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

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Date:   Thu, 30 Sep 2021 11:32:41 -0400
From:   Alan Stern <stern@...land.harvard.edu>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Kuppuswamy Sathyanarayanan 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>,
        Borislav Petkov <bp@...en8.de>, x86@...nel.org,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Andreas Noever <andreas.noever@...il.com>,
        Michael Jamet <michael.jamet@...el.com>,
        Yehezkel Bernat <YehezkelShB@...il.com>,
        "Rafael J . Wysocki" <rafael@...nel.org>,
        Mika Westerberg <mika.westerberg@...ux.intel.com>,
        Jonathan Corbet <corbet@....net>,
        Jason Wang <jasowang@...hat.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Kuppuswamy Sathyanarayanan <knsathya@...nel.org>,
        linux-kernel@...r.kernel.org, linux-pci@...r.kernel.org,
        linux-usb@...r.kernel.org,
        virtualization@...ts.linux-foundation.org
Subject: Re: [PATCH v2 2/6] driver core: Add common support to skip probe for
 un-authorized devices

On Thu, Sep 30, 2021 at 10:48:54AM -0400, Michael S. Tsirkin wrote:
> On Thu, Sep 30, 2021 at 10:43:05AM -0400, Alan Stern wrote:
> > 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.

Does this make sense?

The only way to tell the kernel to block a built-in driver is by 
using some boot-command-line option.  Otherwise the driver's init 
code will run before you have a chance to tell the kernel anything at 
all.

So if you change your mind about whether a driver should be blocked, 
all you have to do is remove the blocking option from the command 
line and reboot.  No kernel rebuild is necessary.

Alan Stern