[v2] usb: core: Fix crash w/ usb_choose_configuration() if no driver

It's possible that usb_choose_configuration() can get called when a
USB device has no driver. In this case the recent commit a87b8e3be926
("usb: core: Allow subclassed USB drivers to override
usb_choose_configuration()") can cause a crash since it dereferenced
the driver structure without checking for NULL. Let's add a check.

A USB device with no driver is an anomaly, so make
usb_choose_configuration() return immediately if there is no driver.

This was seen in the real world when usbguard got ahold of a r8152
device at the wrong time. It can also be simulated via this on a
computer with one r8152-based USB Ethernet adapter:
  cd /sys/bus/usb/drivers/r8152-cfgselector
  to_unbind="$(ls -d *-*)"
  real_dir="$(readlink -f "${to_unbind}")"
  echo "${to_unbind}" > unbind
  cd "${real_dir}"
  echo 0 > authorized
  echo 1 > authorized

Fixes: a87b8e3be926 ("usb: core: Allow subclassed USB drivers to override usb_choose_configuration()")
Signed-off-by: Douglas Anderson <dianders@chromium.org>
---

Changes in v2:
- Return immediately if no driver, as per Alan.

 drivers/usb/core/generic.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

On Mon, Dec 11, 2023 at 07:08:14AM -0800, Douglas Anderson wrote:
> It's possible that usb_choose_configuration() can get called when a
> USB device has no driver. In this case the recent commit a87b8e3be926
> ("usb: core: Allow subclassed USB drivers to override
> usb_choose_configuration()") can cause a crash since it dereferenced
> the driver structure without checking for NULL. Let's add a check.
> 
> A USB device with no driver is an anomaly, so make
> usb_choose_configuration() return immediately if there is no driver.
> 
> This was seen in the real world when usbguard got ahold of a r8152
> device at the wrong time. It can also be simulated via this on a
> computer with one r8152-based USB Ethernet adapter:
>   cd /sys/bus/usb/drivers/r8152-cfgselector
>   to_unbind="$(ls -d *-*)"
>   real_dir="$(readlink -f "${to_unbind}")"
>   echo "${to_unbind}" > unbind
>   cd "${real_dir}"
>   echo 0 > authorized
>   echo 1 > authorized
> 
> Fixes: a87b8e3be926 ("usb: core: Allow subclassed USB drivers to override usb_choose_configuration()")
> Signed-off-by: Douglas Anderson <dianders@chromium.org>
> ---
> 
> Changes in v2:
> - Return immediately if no driver, as per Alan.
> 
>  drivers/usb/core/generic.c | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/usb/core/generic.c b/drivers/usb/core/generic.c
> index dcb897158228..2be1e8901e2f 100644
> --- a/drivers/usb/core/generic.c
> +++ b/drivers/usb/core/generic.c
> @@ -59,7 +59,11 @@ int usb_choose_configuration(struct usb_device *udev)
>  	int num_configs;
>  	int insufficient_power = 0;
>  	struct usb_host_config *c, *best;
> -	struct usb_device_driver *udriver = to_usb_device_driver(udev->dev.driver);
> +	struct usb_device_driver *udriver;
> +
> +	if (!udev->dev.driver)
> +		return -1;

This is a rather unusual condition.  It would be good to put a comment 
just before the test, explaining that if a USB device (not an interface) 
doesn't have a driver then the kernel has no business trying to select 
or install a configuration for it.

Along with the comment, feel free to add:

Reviewed-by: Alan Stern <stern@rowland.harvard.edu>

Alan Stern

> +	udriver = to_usb_device_driver(udev->dev.driver);
>  
>  	if (usb_device_is_owned(udev))
>  		return 0;
> -- 
> 2.43.0.472.g3155946c3a-goog
>

Hi,

On Mon, Dec 11, 2023 at 7:27 AM Alan Stern <stern@rowland.harvard.edu> wrote:
>
> On Mon, Dec 11, 2023 at 07:08:14AM -0800, Douglas Anderson wrote:
> > It's possible that usb_choose_configuration() can get called when a
> > USB device has no driver. In this case the recent commit a87b8e3be926
> > ("usb: core: Allow subclassed USB drivers to override
> > usb_choose_configuration()") can cause a crash since it dereferenced
> > the driver structure without checking for NULL. Let's add a check.
> >
> > A USB device with no driver is an anomaly, so make
> > usb_choose_configuration() return immediately if there is no driver.
> >
> > This was seen in the real world when usbguard got ahold of a r8152
> > device at the wrong time. It can also be simulated via this on a
> > computer with one r8152-based USB Ethernet adapter:
> >   cd /sys/bus/usb/drivers/r8152-cfgselector
> >   to_unbind="$(ls -d *-*)"
> >   real_dir="$(readlink -f "${to_unbind}")"
> >   echo "${to_unbind}" > unbind
> >   cd "${real_dir}"
> >   echo 0 > authorized
> >   echo 1 > authorized
> >
> > Fixes: a87b8e3be926 ("usb: core: Allow subclassed USB drivers to override usb_choose_configuration()")
> > Signed-off-by: Douglas Anderson <dianders@chromium.org>
> > ---
> >
> > Changes in v2:
> > - Return immediately if no driver, as per Alan.
> >
> >  drivers/usb/core/generic.c | 6 +++++-
> >  1 file changed, 5 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/usb/core/generic.c b/drivers/usb/core/generic.c
> > index dcb897158228..2be1e8901e2f 100644
> > --- a/drivers/usb/core/generic.c
> > +++ b/drivers/usb/core/generic.c
> > @@ -59,7 +59,11 @@ int usb_choose_configuration(struct usb_device *udev)
> >       int num_configs;
> >       int insufficient_power = 0;
> >       struct usb_host_config *c, *best;
> > -     struct usb_device_driver *udriver = to_usb_device_driver(udev->dev.driver);
> > +     struct usb_device_driver *udriver;
> > +
> > +     if (!udev->dev.driver)
> > +             return -1;
>
> This is a rather unusual condition.  It would be good to put a comment
> just before the test, explaining that if a USB device (not an interface)
> doesn't have a driver then the kernel has no business trying to select
> or install a configuration for it.
>
> Along with the comment, feel free to add:
>
> Reviewed-by: Alan Stern <stern@rowland.harvard.edu>

Thanks for the quick reply. I've added your comment almost verbatim to
the code and sent out a quick v3 with your tag:

https://lore.kernel.org/r/20231211073237.v3.1.If27eb3bf7812f91ab83810f232292f032f4203e0@changeid