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

[syzbot]

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

la, 2026-03-07 kello 10:59 +0200, Pauli Virtanen kirjoitti:
> 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

syzbot seems to have hit some internal error, another try on upstream
branch instead

#syz test

> > 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);
> >