Re: [PATCH v8 3/4] efi: Load efi_secret module if EFI secret area is populated

Dov Murik
2022-04-12T13:27:13Z

On 12/04/2022 16:08, Ard Biesheuvel wrote:
> On Thu, 31 Mar 2022 at 11:05, Dov Murik <dovmurik@linux.ibm.com> wrote:
>>
>> Hello Ard,
>>
>> On 28/02/2022 15:15, Ard Biesheuvel wrote:
>>> On Mon, 28 Feb 2022 at 14:07, Dov Murik <dovmurik@linux.ibm.com> wrote:
>>>>
>>>>
>>>>
>>>> On 28/02/2022 14:49, Ard Biesheuvel wrote:
>>>>> On Mon, 28 Feb 2022 at 12:43, Dov Murik <dovmurik@linux.ibm.com> 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.
>>>>>>
>>>>>> This will cause the <securityfs>/secrets/coco directory to appear in
>>>>>> guests into which secrets were injected; in other cases, the module is
>>>>>> not loaded.
>>>>>>
>>>>>> Signed-off-by: Dov Murik <dovmurik@linux.ibm.com>
>>>>>> Reviewed-by: Gerd Hoffmann <kraxel@redhat.com>
>>>>>
>>>>> It would be better to simply expose a platform device and associated
>>>>> driver, instead of hooking into the module machinery directly.
>>>>>
>>>>> We already do something similar for the EFI rtc and the efivars
>>>>> subsystem, using platform_device_register_simple()
>>>>>
>>>>
>>>> Thanks Ard, I'll look into this.
>>>>
>>>> I hope the mechanism you suggest allows me to perform complex check to
>>>> see if the device is really there (in this case: check for an efi
>>>> variable, map memory as encrypted, verify it starts with a specific GUID
>>>> -- everything before request_module() in the code below).
>>>>
>>>
>>> There is the device part and the driver part. Some of this belongs in
>>> the core code that registers the platform device, and some of it
>>> belongs in the driver. At this point, it probably does not matter that
>>> much which side does what, as the platform driver simply probes and
>>> can perform whatever check it needs, as long as it can back out
>>> gracefully (although I understand that, in this particular case, there
>>> are reasons why the driver may decide to wipe the secret)
>>
>> I finally got to implement this, it seems like it makes the code simple.
>> Thanks for the advice.
>>
>> Just making sure I understand correctly: in this approach this we rely
>> on udev to load the efi_secret module (aliased as "platform:efi_secret")
>> and call its .probe() function? If there's no udev, the module will not
>> be loaded automatically. Did I understand that correctly?
>>
>
> Apologies, I am swamped in email and only spotted this now.
>
> This looks good to me: is this what you implemented in the end?

Yes indeed. So this old patch 3 was replaced by this much simpler code
in the next iteration (v9):

diff --git a/drivers/firmware/efi/efi.c b/drivers/firmware/efi/efi.c
index 378d044b2463..b92eabc554e6 100644
--- a/drivers/firmware/efi/efi.c
+++ b/drivers/firmware/efi/efi.c
@@ -425,6 +425,9 @@ static int __init efisubsys_init(void)
if (efi_enabled(EFI_DBG) && efi_enabled(EFI_PRESERVE_BS_REGIONS))
efi_debugfs_init();

+ if (efi.coco_secret != EFI_INVALID_TABLE_ADDR)
+ platform_device_register_simple("efi_secret", 0, NULL, 0);
+
return 0;

err_remove_group:

(this is were I missed the #ifdef CONFIG_EFI_COCO_SECRET ).

Thanks again for the suggestion.

-Dov