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

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Message-ID: <20220203061615.wwembqmmpmg77iyj@sirius.home.kraxel.org>
Date:   Thu, 3 Feb 2022 07:16:15 +0100
From:   Gerd Hoffmann <kraxel@...hat.com>
To:     Dov Murik <dovmurik@...ux.ibm.com>
Cc:     linux-efi@...r.kernel.org, Borislav Petkov <bp@...e.de>,
        Ashish Kalra <ashish.kalra@....com>,
        Brijesh Singh <brijesh.singh@....com>,
        Tom Lendacky <thomas.lendacky@....com>,
        Ard Biesheuvel <ardb@...nel.org>,
        James Morris <jmorris@...ei.org>,
        "Serge E. Hallyn" <serge@...lyn.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Greg KH <gregkh@...uxfoundation.org>,
        Andrew Scull <ascull@...gle.com>,
        Dave Hansen <dave.hansen@...el.com>,
        "Dr. David Alan Gilbert" <dgilbert@...hat.com>,
        Lenny Szubowicz <lszubowi@...hat.com>,
        Peter Gonda <pgonda@...gle.com>,
        James Bottomley <jejb@...ux.ibm.com>,
        Tobin Feldman-Fitzthum <tobin@...ux.ibm.com>,
        Jim Cadden <jcadden@....com>,
        Daniele Buono <dbuono@...ux.vnet.ibm.com>,
        linux-coco@...ts.linux.dev, linux-security-module@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v7 4/5] efi: Load efi_secret module if EFI secret area is
 populated

  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