Re: [PATCH] driver: usb: nullify dangling pointer in cdc_ncm_free

Johan Hovold
2022-04-20T06:57:08Z

On Tue, Apr 19, 2022 at 01:47:40PM +0200, Oliver Neukum wrote:
> On 14.04.22 17:01, Andy Shevchenko wrote:
> >
> > Good question. Isn't it the commit 2c9d6c2b871d ("usbnet: run unbind()
> > before unregister_netdev()") which changed the ordering of the
> > interface shutdown and basically makes this race happen? I don't see
> > how we can guarantee that IOCTL won't be called until we quiescence
> > the network device — my understanding that on device surprise removal
> True. The best we could do is introduce a mutex for ioctl() and
> disconnect(). That seems the least preferable solution to me.
> > we have to first shutdown what it created and then unbind the device.

Yes, indeed, commit 2c9d6c2b871d ("usbnet: run unbind() before
unregister_netdev()") is fundamentally broken. You can't just start
freeing driver private data before deregistering the device.

> > If I understand the original issue correctly then the problem is in
> > usbnet->unbind and it should actually be split to two hooks, otherwise
> > it seems every possible IOCTL callback must have some kind of
> > reference counting and keep an eye on the surprise removal.
> >
> > Johan, can you correct me if my understanding is wrong?

That sounds correct. I only noticed that the proposed patch looked
insufficient at best and didn't really look into the backstory until
just now.

> It seems to me that fundamentally the order of actions to handle
> a hotunplug must mirror the order in a hotplug. We can add more hooks
> if that turns out to be necessary for some drivers, but the basic
> reverse mirrored order must be supported and I very much favor
> restoring it as default.

I agree, we need to strive to maintain symmetry. Anything else is likely
broken and at least makes things harder to reason about and maintenance
a pain.

> So I am afraid I have to ask again, whether anybody sees a fundamental
> issue with the attached patch, as opposed to it not being an elegant
> solution?
> It looks to me that we are in a fundamental disagreement on the correct
> order in this question and there is no productive way forward other than
> offering both ways.
>
>     Regards
>         Oliver

> From 2e07ccbd1769889963d129ec474909bdcaa4c64a Mon Sep 17 00:00:00 2001
> From: Oliver Neukum <oneukum@suse.com>
> Date: Thu, 10 Mar 2022 13:18:38 +0100
> Subject: [PATCH] usbnet: split unbind callback
>
> Some devices need to be informed of a disconnect before
> the generic layer is informed, others need their notification
> later to avoid race conditions. Hence we provide two callbacks.
>
> Signed-off-by: Oliver Neukum <oneukum@suse.com>
> ---
> drivers/net/usb/asix_devices.c | 8 ++++----
> drivers/net/usb/smsc95xx.c | 4 ++--
> drivers/net/usb/usbnet.c | 7 +++++--
> include/linux/usb/usbnet.h | 3 +++
> 4 files changed, 14 insertions(+), 8 deletions(-)
>
> diff --git a/drivers/net/usb/asix_devices.c b/drivers/net/usb/asix_devices.c
> index 6ea44e53713a..e6cfa9a39a87 100644
> --- a/drivers/net/usb/asix_devices.c
> +++ b/drivers/net/usb/asix_devices.c
> @@ -808,7 +808,7 @@ static int ax88772_stop(struct usbnet *dev)
> return 0;
> }
>
> -static void ax88772_unbind(struct usbnet *dev, struct usb_interface *intf)
> +static void ax88772_disable(struct usbnet *dev, struct usb_interface *intf)
> {
> struct asix_common_private *priv = dev->driver_priv;
>
> @@ -1214,7 +1214,7 @@ static const struct driver_info hawking_uf200_info = {
> static const struct driver_info ax88772_info = {
> .description = "ASIX AX88772 USB 2.0 Ethernet",
> .bind = ax88772_bind,
> - .unbind = ax88772_unbind,
> + .unbind = ax88772_disable,

These should all be

.disable = ...

but you probably need to split the callback and keep unbind as well for
the actual clean up (freeing resources etc).

> .status = asix_status,
> .reset = ax88772_reset,
> .stop = ax88772_stop,
> @@ -1226,7 +1226,7 @@ static const struct driver_info ax88772_info = {
> static const struct driver_info ax88772b_info = {
> .description = "ASIX AX88772B USB 2.0 Ethernet",
> .bind = ax88772_bind,
> - .unbind = ax88772_unbind,
> + .unbind = ax88772_disable,
> .status = asix_status,
> .reset = ax88772_reset,
> .stop = ax88772_stop,
> @@ -1262,7 +1262,7 @@ static const struct driver_info ax88178_info = {
> static const struct driver_info hg20f9_info = {
> .description = "HG20F9 USB 2.0 Ethernet",
> .bind = ax88772_bind,
> - .unbind = ax88772_unbind,
> + .unbind = ax88772_disable,
> .status = asix_status,
> .reset = ax88772_reset,
> .flags = FLAG_ETHER | FLAG_FRAMING_AX | FLAG_LINK_INTR |
> diff --git a/drivers/net/usb/smsc95xx.c b/drivers/net/usb/smsc95xx.c
> index 5567220e9d16..62db57021f5f 100644
> --- a/drivers/net/usb/smsc95xx.c
> +++ b/drivers/net/usb/smsc95xx.c
> @@ -1211,7 +1211,7 @@ static int smsc95xx_bind(struct usbnet *dev, struct usb_interface *intf)
> return ret;
> }
>
> -static void smsc95xx_unbind(struct usbnet *dev, struct usb_interface *intf)
> +static void smsc95xx_disable(struct usbnet *dev, struct usb_interface *intf)
> {
> struct smsc95xx_priv *pdata = dev->driver_priv;
>
> @@ -1985,7 +1985,7 @@ static int smsc95xx_manage_power(struct usbnet *dev, int on)
> static const struct driver_info smsc95xx_info = {
> .description = "smsc95xx USB 2.0 Ethernet",
> .bind = smsc95xx_bind,
> - .unbind = smsc95xx_unbind,
> + .unbind = smsc95xx_disable,
> .link_reset = smsc95xx_link_reset,
> .reset = smsc95xx_reset,
> .check_connect = smsc95xx_start_phy,
> diff --git a/drivers/net/usb/usbnet.c b/drivers/net/usb/usbnet.c
> index b1f93810a6f3..5249a7d7efa5 100644
> --- a/drivers/net/usb/usbnet.c
> +++ b/drivers/net/usb/usbnet.c
> @@ -1641,8 +1641,8 @@ void usbnet_disconnect (struct usb_interface *intf)
> xdev->bus->bus_name, xdev->devpath,
> dev->driver_info->description);
>
> - if (dev->driver_info->unbind)
> - dev->driver_info->unbind(dev, intf);
> + if (dev->driver_info->disable)
> + dev->driver_info->disable(dev, intf);
>
> net = dev->net;
> unregister_netdev (net);
> @@ -1651,6 +1651,9 @@ void usbnet_disconnect (struct usb_interface *intf)
>
> usb_scuttle_anchored_urbs(&dev->deferred);
>
> + if (dev->driver_info->unbind)
> + dev->driver_info->unbind (dev, intf);
> +
> usb_kill_urb(dev->interrupt);

Don't you need to quiesce all I/O, including stopping the interrupt URB,
before unbind?

> usb_free_urb(dev->interrupt);
> kfree(dev->padding_pkt);
> diff --git a/include/linux/usb/usbnet.h b/include/linux/usb/usbnet.h
> index 8336e86ce606..4d2407f1ae93 100644
> --- a/include/linux/usb/usbnet.h
> +++ b/include/linux/usb/usbnet.h
> @@ -129,6 +129,9 @@ struct driver_info {
> /* cleanup device ... can sleep, but can't fail */
> void (*unbind)(struct usbnet *, struct usb_interface *);
>
> + /* disable device ... can sleep, but can't fail */
> + void (*disable)(struct usbnet *, struct usb_interface *);
> +
> /* reset device ... can sleep */
> int (*reset)(struct usbnet *);

Johan