453d18ef7821ccddaeaa5eb..d0b1882f5b4b77d274580a0e8af7a3d43972615d 100644
--- a/net/bluetooth/6lowpan.c
+++ b/net/bluetooth/6lowpan.c
@@ -775,20 +775,10 @@ static void chan_close_cb(struct l2cap_chan *chan)
 	struct lowpan_btle_dev *dev = NULL;
 	struct lowpan_peer *peer;
 	int err = -ENOENT;
-	bool last = false, remove = true;
+	bool last = false;
 
 	BT_DBG("chan %p conn %p", chan, chan->conn);
 
-	if (chan->conn && chan->conn->hcon) {
-		if (!is_bt_6lowpan(chan->conn->hcon))
-			return;
-
-		/* If conn is set, then the netdev is also there and we should
-		 * not remove it.
-		 */
-		remove = false;
-	}
-
 	spin_lock(&devices_lock);
 
 	list_for_each_entry_rcu(entry, &bt_6lowpan_devices, list) {
@@ -815,10 +805,8 @@ static void chan_close_cb(struct l2cap_chan *chan)
 
 		ifdown(dev->netdev);
 
-		if (remove) {
-			INIT_WORK(&entry->delete_netdev, delete_netdev);
-			schedule_work(&entry->delete_netdev);
-		}
+		INIT_WORK(&entry->delete_netdev, delete_netdev);
+		schedule_work(&entry->delete_netdev);
 	} else {
 		spin_unlock(&devices_lock);
 	}
-- 
2.55.0.679.g6767b8d81c-goog

Re: [PATCH v2 1/2 5.10.y, 5.15.y] Bluetooth: L2CAP: Fix UAF in channel timeout by holding conn ref

[Sasha Levin]

Hey Todd,

I think you tackled only one of the conditionals. The "reinstated NULL check on
conn" in the changelog landed in l2cap_chan_timeout(); l2cap_chan_del() is
identical to v1 and still starts the block with an unguarded deref:

> +	if (!test_and_set_bit(FLAG_DEL, &chan->flags)) {
> +		struct amp_mgr *mgr = chan->conn->hcon->amp_mgr;

5.15 and 5.10 both still ship net/bluetooth/a2mp.c, and a2mp_chan_open() calls
l2cap_chan_del(chan, 0) on the l2cap_ertm_init() failure path before the
channel is added, so chan->conn is NULL there. Pre-patch the if (conn) guard
made that a no-op; as written the backport turns it into a NULL deref. It
needs:

	if (chan->conn && !test_and_set_bit(FLAG_DEL, &chan->flags)) {

-- 
Thanks,
Sasha