linux-kernel - Re: [PATCH RFC 1/1] module: Make use of platform keyring for module signature verify

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Message-ID: <bec46b16778d9292ab90b3e1c71be6c56c8a5a50.camel@HansenPartnership.com>
Date: Thu, 05 Jun 2025 08:05:56 -0400
From: James Bottomley <James.Bottomley@...senPartnership.com>
To: Coiby Xu <coxu@...hat.com>
Cc: Vitaly Kuznetsov <vkuznets@...hat.com>, 
 linux-security-module@...r.kernel.org, linux-integrity@...r.kernel.org, 
 linux-modules@...r.kernel.org, linux-kernel@...r.kernel.org, 
 linux-doc@...r.kernel.org, keyrings@...r.kernel.org, David Howells
 <dhowells@...hat.com>, David Woodhouse <dwmw2@...radead.org>, Jonathan
 Corbet <corbet@....net>, Luis Chamberlain <mcgrof@...nel.org>, Petr Pavlu
 <petr.pavlu@...e.com>, Sami Tolvanen <samitolvanen@...gle.com>, Daniel
 Gomez <da.gomez@...sung.com>, Mimi Zohar <zohar@...ux.ibm.com>, Roberto
 Sassu <roberto.sassu@...wei.com>, Dmitry Kasatkin
 <dmitry.kasatkin@...il.com>,  Eric Snowberg <eric.snowberg@...cle.com>,
 Paul Moore <paul@...l-moore.com>, James Morris <jmorris@...ei.org>,  "Serge
 E. Hallyn" <serge@...lyn.com>, Peter Jones <pjones@...hat.com>, Robert
 Holmes <robeholmes@...il.com>, Jeremy Cline <jcline@...hat.com>, Gerd
 Hoffmann <kraxel@...hat.com>
Subject: Re: [PATCH RFC 1/1] module: Make use of platform keyring for module
 signature verify

On Thu, 2025-06-05 at 16:34 +0800, Coiby Xu wrote:
> On Tue, Jun 03, 2025 at 09:03:22AM -0400, James Bottomley wrote:
> > On Tue, 2025-06-03 at 10:52 +0200, Vitaly Kuznetsov wrote:
> > > James Bottomley <James.Bottomley@...senPartnership.com> writes:
> > [...]
> > > > Also, are you sure a config option is the right thing? 
> > > > Presumably Red Hat wants to limit its number of kernels and the
> > > > design of just linking the machine keyring (i.e. MoK) was for
> > > > the use case where trust is being pivoted away from db by shim,
> > > > so users don't want to trust the db keys they don't control. 
> > > > If the same kernel gets used for both situations (trusted and
> > > > untrusted db) you might want a runtime means to distinguish
> > > > them.
> > > 
> > > I was not personally involved when RH put the patch downstream
> > > (and wasn't very successful in getting the background story) but
> > > it doesn't even have an additional Kconfig, e.g.:
> > > https://gitlab.com/redhat/centos-stream/src/kernel/centos-stream-10/-/commit/03d4694fa6511132989bac0da11fa677ea5d29f6
> > > so apparently there's no desire to limit anything, basically,
> > > .platform is always trusted on Fedora/RHEL systems (for a long
> > > time already).
> > 
> > It sounds like that's just distro politics:  RH wants to enable
> > binary modules (by allowing them to be signed) but doesn't want to
> > be seen to be signing them (so they can't be signed with the
> > embedded RH key) so that gamers can have performant graphics
> > drivers and the like.  Thus it mixes in the db keyring, which
> > usually contains several Microsoft certificates and also one from
> > the ODM manufacturer, so now it can send would be shippers of
> > binary modules to those groups to get them signed. If you only have
> > the built in and MoK keyrings, the only possible signers are either
> > RH or the machine owner ... who isn't a single entity to deal
> > with.  Personally I think this is a bit daft: Debian manages an out
> > of tree module infrastructure using DKMS and MoK signing, so I
> > can't see why RH can't get it to work in the same way.
> 
> It's interesting to find that although Debian's wiki page [1] only
> mentions DKMS and MOK, it actually has the same downstream kernel
> patch [2][3] as Fedora/RHEL to allow using db keys to verify kernel
> modules. 
> [1] https://wiki.debian.org/SecureBoot
> [2]
> https://salsa.debian.org/kernel-team/linux/-/blob/debian/latest/debian/patches/features/all/db-mok-keyring/KEYS-Make-use-of-platform-keyring-for-module-signature.patch?ref_type=heads
> [3]
> https://sources.debian.org/patches/linux/6.12.30-1/features/all/db-mok-keyring/KEYS-Make-use-of-platform-keyring-for-module-signature.patch/
> 

Well if you read the attached bug reports:

https://bugs.debian.org/935945
https://bugs.debian.org/1030200

You can see that it's people trying to get an external module to work
(actually zfs locally signed) by adding keys to MoK and it failed
because of a configuration error (CONFIG_INTEGRITY_MACHINE_KEYRING
wasn't set).  They added this patch as part of the thrashing around
trying to fix the problem because they found it in Fedora.

Regards,

James