linux-kernel - Re: [PATCH] Bluetooth: fix double free in hci_req_sync

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Date: Mon, 24 Jun 2024 09:54:56 -0400
From: Luiz Augusto von Dentz <luiz.dentz@...il.com>
To: Edward Adam Davis <eadavis@...com>
Cc: pav@....fi, johan.hedberg@...il.com, linux-bluetooth@...r.kernel.org, 
	linux-kernel@...r.kernel.org, marcel@...tmann.org, 
	syzbot+35ebc808442df6420eae@...kaller.appspotmail.com, 
	syzkaller-bugs@...glegroups.com
Subject: Re: [PATCH] Bluetooth: fix double free in hci_req_sync_complete

Hi Edward,

On Sun, Jun 23, 2024 at 8:02 AM Edward Adam Davis <eadavis@...com> wrote:
>
> On Sun, 23 Jun 2024 13:30:50 +0300, Pauli Virtanen wrote:
> > > cpu1                       cpu2
> > > ====                       ====
> > >                            sock_ioctl
> > >                            sock_do_ioctl
> > >                            hci_sock_ioctl
> > > hci_rx_work                hci_dev_cmd
> > > hci_event_packet           hci_req_sync
> > > req_complete_skb           __hci_req_sync
> > > hci_req_sync_complete
> > >
> > > If hci_rx_work executes before __hci_req_sync releases req_skb, everything
> > > is normal, otherwise it will result in double free of req_skb.
> > >
> > > Adding NULL check of req_skb before releasing it can avoid double free.
> >
> > Do you understand why?
> >
> > kfree_skb(NULL) is allowed, so this is logically a no-op.
> >
> > Probably it perturbs the timings so syzkaller repro no longer hits the
> > race window, ie doesn't fix the issue.
> Good, even if you already know race, let me ask you a question: how to reduce race window?

We actually need to stop using hci_req_sync since that has been
deprecated, instead it shall use hci_cmd_sync_submit, we could for the
time being just make hci_req_sync use hci_cmd_sync_submit to avoid
such races.

> ``
> Edward
>


-- 
Luiz Augusto von Dentz