Re: [PATCH v7 4/5] efi: Load efi_secret module if EFI secret area is populated

From: Gerd Hoffmann <kraxel@redhat.com>
To: Dov Murik <dovmurik@linux.ibm.com>
Cc: linux-efi@vger.kernel.org, Borislav Petkov <bp@suse.de>,
	Ashish Kalra <ashish.kalra@amd.com>,
	Brijesh Singh <brijesh.singh@amd.com>,
	Tom Lendacky <thomas.lendacky@amd.com>,
	Ard Biesheuvel <ardb@kernel.org>,
	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>,
	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@vger.kernel.org
Subject: Re: [PATCH v7 4/5] efi: Load efi_secret module if EFI secret area is populated
Date: Thu, 3 Feb 2022 07:16:15 +0100	[thread overview]
Message-ID: <20220203061615.wwembqmmpmg77iyj@sirius.home.kraxel.org> (raw)
In-Reply-To: <cb548aa2-1ac3-46e7-91e4-f57a4fd63754@linux.ibm.com>

  Hi,

> > I think the module should fail noisily.  See above for autoload.  In
> > case the module is loaded (either manually by the admin, or because
> > efi.coco_secret != EFI_INVALID_TABLE_ADDR) and it can't actually load
> > the secrets we want know why ...
> 
> Note that the AmdSev build of OVMF always publishes
> LINUX_EFI_COCO_SECRET_TABLE_GUID in the EFI table.  Even when
> LAUNCH_SECRET was not executed.  In such cases the secret area will be
> empty.

Hmm, ok.  Why?  I assume the secret area is filled by the host and ovmf
doesn't even look at it?

> If we keep only the 'efi.coco_secret != EFI_INVALID_TABLE_ADDR' check,
> we'll get errors from efi_secret for every VM launch that doesn't
> undergo LAUNCH_SECRET.  I don't think that's good.

Well, if that is a common case the module could either print nothing or
log KERN_INFO level instead of KERN_ERROR.

> If we *do* want to check that the area starts with
> EFI_SECRET_TABLE_HEADER_GUID (like I think we should), we need all the
> checks before that, like checking that the area is big enough, and that
> all the memremap()s succeed -- before actually comparing the
> header_guid.  The checks are basically prerequisites for calling
> efi_guidcmp() safely.

It is still not fully clear to me why you want do that check twice.

take care,
  Gerd