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

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Date:   Wed, 2 Feb 2022 15:31:28 +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

On Wed, Feb 02, 2022 at 01:08:43PM +0200, Dov Murik wrote:
> 
> 
> On 02/02/2022 10:47, Gerd Hoffmann wrote:
> > On Tue, Feb 01, 2022 at 12:44:12PM +0000, Dov Murik 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.
> > 
> >> +	area = memremap(efi.coco_secret, sizeof(*area), MEMREMAP_WB);
> >> +	if (!area) {
> >> +		pr_err("Failed to map confidential computing secret area descriptor\n");
> >> +		return -ENOMEM;
> >> +	}
> >> +	if (!area->base_pa || area->size < sizeof(*header_guid))
> >> +		goto unmap_desc;
> >> +
> >> +	header_guid = (void __force *)ioremap_encrypted(area->base_pa, sizeof(*header_guid));
> >> +	if (!header_guid) {
> >> +		pr_err("Failed to map secret area\n");
> >> +		ret = -ENOMEM;
> >> +		goto unmap_desc;
> >> +	}
> >> +	if (efi_guidcmp(*header_guid, EFI_SECRET_TABLE_HEADER_GUID))
> >> +		goto unmap_encrypted;
> > 
> > Why these sanity checks are here and not in the efi_secret module?
> 
> The same checks indeed appear in the efi_secret module (see in patch 3:
> efi_secret_map_area() and the beginning of efi_secret_securityfs_setup()).
> 
> However, in the efi_secret module, the checks are noisy, because they
> expect the secret area to be populated.  For example:
> 
> +	if (efi.coco_secret == EFI_INVALID_TABLE_ADDR) {
> +		pr_err("Secret area address is not available\n");
> +		return -EINVAL;
> +	}

Note I explicitly excluded that check ;)

Checking whenever efi.coco_secret looks valid and only try load
efi_secret if that is the case (and otherwise stay silent) makes
perfect sense.  The other checks should be dropped IMHO.

> Another approach could be to just try to load the module anyway, and
> the module will fail (silently? noisily?) if there's no designated
> secret area or it's not populated.  I feel that will be harder to
> understand what's going on.

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 ...

take care,
  Gerd