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From:   Ard Biesheuvel <ardb@kernel.org>
Date:   Tue, 12 Apr 2022 15:08:25 +0200
Message-ID: <CAMj1kXHfeS9WzHkOyM3JGfgVLK8GQ78KBArD19NxcCMn-SDPKg@mail.gmail.com>
Subject: Re: [PATCH v8 3/4] efi: Load efi_secret module if EFI secret area is populated
To:     Dov Murik <dovmurik@linux.ibm.com>
Cc:     linux-efi <linux-efi@vger.kernel.org>,
        Gerd Hoffmann <kraxel@redhat.com>,
        Borislav Petkov <bp@suse.de>,
        Ashish Kalra <ashish.kalra@amd.com>,
        Brijesh Singh <brijesh.singh@amd.com>,
        Tom Lendacky <thomas.lendacky@amd.com>,
        James Morris <jmorris@namei.org>,
        "Serge E. Hallyn" <serge@hallyn.com>,
        Andi Kleen <ak@linux.intel.com>,
        Greg KH <gregkh@linuxfoundation.org>,
        Andrew Scull <ascull@google.com>,
        Dave Hansen <dave.hansen@intel.com>,
        "Dr. David Alan Gilbert" <dgilbert@redhat.com>,
        Lenny Szubowicz <lszubowi@redhat.com>,
        Peter Gonda <pgonda@google.com>,
        Matthew Garrett <mjg59@srcf.ucam.org>,
        James Bottomley <jejb@linux.ibm.com>,
        Tobin Feldman-Fitzthum <tobin@linux.ibm.com>,
        Jim Cadden <jcadden@ibm.com>,
        Daniele Buono <dbuono@linux.vnet.ibm.com>,
        linux-coco@lists.linux.dev, linux-security-module@vger.kernel.org,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
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On Thu, 31 Mar 2022 at 11:05, Dov Murik <dovmurik@linux.ibm.com> wrote:
>
> Hello Ard,
>
> On 28/02/2022 15:15, Ard Biesheuvel wrote:
> > On Mon, 28 Feb 2022 at 14:07, Dov Murik <dovmurik@linux.ibm.com> wrote:
> >>
> >>
> >>
> >> On 28/02/2022 14:49, Ard Biesheuvel wrote:
> >>> On Mon, 28 Feb 2022 at 12:43, Dov Murik <dovmurik@linux.ibm.com> wrote:
> >>>>
> >>>> If the efi_secret module is built, register a late_initcall in the EFI
> >>>> driver which checks whether the EFI secret area is available and
> >>>> populated, and then requests to load the efi_secret module.
> >>>>
> >>>> This will cause the <securityfs>/secrets/coco directory to appear in
> >>>> guests into which secrets were injected; in other cases, the module is
> >>>> not loaded.
> >>>>
> >>>> Signed-off-by: Dov Murik <dovmurik@linux.ibm.com>
> >>>> Reviewed-by: Gerd Hoffmann <kraxel@redhat.com>
> >>>
> >>> It would be better to simply expose a platform device and associated
> >>> driver, instead of hooking into the module machinery directly.
> >>>
> >>> We already do something similar for the EFI rtc and the efivars
> >>> subsystem, using platform_device_register_simple()
> >>>
> >>
> >> Thanks Ard, I'll look into this.
> >>
> >> I hope the mechanism you suggest allows me to perform complex check to
> >> see if the device is really there (in this case: check for an efi
> >> variable, map memory as encrypted, verify it starts with a specific GUID
> >> -- everything before request_module() in the code below).
> >>
> >
> > There is the device part and the driver part. Some of this belongs in
> > the core code that registers the platform device, and some of it
> > belongs in the driver. At this point, it probably does not matter that
> > much which side does what, as the platform driver simply probes and
> > can perform whatever check it needs, as long as it can back out
> > gracefully (although I understand that, in this particular case, there
> > are reasons why the driver may decide to wipe the secret)
>
> I finally got to implement this, it seems like it makes the code simple.
> Thanks for the advice.
>
> Just making sure I understand correctly: in this approach this we rely
> on udev to load the efi_secret module (aliased as "platform:efi_secret")
> and call its .probe() function?  If there's no udev, the module will not
> be loaded automatically.  Did I understand that correctly?
>

Apologies, I am swamped in email and only spotted this now.

This looks good to me: is this what you implemented in the end?