commands/bli: Set LoaderTpm2ActivePcrBanks runtime variable

It turns out checking from userspace is not 100% reliable to figure out
whether the firmware had TPM2 support enabled or not. For example with
EDK2 arm64, the default upstream build config bundles TPM2 support with
SecureBoot support, so if the latter is disabled, TPM2 is also unavailable.
But still, the ACPI TPM2 table is created just as if it was enabled. So,
/sys/firmware/acpi/tables/TPM2 exists and looks correct but there are no
measurements, neither the firmware nor the loader/stub can do them, and
/sys/kernel/security/tpm0/binary_bios_measurements does not exist.
So, userspace cannot really tell what was going on in UEFI mode.

The loader can use the apposite UEFI protocol to check, which is a more
definitive answer. Export the bitmask with the list of active banks as-is.
If it's not 0, then in userspace we can be sure a working TPM2 was available
in UEFI mode.

systemd-boot and systemd-stub v258 (current main) set this variable and
userspace portion consumes it to be able to tell what was available in
the firmware context.

Signed-off-by: Luca Boccassi <luca.boccassi@gmail.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>

grub-core/commands/efi/tpm.c

@@ -332,3 +332,36 @@ grub_tpm_present (void)
       return grub_tpm2_present (tpm);
     }
 }
+
+grub_uint32_t
+grub_tpm2_active_pcr_banks (void)
+{
+  grub_efi_handle_t tpm_handle;
+  grub_efi_uint8_t protocol_version;
+  grub_efi_tpm2_protocol_t *tpm;
+  grub_efi_uint32_t active_pcr_banks = 0;
+
+  if (!grub_tpm_handle_find (&tpm_handle, &protocol_version))
+    return 0;
+
+  if (protocol_version == 1)
+    return 0; /* We report TPM2 status */
+
+  tpm = grub_efi_open_protocol (tpm_handle, &tpm2_guid,
+				GRUB_EFI_OPEN_PROTOCOL_GET_PROTOCOL);
+  if (tpm == NULL)
+    {
+      grub_dprintf ("tpm", "Cannot open TPM2 protocol\n");
+      return 0;
+    }
+
+  if (grub_tpm2_present (tpm))
+    {
+      grub_efi_status_t status = tpm->get_active_pcr_banks (tpm, &active_pcr_banks);
+
+      if (status != GRUB_EFI_SUCCESS)
+	return 0; /* Assume none available if the call fails. */
+    }
+
+  return active_pcr_banks;
+}