Linux Virtualization - Sep 2021 - [PATCH v2 2/6] driver core: Add common support to skip probe for un-authorized devices

On Wed, Sep 29, 2021 at 06:05:07PM -0700, Kuppuswamy Sathyanarayanan
wrote:
> While the common case for device-authorization is to skip probe of
> unauthorized devices, some buses may still want to emit a message on
> probe failure (Thunderbolt), or base probe failures on the
> authorization status of a related device like a parent (USB). So add
> an option (has_probe_authorization) in struct bus_type for the bus
> driver to own probe authorization policy.
> 
> Reviewed-by: Dan Williams <dan.j.williams at intel.com>
> Signed-off-by: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy at
linux.intel.com>


So what e.g. the PCI patch
https://lore.kernel.org/all/CACK8Z6E8pjVeC934oFgr=VB3pULx_GyT2NkzAogdRQJ9TKSX9A
at mail.gmail.com/
actually proposes is a list of
allowed drivers, not devices. Doing it at the device level
has disadvantages, for example some devices might have a legacy
unsafe driver, or an out of tree driver. It also does not
address drivers that poke at hardware during init.

Accordingly, I think the right thing to do is to skip
driver init for disallowed drivers, not skip probe
for specific devices.

> ---
>  drivers/base/dd.c            | 5 +++++
>  drivers/thunderbolt/domain.c | 1 +
>  drivers/usb/core/driver.c    | 1 +
>  include/linux/device/bus.h   | 4 ++++
>  4 files changed, 11 insertions(+)
> 
> diff --git a/drivers/base/dd.c b/drivers/base/dd.c
> index 68ea1f949daa..0cd03ac7d3b1 100644
> --- a/drivers/base/dd.c
> +++ b/drivers/base/dd.c
> @@ -544,6 +544,11 @@ static int really_probe(struct device *dev, struct
device_driver *drv)
>  			   !drv->suppress_bind_attrs;
>  	int ret;
>  
> +	if (!dev->authorized &&
!dev->bus->has_probe_authorization) {
> +		dev_dbg(dev, "Device is not authorized\n");
> +		return -ENODEV;
> +	}
> +
>  	if (defer_all_probes) {
>  		/*
>  		 * Value of defer_all_probes can be set only by
> diff --git a/drivers/thunderbolt/domain.c b/drivers/thunderbolt/domain.c
> index 3e39686eff14..6de8a366b796 100644
> --- a/drivers/thunderbolt/domain.c
> +++ b/drivers/thunderbolt/domain.c
> @@ -321,6 +321,7 @@ struct bus_type tb_bus_type = {
>  	.probe = tb_service_probe,
>  	.remove = tb_service_remove,
>  	.shutdown = tb_service_shutdown,
> +	.has_probe_authorization = true,
>  };
>  
>  static void tb_domain_release(struct device *dev)
> diff --git a/drivers/usb/core/driver.c b/drivers/usb/core/driver.c
> index fb476665f52d..f57b5a7a90ca 100644
> --- a/drivers/usb/core/driver.c
> +++ b/drivers/usb/core/driver.c
> @@ -2028,4 +2028,5 @@ struct bus_type usb_bus_type = {
>  	.match =	usb_device_match,
>  	.uevent =	usb_uevent,
>  	.need_parent_lock =	true,
> +	.has_probe_authorization = true,
>  };
> diff --git a/include/linux/device/bus.h b/include/linux/device/bus.h
> index 062777a45a74..571a2f6e7c1d 100644
> --- a/include/linux/device/bus.h
> +++ b/include/linux/device/bus.h
> @@ -69,6 +69,9 @@ struct fwnode_handle;
>   * @lock_key:	Lock class key for use by the lock validator
>   * @need_parent_lock:	When probing or removing a device on this bus, the
>   *			device core should lock the device's parent.
> + * @has_probe_authorization: Set true to indicate to the driver-core to
skip
> + *			     the authorization checks and let bus drivers
> + *			     handle it locally.
>   *
>   * A bus is a channel between the processor and one or more devices. For
the
>   * purposes of the device model, all devices are connected via a bus, even
if
> @@ -112,6 +115,7 @@ struct bus_type {
>  	struct lock_class_key lock_key;
>  
>  	bool need_parent_lock;
> +	bool has_probe_authorization;
>  };
>  
>  extern int __must_check bus_register(struct bus_type *bus);
> -- 
> 2.25.1

On Thu, Sep 30, 2021 at 06:59:36AM -0400, Michael S. Tsirkin
wrote:
> On Wed, Sep 29, 2021 at 06:05:07PM -0700, Kuppuswamy Sathyanarayanan wrote:
> > While the common case for device-authorization is to skip probe of
> > unauthorized devices, some buses may still want to emit a message on
> > probe failure (Thunderbolt), or base probe failures on the
> > authorization status of a related device like a parent (USB). So add
> > an option (has_probe_authorization) in struct bus_type for the bus
> > driver to own probe authorization policy.
> > 
> > Reviewed-by: Dan Williams <dan.j.williams at intel.com>
> > Signed-off-by: Kuppuswamy Sathyanarayanan
<sathyanarayanan.kuppuswamy at linux.intel.com>
> 
> 
> 
> So what e.g. the PCI patch
>
https://lore.kernel.org/all/CACK8Z6E8pjVeC934oFgr=VB3pULx_GyT2NkzAogdRQJ9TKSX9A
at mail.gmail.com/
> actually proposes is a list of
> allowed drivers, not devices. Doing it at the device level
> has disadvantages, for example some devices might have a legacy
> unsafe driver, or an out of tree driver. It also does not
> address drivers that poke at hardware during init.
Doing it at a device level is the only sane way to do this.

A user needs to say "this device is allowed to be controlled by this
driver".  This is the trust model that USB has had for over a decade and
what thunderbolt also has.
> Accordingly, I think the right thing to do is to skip
> driver init for disallowed drivers, not skip probe
> for specific devices.
What do you mean by "driver init"?  module_init()?

No driver should be touching hardware in their module init call.  They
should only be touching it in the probe callback as that is the only
time they are ever allowed to talk to hardware.  Specifically the device
that has been handed to them.

If there are in-kernel PCI drivers that do not do this, they need to be
fixed today.

We don't care about out-of-tree drivers for obvious reasons that we have
no control over them.

thanks,

greg k-h