netdev - Re: [PATCH] drivers: net: remove a dangling pointer in peak_usb_create

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Date:   Fri, 21 Jan 2022 11:36:52 +0800
From:   Dongliang Mu <mudongliangabcd@...il.com>
To:     Pavel Skripkin <paskripkin@...il.com>
Cc:     Dongliang Mu <dzm91@...t.edu.cn>,
        Wolfgang Grandegger <wg@...ndegger.com>,
        Marc Kleine-Budde <mkl@...gutronix.de>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Stephane Grosjean <s.grosjean@...k-system.com>,
        Stefan Mätje <stefan.maetje@....eu>,
        Vincent Mailhol <mailhol.vincent@...adoo.fr>,
        linux-can@...r.kernel.org,
        "open list:NETWORKING [GENERAL]" <netdev@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] drivers: net: remove a dangling pointer in peak_usb_create_dev

On Fri, Jan 21, 2022 at 8:09 AM Dongliang Mu <mudongliangabcd@...il.com> wrote:
>
> On Thu, Jan 20, 2022 at 10:27 PM Pavel Skripkin <paskripkin@...il.com> wrote:
> >
> > Hi Dongliang,
> >
> > On 1/20/22 16:05, Dongliang Mu wrote:
> > > From: Dongliang Mu <mudongliangabcd@...il.com>
> > >
> > > The error handling code of peak_usb_create_dev forgets to reset the
> > > next_siblings of previous entry.
> > >
> > > Fix this by nullifying the (dev->prev_siblings)->next_siblings in the
> > > error handling code.
> > >
> > > Signed-off-by: Dongliang Mu <mudongliangabcd@...il.com>
> > > ---
> > >   drivers/net/can/usb/peak_usb/pcan_usb_core.c | 3 +++
> > >   1 file changed, 3 insertions(+)
> > >
> > > diff --git a/drivers/net/can/usb/peak_usb/pcan_usb_core.c b/drivers/net/can/usb/peak_usb/pcan_usb_core.c
> > > index b850ff8fe4bd..f858810221b6 100644
> > > --- a/drivers/net/can/usb/peak_usb/pcan_usb_core.c
> > > +++ b/drivers/net/can/usb/peak_usb/pcan_usb_core.c
> > > @@ -894,6 +894,9 @@ static int peak_usb_create_dev(const struct peak_usb_adapter *peak_usb_adapter,
> > >               dev->adapter->dev_free(dev);
> > >
> > >   lbl_unregister_candev:
> > > +     /* remove the dangling pointer in next_siblings */
> > > +     if (dev->prev_siblings)
> > > +             (dev->prev_siblings)->next_siblings = NULL;
> > >       unregister_candev(netdev);
> > >
> > >   lbl_restore_intf_data:
> >
> >
> > Is this pointer used somewhere? I see, that couple of
> > struct peak_usb_adapter::dev_free() functions use it, but
> > peak_usb_disconnect() sets dev->next_siblings to NULL before calling
> > ->dev_free().
> >
> > Do you have a calltrace or oops log?
>
> Hi Pavel,
>
> I have no calltrace or log since this dangling pointer may not be
> dereferenced in the following code. But I am not sure. So the commit
> title of this patch is "remove a dangling pointer in
> peak_usb_create_dev".

BTW, as you mentioned, dev->next_siblings is used in struct
peak_usb_adapter::dev_free() (i.e., pcan_usb_fd_free or
pcan_usb_pro_free), how about the following path?

peak_usb_probe
-> peak_usb_create_dev (goto adap_dev_free;)
   -> dev->adapter->dev_free()
      -> pcan_usb_fd_free or pcan_usb_pro_free (This function uses
next_siblings as condition elements)

static void pcan_usb_fd_free(struct peak_usb_device *dev)
{
        /* last device: can free shared objects now */
        if (!dev->prev_siblings && !dev->next_siblings) {
                struct pcan_usb_fd_device *pdev =
                        container_of(dev, struct pcan_usb_fd_device, dev);

                /* free commands buffer */
                kfree(pdev->cmd_buffer_addr);

                /* free usb interface object */
                kfree(pdev->usb_if);
        }
}

If next_siblings is not NULL, will it lead to the missing free of
cmd_buffer_addr and usb_if?

Please let me know if I made any mistakes.

> >
> >
> >
> >
> > With regards,
> > Pavel Skripkin