kern/efi/sb: Reject non-kernel files in the shim_lock verifier

We must not allow other verifiers to pass things like the GRUB modules.
Instead of maintaining a blocklist, maintain an allowlist of things
that we do not care about.

This allowlist really should be made reusable, and shared by the
lockdown verifier, but this is the minimal patch addressing
security concerns where the TPM verifier was able to mark modules
as verified (or the OpenPGP verifier for that matter), when it
should not do so on shim-powered secure boot systems.

Fixes: CVE-2022-28735

Signed-off-by: Julian Andres Klode <julian.klode@canonical.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>

grub-core/kern/efi/sb.c

@@ -119,10 +119,11 @@ shim_lock_verifier_init (grub_file_t io __attribute__ ((unused)),
 			 void **context __attribute__ ((unused)),
 			 enum grub_verify_flags *flags)
 {
-  *flags = GRUB_VERIFY_FLAGS_SKIP_VERIFICATION;
+  *flags = GRUB_VERIFY_FLAGS_NONE;
 
   switch (type & GRUB_FILE_TYPE_MASK)
     {
+    /* Files we check. */
     case GRUB_FILE_TYPE_LINUX_KERNEL:
     case GRUB_FILE_TYPE_MULTIBOOT_KERNEL:
     case GRUB_FILE_TYPE_BSD_KERNEL:
@@ -130,11 +131,43 @@ shim_lock_verifier_init (grub_file_t io __attribute__ ((unused)),
     case GRUB_FILE_TYPE_PLAN9_KERNEL:
     case GRUB_FILE_TYPE_EFI_CHAINLOADED_IMAGE:
       *flags = GRUB_VERIFY_FLAGS_SINGLE_CHUNK;
+      return GRUB_ERR_NONE;
 
-      /* Fall through. */
+    /* Files that do not affect secureboot state. */
+    case GRUB_FILE_TYPE_NONE:
+    case GRUB_FILE_TYPE_LOOPBACK:
+    case GRUB_FILE_TYPE_LINUX_INITRD:
+    case GRUB_FILE_TYPE_OPENBSD_RAMDISK:
+    case GRUB_FILE_TYPE_XNU_RAMDISK:
+    case GRUB_FILE_TYPE_SIGNATURE:
+    case GRUB_FILE_TYPE_PUBLIC_KEY:
+    case GRUB_FILE_TYPE_PUBLIC_KEY_TRUST:
+    case GRUB_FILE_TYPE_PRINT_BLOCKLIST:
+    case GRUB_FILE_TYPE_TESTLOAD:
+    case GRUB_FILE_TYPE_GET_SIZE:
+    case GRUB_FILE_TYPE_FONT:
+    case GRUB_FILE_TYPE_ZFS_ENCRYPTION_KEY:
+    case GRUB_FILE_TYPE_CAT:
+    case GRUB_FILE_TYPE_HEXCAT:
+    case GRUB_FILE_TYPE_CMP:
+    case GRUB_FILE_TYPE_HASHLIST:
+    case GRUB_FILE_TYPE_TO_HASH:
+    case GRUB_FILE_TYPE_KEYBOARD_LAYOUT:
+    case GRUB_FILE_TYPE_PIXMAP:
+    case GRUB_FILE_TYPE_GRUB_MODULE_LIST:
+    case GRUB_FILE_TYPE_CONFIG:
+    case GRUB_FILE_TYPE_THEME:
+    case GRUB_FILE_TYPE_GETTEXT_CATALOG:
+    case GRUB_FILE_TYPE_FS_SEARCH:
+    case GRUB_FILE_TYPE_LOADENV:
+    case GRUB_FILE_TYPE_SAVEENV:
+    case GRUB_FILE_TYPE_VERIFY_SIGNATURE:
+      *flags = GRUB_VERIFY_FLAGS_SKIP_VERIFICATION;
+      return GRUB_ERR_NONE;
 
+    /* Other files. */
     default:
-      return GRUB_ERR_NONE;
+      return grub_error (GRUB_ERR_ACCESS_DENIED, N_("prohibited by secure boot policy"));
     }
 }
 

include/grub/verify.h

@@ -24,6 +24,7 @@
 
 enum grub_verify_flags
   {
+    GRUB_VERIFY_FLAGS_NONE		= 0,
     GRUB_VERIFY_FLAGS_SKIP_VERIFICATION	= 1,
     GRUB_VERIFY_FLAGS_SINGLE_CHUNK	= 2,
     /* Defer verification to another authority. */