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

[Michael S. Tsirkin]

On Thu, Sep 30, 2021 at 10:43:05AM -0400, Alan Stern wrote:
> On Thu, Sep 30, 2021 at 03:52:52PM +0200, Greg Kroah-Hartman wrote:
> > 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@xxxxxxxxx>
> > > > Signed-off-by: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@xxxxxxxxxxxxxxx>
> > > 
> > > 
> > > 
> > > So what e.g. the PCI patch
> > > https://lore.kernel.org/all/CACK8Z6E8pjVeC934oFgr=VB3pULx_GyT2NkzAogdRQJ9TKSX9A@xxxxxxxxxxxxxx/
> > > 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.
> 
> I don't see any point in talking about "untrusted drivers".  If a 
> driver isn't trusted then it doesn't belong in your kernel.  Period.  
> When you load a driver into your kernel, you are implicitly trusting 
> it (aside from limitations imposed by security modules).  The code 
> it contains, the module_init code in particular, runs with full 
> superuser permissions.
> 
> What use is there in loading a driver but telling the kernel "I don't 
> trust this driver, so don't allow it to probe any devices"?  Why not 
> just blacklist it so that it never gets modprobed in the first place?
> 
> Alan Stern

When the driver is built-in, it seems useful to be able to block it
without rebuilding the kernel. This is just flipping it around
and using an allow-list for cases where you want to severly
limit the available functionality.


-- 
MST