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

Dov Murik
2022-02-02T15:09:33Z

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