efi: Allow the "db" UEFI variable to be suppressed

If a user tells shim to not use the certs/hashes in the UEFI db variable
for verification purposes, shim will set a UEFI variable called
MokIgnoreDB. Have the uefi import code look for this and ignore the db
variable if it is found.

[[e-mail]: removed reference to "secondary" keyring comment]
Signed-off-by: Josh Boyer <[e-mail]>
Signed-off-by: David Howells <[e-mail]>
Acked-by: Nayna Jain <[e-mail]>
Acked-by: Serge Hallyn <[e-mail]>
Reviewed-by: James Morris <[e-mail]>
Signed-off-by: Mimi Zohar <[e-mail]>

security/integrity/platform_certs/load_uefi.c

···
 static efi_guid_t efi_cert_sha256_guid __initdata = EFI_CERT_SHA256_GUID;
 
 /*
+ * Look to see if a UEFI variable called MokIgnoreDB exists and return true if
+ * it does.
+ *
+ * This UEFI variable is set by the shim if a user tells the shim to not use
+ * the certs/hashes in the UEFI db variable for verification purposes.  If it
+ * is set, we should ignore the db variable also and the true return indicates
+ * this.
+ */
+static __init bool uefi_check_ignore_db(void)
+{
+	efi_status_t status;
+	unsigned int db = 0;
+	unsigned long size = sizeof(db);
+	efi_guid_t guid = EFI_SHIM_LOCK_GUID;
+
+	status = efi.get_variable(L"MokIgnoreDB", &guid, NULL, &size, &db);
+	return status == EFI_SUCCESS;
+}
+
+/*
  * Get a certificate list blob from the named EFI variable.
  */
 static __init void *get_cert_list(efi_char16_t *name, efi_guid_t *guid,
···
 }
 
 /*
- * Load the certs contained in the UEFI databases
+ * Load the certs contained in the UEFI databases into the platform trusted
+ * keyring and the UEFI blacklisted X.509 cert SHA256 hashes into the blacklist
+ * keyring.
  */
 static int __init load_uefi_certs(void)
 {
···
 	/* Get db, MokListRT, and dbx.  They might not exist, so it isn't
 	 * an error if we can't get them.
 	 */
-	db = get_cert_list(L"db", &secure_var, &dbsize);
-	if (!db) {
-		pr_err("Couldn't get UEFI db list\n");
-	} else {
-		rc = parse_efi_signature_list("UEFI:db",
-					      db, dbsize, get_handler_for_db);
-		if (rc)
-			pr_err("Couldn't parse db signatures: %d\n", rc);
-		kfree(db);
+	if (!uefi_check_ignore_db()) {
+		db = get_cert_list(L"db", &secure_var, &dbsize);
+		if (!db) {
+			pr_err("MODSIGN: Couldn't get UEFI db list\n");
+		} else {
+			rc = parse_efi_signature_list("UEFI:db",
+					db, dbsize, get_handler_for_db);
+			if (rc)
+				pr_err("Couldn't parse db signatures: %d\n",
+				       rc);
+			kfree(db);
+		}
 	}
 
 	mok = get_cert_list(L"MokListRT", &mok_var, &moksize);