repeating my comment
<https://bugzilla.redhat.com/show_bug.cgi?id=1871403#c2> here:

"Custom Mode" is not defined in the UEFI specification, it is an
edk2-specific Secure Boot operational mode.

"Custom Mode" means that updates to the PK, KEK, db, and dbx
authenticated UEFI variables are not verified.

Therefore, you certainly do *not* want to boot a guest operating system
in "Custom Mode" (as long as you care for Secure Boot at all, that is,
with that particular guest OS). Because in "Custom Mode", the guest OS
could trivially replace the certificates and hashes that control Secure
Boot.

"Custom Mode" is therefore a shortcut for a "trusted agent" to enroll
PK, KEK, db, dbx, without being subject to verification against existent
PK, KEK, etc contents.

Thus, "Custom Mode" is temporary. Once you have your certificates
enrolled, the SB operational mode flips back to "Standard Mode" --
meaning that *further* updates against those authenticated UEFI
variables will be verified (as they should).


The visual form on which you manipulate these settings in the Setup TUI
is implemented in edk2 by the
"SecurityPkg/VariableAuthenticated/SecureBootConfigDxe" driver. In
"SecureBootConfigImpl.c", function SecureBootCallback(), which is sort
of the "user action handler" for the form
(EFI_HII_ACCESS_FORM_CALLBACK), we have the following snippet:

>|   } else if (Action == EFI_BROWSER_ACTION_FORM_CLOSE) {
>|     //
>|     // Force the platform back to Standard Mode once user leave the setup screen.
>|     //
>|     GetVariable2 (EFI_CUSTOM_MODE_NAME, &gEfiCustomModeEnableGuid, (VOID**)&SecureBootMode, NULL);
>|     if (NULL != SecureBootMode && *SecureBootMode == CUSTOM_SECURE_BOOT_MODE) {
>|       IfrNvData->SecureBootMode = STANDARD_SECURE_BOOT_MODE;
>|       SetSecureBootMode(STANDARD_SECURE_BOOT_MODE);
>|     }

(This comes from historical commit f71ed839e1b5, dated 2013-01-09, which
says "Set the secure boot state to Standard Mode when user leaving
secure boot setup page".)

Another example, from "OvmfPkg/EnrollDefaultKeys/EnrollDefaultKeys.c":

>|   //
>|   // Enter Custom Mode so we can enroll PK, KEK, db, and dbx without signature
>|   // checks on those variable writes.
>|   //
>| ...
>|   //
>|   // Leave Custom Mode, so that updates to PK, KEK, db, and dbx require valid
>|   // signatures.
>|   //
>| ...

So the behavior you are seeing is by design -- "Custom Mode" is not a
permanent setting, but a temporary mode for agents such as
SecureBootConfigDxe, and OVMF's "EnrollDefaultKeys.efi".


At the end of "EnrollDefaultKeys/EnrollDefaultKeys.c", there is a
"diagram" which attempts to organize the SB-related "control variables"
into a tree:

>|   //                                 [SetupMode]
>|   //                        (read-only, standardized by UEFI)
>|   //                                /                \_
>|   //                               0               1, default
>|   //                              /                    \_
>|   //                      PK enrolled                   no PK enrolled yet,
>|   //              (this is called "User Mode")          PK enrollment possible
>|   //                             |
>|   //                             |
>|   //                     [SecureBootEnable]
>|   //         (read-write, edk2-specific, boot service only)
>|   //                /                           \_
>|   //               0                         1, default
>|   //              /                               \_
>|   //       [SecureBoot]=0                     [SecureBoot]=1
>|   // (read-only, standardized by UEFI)  (read-only, standardized by UEFI)
>|   //     images are not verified         images are verified, platform is
>|   //                                      operating in Secure Boot mode
>|   //                                                 |
>|   //                                                 |
>|   //                                           [CustomMode]
>|   //                          (read-write, edk2-specific, boot service only)
>|   //                                /                           \_
>|   //                          0, default                         1
>|   //                              /                               \_
>|   //                      PK, KEK, db, dbx                PK, KEK, db, dbx
>|   //                    updates are verified          updates are not verified

"Standard mode" is just a symbolic name for CustomMode=0.

This upstream tracker does not have a NOTABUG resolution, so I'm closing
this ticket as INVALID. Please reopen it if you encounter further
issues. Thanks!

Thanks for the detailed explanation.