Re: Question about signed uefi vars at OS level


Rafael Machado
 

Hi James, thanks for the answer.

I will try to explain my scenario in simple words.
In my case, what I would like to do is to create a runtime uefi var, that would be changed only by one .exe I have developed. 
So other .exe would not be able to perform changes at this uefi var.

Any ideia?

Thanks
Rafael



On Tue, Jul 26, 2022, 10:17 AM James Bottomley <James.Bottomley@...> wrote:
On Tue, 2022-07-26 at 10:09 -0300, Rafael Machado wrote:
> Hey everyone
>
> I have a question for the experts.
>
> Suppose I have a BIOS feature that can be set from the OS via some OS
> application (.exe) that calls the runtime services set variable ().
>
> To set this feature I have a UEFI var, that during DXE is processed
> by some uefi module.
>
> In case I define this UEFI var as signed var
> (EFI_VARIABLE_AUTHENTICATED_WRITE_ACCESS or
> EFI_VARIABLE_TIME_BASED_AUTHENTICATED_WRITE_ACCES), at my OS
> application I will have to add the signing key, so it would be
> possible to create new signed data to change the uefi variable as
> needed from the OS level.
>
> So my question is:
> What is the correct way of creating a UEFI variable that is protected
> and that can be changed, by authorized person only, from OS level
> without the need of embedding my secret at the OS application (.exe)?

You don't give your use case, so it's hard to answer the above.
However, the signing process of the update must be guarded because of
the need to keep the key secret, so update bundles are usually created
away from the system to be updated to preserve this.  If you want your
application to make arbitrary updates while it's running, you probably
don't want to be using signed variables.

James


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