2021-03-23 03:49:51

by Lv Yunlong

Subject: [PATCH] usb: Add data checks in usbtmc_disconnect

In usbtmc_disconnect, data is got from intf with the
initial reference. There is no refcount inc operation
before usbmc_free_int(data). In usbmc_free_int(data),
the data may be freed.

But later in usbtmc_disconnect, there is another put
function of data. I think it is better to add necessary
checks to avoid the data being put twice. It could cause
errors in race.

Signed-off-by: Lv Yunlong <[email protected]>
---
drivers/usb/class/usbtmc.c | 4 +++-
1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/drivers/usb/class/usbtmc.c b/drivers/usb/class/usbtmc.c
index 74d5a9c5238a..e0438cb46386 100644
--- a/drivers/usb/class/usbtmc.c
+++ b/drivers/usb/class/usbtmc.c
@@ -2494,7 +2494,9 @@ static void usbtmc_disconnect(struct usb_interface *intf)
}
mutex_unlock(&data->io_mutex);
usbtmc_free_int(data);
- kref_put(&data->kref, usbtmc_delete);
+
+ if (data->iin_ep_present && data->iin_urb)
+ kref_put(&data->kref, usbtmc_delete);
}

static void usbtmc_draw_down(struct usbtmc_file_data *file_data)
--
2.25.1

2021-03-23 07:18:56

by Greg KH

Subject: Re: [PATCH] usb: Add data checks in usbtmc_disconnect

On Mon, Mar 22, 2021 at 08:47:17PM -0700, Lv Yunlong wrote:
> In usbtmc_disconnect, data is got from intf with the
> initial reference. There is no refcount inc operation
> before usbmc_free_int(data). In usbmc_free_int(data),
> the data may be freed.
>
> But later in usbtmc_disconnect, there is another put
> function of data. I think it is better to add necessary
> checks to avoid the data being put twice. It could cause
> errors in race.
>
> Signed-off-by: Lv Yunlong <[email protected]>
> ---
> drivers/usb/class/usbtmc.c | 4 +++-
> 1 file changed, 3 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/usb/class/usbtmc.c b/drivers/usb/class/usbtmc.c
> index 74d5a9c5238a..e0438cb46386 100644
> --- a/drivers/usb/class/usbtmc.c
> +++ b/drivers/usb/class/usbtmc.c
> @@ -2494,7 +2494,9 @@ static void usbtmc_disconnect(struct usb_interface *intf)
> }
> mutex_unlock(&data->io_mutex);
> usbtmc_free_int(data);
> - kref_put(&data->kref, usbtmc_delete);
> +
> + if (data->iin_ep_present && data->iin_urb)
> + kref_put(&data->kref, usbtmc_delete);

What protects the data from changing right after the check and right
before the kref_put() call?

krefs need a lock somewhere to protect from races like this, please fix
that logic instead.

thanks,

greg k-h

2021-03-23 09:12:17

by Lv Yunlong

Subject: Re: Re: [PATCH] usb: Add data checks in usbtmc_disconnect

> -----原始邮件-----
> 发件人: "Greg KH" <[email protected]>
> 发送时间: 2021-03-23 15:16:55 (星期二)
> 收件人: "Lv Yunlong" <[email protected]>
> 抄送: [email protected], [email protected]
> 主题: Re: [PATCH] usb: Add data checks in usbtmc_disconnect
>
> On Mon, Mar 22, 2021 at 08:47:17PM -0700, Lv Yunlong wrote:
> > In usbtmc_disconnect, data is got from intf with the
> > initial reference. There is no refcount inc operation
> > before usbmc_free_int(data). In usbmc_free_int(data),
> > the data may be freed.
> >
> > But later in usbtmc_disconnect, there is another put
> > function of data. I think it is better to add necessary
> > checks to avoid the data being put twice. It could cause
> > errors in race.
> >
> > Signed-off-by: Lv Yunlong <[email protected]>
> > ---
> > drivers/usb/class/usbtmc.c | 4 +++-
> > 1 file changed, 3 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/usb/class/usbtmc.c b/drivers/usb/class/usbtmc.c
> > index 74d5a9c5238a..e0438cb46386 100644
> > --- a/drivers/usb/class/usbtmc.c
> > +++ b/drivers/usb/class/usbtmc.c
> > @@ -2494,7 +2494,9 @@ static void usbtmc_disconnect(struct usb_interface *intf)
> > }
> > mutex_unlock(&data->io_mutex);
> > usbtmc_free_int(data);
> > - kref_put(&data->kref, usbtmc_delete);
> > +
> > + if (data->iin_ep_present && data->iin_urb)
> > + kref_put(&data->kref, usbtmc_delete);
>
> What protects the data from changing right after the check and right
> before the kref_put() call?
>
> krefs need a lock somewhere to protect from races like this, please fix
> that logic instead.
>
> thanks,
>
> greg k-h

Thanks for your reminder. I think there is something wrong with my patch.

The check condition before kref_put() added is because the data will not be
freed only when (data->iin_ep_present && data->iin_urb) is true in
usbtmc_free_int(data). But i ignored that the data may be already be freed
in usbtmc_free_int().

I will submit a PATCH v2 later. Thanks.