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

[syzbot]

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+6df45d...@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-b...@vger.kernel.org>
To: "Luiz Augusto von Dentz" <luiz....@gmail.com>
To: "Marcel Holtmann" <mar...@holtmann.org>
To: "Dmitry Antipov" <dman...@yandex.ru>
Cc: <linux-...@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...@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
--
This is an AI-generated patch subject to moderation.
Reply with '#syz upstream' to Sign-off the patch as a human author
and send it to the upstream kernel mailing lists.
Reply with '#syz reject' to reject it ('#syz unreject' to undo).

See https://goo.gle/syzbot-ai-patches for information about AI-generated patches.
You can comment on the patch as usual, syzbot will try to address
the comments and send a new version of the patch if necessary.
syzbot engineers can be reached at syzk...@googlegroups.com.

[Krystian Kaniewski]

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.

[syzbot]

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.

This is caused by a race condition between hci_conn_del_sysfs() and

Link: https://syzkaller.appspot.com/ai_job?id=f1c0e740-db21-40af-a9ff-84db0fd8b8bd

To: <linux-b...@vger.kernel.org>
To: "Luiz Augusto von Dentz" <luiz....@gmail.com>
To: "Marcel Holtmann" <mar...@holtmann.org>
To: "Dmitry Antipov" <dman...@yandex.ru>
Cc: <linux-...@vger.kernel.org>

---

v3:
- Removed the quoted crash trace from the commit description.

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().

https://lore.kernel.org/all/7649937b-a96e-44a1...@mail.kernel.org/T/

[Krystian Kaniewski]

#syz upstream