[edk2-devel] [PATCH] SecurityPkg: Don't Verify the enrolled PK in setup mode

[Laszlo Ersek]

Hi,

On 07/10/19 10:50, Wang, Jian J wrote:
> Hi Derek,
> 
> Please file a Bugzilla for this issue. With it addressed,
> 
>     Reviewed-by: Jian J Wang jian.j.wang at intel.com<mailto:jian.j.wang at intel.com>

I saw this patch as soon as it was posted, and I've been hoping for a
deeper discussion on the list. (I didn't ask my questions immediately
because I felt they might have been somewhat naive.) So I guess I have
to ask them now :)


(1) What is the exact failure (or spec non-conformance) scenario?

Is it the problem that, currently, even if SetupMode is 1, CustomMode
also needs to be set, for enrolling a self-signed PK?

(Looking at the patch itself, it looks like the subcondition that is
*not* the CustomMode check is relaxed; so that seems to support my
question.)


(2) Can we please confirm that the code will continue to enforce
self-signedness?

I checked the spec, and I'm a bit worried about the spec language
proper. Page 246 in UEFI-2.8 says,

  3. If the variable SetupMode==1, and the variable is a secure boot
     policy variable, then the firmware implementation shall consider
     the checks in the following steps 4 and 5 to have passed, and
     proceed with updating the variable value as outlined below.

  4. Verify the signature by:
     — extracting the EFI_VARIABLE_AUTHENTICATION_2 descriptor from the
       Data buffer;
     — using the descriptor contents and other parameters to (a)
       construct the input to the digest algorithm; (b) computing the
       digest; and (c) comparing the digest with the result of applying
       the signer’s public key to the signature.

In other words, step#4 seems to stand for verifying self-signedness, and
step#3 appears to require that *not even* self-signedness be enforced,
when in SetupMode.

Honestly, I think that step#4 should never be skipped. In other words,
self-signedness should be unconditional.

A certificate is basically a signed statement that a particular public
key belongs to a particular entity (such as "FooBar"). If *not even*
FooBar signs that statement, then the statement has *zero* credibility.
So why should we accept it for any purpose at all?

Speaking in terms of "PK" (UEFI Platform Key), specifically: what good
would a platform vendor be that failed to sign its own PK?

So, I'd like to understand:

(2a) whether skipping step#4 in SetupMode is a bug in the spec --
because, I think it is;

(2b) whether the edk2 code would continue enforcing self-signedness on
X509 certificates, if the proposed patch were applied.

Thanks!
Laszlo

> From: devel at edk2.groups.io [mailto:devel at edk2.groups.io] On Behalf Of Zhang, Chao B
> Sent: Tuesday, July 09, 2019 11:39 PM
> To: devel at edk2.groups.io; derek.lin2 at hpe.com
> Subject: Re: [edk2-devel] [PATCH] SecurityPkg: Don't Verify the enrolled PK in setup mode
> 
> Hi Derek:
>    The patch is good to me.
>    Reviewed-by : Chao Zhang <chao.b.zhang at intel.com<mailto:chao.b.zhang at intel.com>>
> 
> From: devel at edk2.groups.io<mailto:devel at edk2.groups.io> [mailto:devel at edk2.groups.io] On Behalf Of derek.lin2 at hpe.com<mailto:derek.lin2 at hpe.com>
> Sent: Tuesday, July 2, 2019 1:25 PM
> To: devel at edk2.groups.io<mailto:devel at edk2.groups.io>
> Subject: [edk2-devel] [PATCH] SecurityPkg: Don't Verify the enrolled PK in setup mode
> 
> Patch is attached from group.io.
> Since ECR785, which is added UEFI 2.3.1 errata A, enrolling a PK in setup mode doesn't need to verify the PK.
> Below is the sentence about it in UEFI spec
> ```
> 3. If the firmware is in setup mode and the variable is one of:
> - The global PK variable;
> - The global KEK variable;
> - The "db" variable with GUID EFI_IMAGE_SECURITY_DATABASE_GUID; or
> - The "dbx" variable with GUID EFI_IMAGE_SECURITY_DATABASE_GUID,
> then the firmware implementation shall consider the checks in the following steps 4 and 5 to
> have passed, and proceed with updating the variable value as outlined below.
> ```
> The step 4 is to verify the signature and the step 5 is to verify the cert.
> 
> After this change, when system is in Setup mode, setting a PK does not require authenticated variable descriptor.
> 
> Signed-off-by: Derek Lin <derek.lin2 at hpe.com<mailto:derek.lin2 at hpe.com>>
> Signed-off-by: cinnamon shia <cinnamon.shia at hpe.com<mailto:cinnamon.shia at hpe.com>>
> 
> 
> 
> 
> 


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