On 28. Feb 2023, at 07:12, Gerd Hoffmann <kraxel@redhat.com> wrote:

Hi,

(a) the SecureBoot variable is not present (EFI_NOT_FOUND) according to

the return value, or

@Maintainers Would there be any objection to drop this and skip the SB checks only when explicitly disabled?

Please explicitly respond even if not, so we don't end up with everyone silently agreeing, but forgetting about the patch after. Thanks! :)

I hold back v2, waiting for an answer here.

- if (*SecureBoot == SECURE_BOOT_MODE_DISABLE) {

- FreePool (SecureBoot);

+ if ((VarStatus == EFI_SUCCESS) && (SecureBoot == SECURE_BOOT_MODE_DISABLE)) {

I would check the attributes here as well. They should be BS | RT, but

explicitly not NV. This would force the SB checks in case a malicious

actor somehow managed to store a persistent disable-value variable (be

that a bug, physical access, or other means).

Like this (incremental fixup)?

Sorry, I formulated it a bit vague - what I meant is that the attributes should be exactly BS | RT (i.e., equal to), but I see how adding it must not be NV sounds like it should be just those three Bits checked. Otherwise, yes, thanks a lot!

It’s a read-only status reporting variable, so even with future changes, setting any of the other attributes wouldn’t make much sense. BS | RT is what the spec currently dictates (there is a table here: https://uefi.org/specs/UEFI/2.10/03_Boot_Manager.html).

Do we have macros for variable attribute checking?
Havn't seen anything while skimming variable-related headers ...

Don’t think so. Not sure there is much attribute-checking done to begin with outside VarCheckLib. The only stack I’ve seen doing this extensively is recent-years Mac EFI.

take care,
Gerd

diff --git a/SecurityPkg/Library/DxeImageVerificationLib/DxeImageVerificationLib.c b/SecurityPkg/Library/DxeImageVerificationLib/DxeImageVerificationLib.c
index f29a27e5a053..79c784f77ac8 100644
--- a/SecurityPkg/Library/DxeImageVerificationLib/DxeImageVerificationLib.c
+++ b/SecurityPkg/Library/DxeImageVerificationLib/DxeImageVerificationLib.c
@@ -1688,6 +1688,7 @@ DxeImageVerificationHandler (
  EFI_STATUS                    HashStatus;
  EFI_STATUS                    DbStatus;
  EFI_STATUS                    VarStatus;
+ UINT32                        VarAttr;
  BOOLEAN                       IsFound;

  SignatureList     = NULL;
@@ -1745,7 +1746,7 @@ DxeImageVerificationHandler (
  }

  SecureBootSize = sizeof (SecureBoot);
- VarStatus      = gRT->GetVariable (EFI_SECURE_BOOT_MODE_NAME, &gEfiGlobalVariableGuid, NULL, &SecureBootSize, &SecureBoot);
+ VarStatus      = gRT->GetVariable (EFI_SECURE_BOOT_MODE_NAME, &gEfiGlobalVariableGuid, &VarAttr, &SecureBootSize, &SecureBoot);
  //
  // Skip verification if SecureBoot variable doesn't exist.
  //
@@ -1756,7 +1757,12 @@ DxeImageVerificationHandler (
  //
  // Skip verification if SecureBoot is disabled but not AuditMode
  //
- if ((VarStatus == EFI_SUCCESS) && (SecureBoot == SECURE_BOOT_MODE_DISABLE)) {
+ if ((VarStatus == EFI_SUCCESS) &&
+      !(VarAttr & EFI_VARIABLE_NON_VOLATILE) &&
+      (VarAttr & EFI_VARIABLE_BOOTSERVICE_ACCESS) &&
+      (VarAttr & EFI_VARIABLE_RUNTIME_ACCESS) &&
+      (SecureBoot == SECURE_BOOT_MODE_DISABLE))
+ {
    return EFI_SUCCESS;
  }