Forwarded: Re: [RESEND] Bluetooth: L2CAP: Fix use-after-free in l2cap_unregister_user

[syzbot]

For archival purposes, forwarding an incoming command email to
linux-kernel@xxxxxxxxxxxxxxx, syzkaller-bugs@xxxxxxxxxxxxxxxx.

***

Subject: Re: [RESEND] Bluetooth: L2CAP: Fix use-after-free in l2cap_unregister_user
Author: pav@xxxxxx

pe, 2026-03-06 kello 16:04 -0500, Luiz Augusto von Dentz kirjoitti:
> From: Shaurya Rane <ssrane_b23@xxxxxxxxxxxxx>
> 
> After commit ab4eedb790ca ("Bluetooth: L2CAP: Fix corrupted list in
> hci_chan_del"), l2cap_conn_del() uses conn->lock to protect access to
> conn->users and conn->hchan. However, l2cap_register_user() and
> l2cap_unregister_user() still use hci_dev_lock(), creating a race
> condition where these functions can access conn->users and conn->hchan
> concurrently with l2cap_conn_del().

AFAIK the above text from the original submitter is a bit inaccurate,
as l2cap_conn_del() is called with hdev lock held, so conn->users/hchan
should be safe.

However, using conn->mutex should fix the use-after-free in

	conn->hcon->hdev
	hci_dev_lock(hdev);
	hci_dev_unlock(hdev);

by making l2cap_unregister_user() safe to call after the hcon/hdev are
no longer alive.

The change looks OK to me, but probably worth to double check with
syzbot it fixes the original issue

#syz test git://git.kernel.org/pub/scm/linux/kernel/git/bluetooth/bluetooth-next.git master

> This can lead to use-after-free and list corruption bugs, as reported
> by syzbot.
> 
> Fix this by changing l2cap_register_user() and l2cap_unregister_user()
> to use conn->lock instead of hci_dev_lock(), ensuring consistent locking
> for the l2cap_conn structure.
> Reported-by: syzbot+14b6d57fb728e27ce23c@xxxxxxxxxxxxxxxxxxxxxxxxx
> Closes: https://syzkaller.appspot.com/bug?extid=14b6d57fb728e27ce23c
> Fixes: ab4eedb790ca ("Bluetooth: L2CAP: Fix corrupted list in hci_chan_del")
> Signed-off-by: Shaurya Rane <ssrane_b23@xxxxxxxxxxxxx>
> Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@xxxxxxxxx>
> ---
>  net/bluetooth/l2cap_core.c | 20 ++++++++------------
>  1 file changed, 8 insertions(+), 12 deletions(-)
> 
> diff --git a/net/bluetooth/l2cap_core.c b/net/bluetooth/l2cap_core.c
> index 14131e427efd..6606d7f12534 100644
> --- a/net/bluetooth/l2cap_core.c
> +++ b/net/bluetooth/l2cap_core.c
> @@ -1678,17 +1678,15 @@ static void l2cap_info_timeout(struct work_struct *work)
>  
>  int l2cap_register_user(struct l2cap_conn *conn, struct l2cap_user *user)
>  {
> -	struct hci_dev *hdev = conn->hcon->hdev;
>  	int ret;
>  
>  	/* We need to check whether l2cap_conn is registered. If it is not, we
> -	 * must not register the l2cap_user. l2cap_conn_del() is unregisters
> -	 * l2cap_conn objects, but doesn't provide its own locking. Instead, it
> -	 * relies on the parent hci_conn object to be locked. This itself relies
> -	 * on the hci_dev object to be locked. So we must lock the hci device
> -	 * here, too. */
> +	 * must not register the l2cap_user. l2cap_conn_del() unregisters
> +	 * l2cap_conn objects under conn->lock, and we use the same lock here
> +	 * to protect access to conn->users and conn->hchan.
> +	 */
>  
> -	hci_dev_lock(hdev);
> +	mutex_lock(&conn->lock);
>  
>  	if (!list_empty(&user->list)) {
>  		ret = -EINVAL;
> @@ -1709,16 +1707,14 @@ int l2cap_register_user(struct l2cap_conn *conn, struct l2cap_user *user)
>  	ret = 0;
>  
>  out_unlock:
> -	hci_dev_unlock(hdev);
> +	mutex_unlock(&conn->lock);
>  	return ret;
>  }
>  EXPORT_SYMBOL(l2cap_register_user);
>  
>  void l2cap_unregister_user(struct l2cap_conn *conn, struct l2cap_user *user)
>  {
> -	struct hci_dev *hdev = conn->hcon->hdev;
> -
> -	hci_dev_lock(hdev);
> +	mutex_lock(&conn->lock);
>  
>  	if (list_empty(&user->list))
>  		goto out_unlock;
> @@ -1727,7 +1723,7 @@ void l2cap_unregister_user(struct l2cap_conn *conn, struct l2cap_user *user)
>  	user->remove(conn, user);
>  
>  out_unlock:
> -	hci_dev_unlock(hdev);
> +	mutex_unlock(&conn->lock);
>  }
>  EXPORT_SYMBOL(l2cap_unregister_user);
>