Re: [PATCH RFC v2] Bluetooth: hci_sysfs: Fix NULL pointer dereference in device_del()

[Krystian Kaniewski <krystianmkaniewski@gmail.com>]

Remove the quoted crash trace or replace it with the retained report 
trace. The retained report shows `PID: 14113 Comm: kbnepd bnep0` and 
faults in `klist_put()` from `device_del()` during `bnep_session`. The 
current quote instead shows `PID: 21636 Comm: syz-executor` and faults 
through `device_move()`, so it is not the actual report and contradicts 
the corrected subject and ordering described below it. Keep the code 
diff, corrected `device_del()` race explanation, `Fixes` tag, 
provenance, and report links unchanged.

On 9/2/2026 6:57 PM, syzbot wrote:
> A NULL pointer dereference in klist_put() occurs when a child device (such
> as a BNEP network device in bnep_session) is concurrently being
> unregistered while hci_conn_del_sysfs() reparents child devices:
>
> Oops: general protection fault, probably for non-canonical address
> 0xdffffc000000000b: 0000 [#1] SMP KASAN NOPTI
> KASAN: null-ptr-deref in range [0x0000000000000058-0x000000000000005f]
> CPU: 1 UID: 0 PID: 21636 Comm: syz-executor Not tainted syzkaller #1
> PREEMPT(full)
> Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS
> 1.16.3-debian-1.16.3-2 04/01/2014
> RIP: 0010:klist_put lib/klist.c:212 [inline]
> RIP: 0010:klist_del lib/klist.c:230 [inline]
> RIP: 0010:klist_remove+0x156/0x340 lib/klist.c:249
> ...
> Call Trace:
>   <TASK>
>   device_move+0x18e/0x720 drivers/base/core.c:4702
>   hci_conn_del_sysfs+0xb8/0x1a0 net/bluetooth/hci_sysfs.c:75
>   hci_conn_cleanup net/bluetooth/hci_conn.c:170 [inline]
>   hci_conn_del+0xc3d/0x1200 net/bluetooth/hci_conn.c:1318
>   hci_conn_hash_flush+0x189/0x260 net/bluetooth/hci_conn.c:2747
>   hci_dev_close_sync+0x7fc/0x10c0 net/bluetooth/hci_sync.c:5593
>   hci_dev_do_close net/bluetooth/hci_core.c:502 [inline]
>   hci_unregister_dev+0x232/0x5b0 net/bluetooth/hci_core.c:2681
>   vhci_release+0x16d/0x1c0 drivers/bluetooth/hci_vhci.c:700
>   ...
>   </TASK>
>
> This crash is caused by a race condition between hci_conn_del_sysfs() and
> concurrent child device unregistration (e.g. bnep_session calling
> unregister_netdev()). During device unregistration, device_del() snapshots
> a non-NULL parent pointer. Concurrently, hci_conn_del_sysfs() finds the
> child device using device_find_any_child() and calls device_move() to
> reparent it to NULL, which removes the node from its parent's klist and
> clears knode_parent. Subsequently, device_del() calls
> klist_del(&dev->p->knode_parent) using the stale parent snapshot, causing
> klist_put() to dereference knode_klist(n)->put on an already removed node,
> resulting in a NULL pointer dereference.
>
> This race was introduced by commit 27aabf27fd01 ("Bluetooth: fix
> use-after-free in device_for_each_child()"), which replaced
> device_find_child(..., __match_tty) with device_find_any_child() in
> hci_conn_del_sysfs(). That change was intended to avoid a use-after-free
> where conn->dev outlived its parent hdev->dev when child devices held
> references to conn->dev, because conn->dev only held a reference to
> hdev->dev while registered in sysfs.
>
> Fix the issue properly by taking an explicit reference to the parent device
> with get_device(&hdev->dev) in hci_conn_init_sysfs() and dropping it with
> put_device(parent) in bt_link_release() when the conn device is freed. This
> ensures that hdev->dev remains valid for the entire lifecycle of conn->dev,
> resolving the underlying use-after-free. With the parent reference held
> properly, restore the __match_tty filter in hci_conn_del_sysfs() so that
> device_move() is only invoked on persistent RFCOMM TTY devices as
> originally intended, eliminating the race condition with unregistering
> network devices.
>
> Fixes: 27aabf27fd01 ("Bluetooth: fix use-after-free in device_for_each_child()")
> Assisted-by: Gemini:gemini-3.7-flash syzbot
> Reported-by: syzbot+6df45dd3d03e1a9aca96@syzkaller.appspotmail.com
> Closes: https://syzkaller.appspot.com/bug?extid=6df45dd3d03e1a9aca96
> Link: https://syzkaller.appspot.com/ai_job?id=03299926-ecd5-4848-b5f8-a253b09af490
> To: <linux-bluetooth@vger.kernel.org>
> To: "Luiz Augusto von Dentz" <luiz.dentz@gmail.com>
> To: "Marcel Holtmann" <marcel@holtmann.org>
> To: "Dmitry Antipov" <dmantipov@yandex.ru>
> Cc: <linux-kernel@vger.kernel.org>
>
> ---
> v2:
> - Update subject line to reflect the NULL pointer dereference in device_del().
> - Replace trace with the actual crash report trace.
> - Clarify the race condition ordering between device_del() and concurrent hci_conn_del_sysfs().
>
> v1:
> https://lore.kernel.org/all/21c8fde7-0caa-4f2c-ae68-4bd3443ff0b5@mail.kernel.org/T/
> ---
> diff --git a/net/bluetooth/hci_sysfs.c b/net/bluetooth/hci_sysfs.c
> index 8957ce7c2..c2065abf7 100644
> --- a/net/bluetooth/hci_sysfs.c
> +++ b/net/bluetooth/hci_sysfs.c
> @@ -13,7 +13,10 @@ static const struct class bt_class = {
>   static void bt_link_release(struct device *dev)
>   {
>   	struct hci_conn *conn = to_hci_conn(dev);
> +	struct device *parent = dev->parent;
> +
>   	kfree(conn);
> +	put_device(parent);
>   }
>   
>   static const struct device_type bt_link = {
> @@ -21,6 +24,16 @@ static const struct device_type bt_link = {
>   	.release = bt_link_release,
>   };
>   
> +/*
> + * The rfcomm tty device will possibly retain even when conn
> + * is down, and sysfs doesn't support move zombie device,
> + * so we should move the device before conn device is destroyed.
> + */
> +static int __match_tty(struct device *dev, const void *data)
> +{
> +	return !strncmp(dev_name(dev), "rfcomm", 6);
> +}
> +
>   void hci_conn_init_sysfs(struct hci_conn *conn)
>   {
>   	struct hci_dev *hdev = conn->hdev;
> @@ -29,7 +42,7 @@ void hci_conn_init_sysfs(struct hci_conn *conn)
>   
>   	conn->dev.type = &bt_link;
>   	conn->dev.class = &bt_class;
> -	conn->dev.parent = &hdev->dev;
> +	conn->dev.parent = get_device(&hdev->dev);
>   
>   	device_initialize(&conn->dev);
>   }
> @@ -69,7 +82,7 @@ void hci_conn_del_sysfs(struct hci_conn *conn)
>   	while (1) {
>   		struct device *dev;
>   
> -		dev = device_find_any_child(&conn->dev);
> +		dev = device_find_child(&conn->dev, NULL, __match_tty);
>   		if (!dev)
>   			break;
>   		device_move(dev, NULL, DPM_ORDER_DEV_LAST);
>
>
> base-commit: cee9395acd8043be0644b25c34bfa86623f2b935