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 17:09:03 +0200
From:   Dov Murik <dovmurik@...ux.ibm.com>
To:     Gerd Hoffmann <kraxel@...hat.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, Dov Murik <dovmurik@...ux.ibm.com>
Subject: Re: [PATCH v7 4/5] efi: Load efi_secret module if EFI secret area is
 populated



On 02/02/2022 16:31, Gerd Hoffmann wrote:
> 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 ...
> 

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.

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.

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.

-Dov