linux-kernel - Re: [PATCH v9 10/10] docs: Include modules.builtin.alias

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Date:   Mon, 19 Dec 2022 14:07:47 -0800
From:   Luis Chamberlain <mcgrof@...nel.org>
To:     Allen Webb <allenwebb@...gle.com>
Cc:     "linux-modules@...r.kernel.org" <linux-modules@...r.kernel.org>,
        "linux-usb@...r.kernel.org" <linux-usb@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>
Subject: Re: [PATCH v9 10/10] docs: Include modules.builtin.alias

On Mon, Dec 19, 2022 at 03:40:42PM -0600, Allen Webb wrote:
> On Mon, Dec 19, 2022 at 3:23 PM Luis Chamberlain <mcgrof@...nel.org> wrote:
> >
> > On Mon, Dec 19, 2022 at 02:46:18PM -0600, Allen Webb wrote:
> > > Update the documentation to include the presense and use case of
> > > modules.builtin.alias.
> > >
> > > Signed-off-by: Allen Webb <allenwebb@...gle.com>
> > > ---
> > >  Documentation/kbuild/kbuild.rst | 6 ++++++
> > >  1 file changed, 6 insertions(+)
> > >
> > > diff --git a/Documentation/kbuild/kbuild.rst b/Documentation/kbuild/kbuild.rst
> > > index 08f575e6236c..1c7c02040a54 100644
> > > --- a/Documentation/kbuild/kbuild.rst
> > > +++ b/Documentation/kbuild/kbuild.rst
> > > @@ -17,6 +17,12 @@ modules.builtin
> > >  This file lists all modules that are built into the kernel. This is used
> > >  by modprobe to not fail when trying to load something builtin.
> > >
> > > +modules.builtin.alias
> > > +---------------------
> > > +This file lists all match-id based aliases for modules built into the kernel.
> > > +These are intended to enable userspace to make authorization decisions based
> > > +on which modules are likely to be bound to a device after it is authorized.
> >
> > What is an example? This sounds obscure.
> 
> Many of the devices that match the usb_storage driver only specify the
> vendor id, product id, and device id (VID:PID:D) and do not match
> against device class, interface class, etc. Here are some examples
> from modules.alias: A grep for wildcards in these fields yields 6136
> matches:
> grep 'dc\*dsc\*dp\*ic\*isc\*ip\*in\*'
> /lib/modules/5.19.11-1rodete1-amd64/modules.alias | wc -l
> 6136
> 
> To write USBGuard policy that only authorizes devices that bind to a
> particular module the policy needs to be aware of all these VID:PID:D
> which can change between kernel versions.
> 
> This is done at runtime rather than excluding modules from the build
> because some devices are not needed at or before login or when a
> device is locked. By not authorizing new devices that would bind to a
> set of modules, these modules become unreachable to an attacker who
> seeks to exploit kernel bugs in those modules.
> 
> I could add this detail to the documentation file, but I was trying to
> keep the description to about the same length as the others around it.

How about the second sentence you wrote say something like:

An example usage of the built-in aliases is to enable software such as
USBGuard to enable / disable specific devices outside of just the
vendor, product and device ID. This allows more flexible security policies
in userspace.

  Luis