efi: Only print errors about failing to get certs if EFI vars are found

If CONFIG_LOAD_UEFI_KEYS is enabled, the kernel attempts to load the certs
from the db, dbx and MokListRT EFI variables into the appropriate keyrings.

But it just assumes that the variables will be present and prints an error
if the certs can't be loaded, even when is possible that the variables may
not exist. For example the MokListRT variable will only be present if shim
is used.

So only print an error message about failing to get the certs list from an
EFI variable if this is found. Otherwise these printed errors just pollute
the kernel log ring buffer with confusing messages like the following:

[    5.427251] Couldn't get size: 0x800000000000000e
[    5.427261] MODSIGN: Couldn't get UEFI db list
[    5.428012] Couldn't get size: 0x800000000000000e
[    5.428023] Couldn't get UEFI MokListRT
Reported-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>
Tested-by: Hans de Goede <hdegoede@redhat.com>
Acked-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Mimi Zohar <zohar@linux.ibm.com>

security/integrity/platform_certs/load_uefi.c

@@ -35,16 +35,18 @@ static __init bool uefi_check_ignore_db(void)
  * Get a certificate list blob from the named EFI variable.
  */
 static __init void *get_cert_list(efi_char16_t *name, efi_guid_t *guid,
-				  unsigned long *size)
+				  unsigned long *size, efi_status_t *status)
 {
-	efi_status_t status;
 	unsigned long lsize = 4;
 	unsigned long tmpdb[4];
 	void *db;
 
-	status = efi.get_variable(name, guid, NULL, &lsize, &tmpdb);
-	if (status != EFI_BUFFER_TOO_SMALL) {
-		pr_err("Couldn't get size: 0x%lx\n", status);
+	*status = efi.get_variable(name, guid, NULL, &lsize, &tmpdb);
+	if (*status == EFI_NOT_FOUND)
+		return NULL;
+
+	if (*status != EFI_BUFFER_TOO_SMALL) {
+		pr_err("Couldn't get size: 0x%lx\n", *status);
 		return NULL;
 	}
 
@@ -52,10 +54,10 @@ static __init void *get_cert_list(efi_char16_t *name, efi_guid_t *guid,
 	if (!db)
 		return NULL;
 
-	status = efi.get_variable(name, guid, NULL, &lsize, db);
-	if (status != EFI_SUCCESS) {
+	*status = efi.get_variable(name, guid, NULL, &lsize, db);
+	if (*status != EFI_SUCCESS) {
 		kfree(db);
-		pr_err("Error reading db var: 0x%lx\n", status);
+		pr_err("Error reading db var: 0x%lx\n", *status);
 		return NULL;
 	}
 
@@ -74,6 +76,7 @@ static int __init load_uefi_certs(void)
 	efi_guid_t mok_var = EFI_SHIM_LOCK_GUID;
 	void *db = NULL, *dbx = NULL, *mok = NULL;
 	unsigned long dbsize = 0, dbxsize = 0, moksize = 0;
+	efi_status_t status;
 	int rc = 0;
 
 	if (!efi.get_variable)
@@ -83,8 +86,11 @@ static int __init load_uefi_certs(void)
 	 * an error if we can't get them.
 	 */
 	if (!uefi_check_ignore_db()) {
-		db = get_cert_list(L"db", &secure_var, &dbsize);
+		db = get_cert_list(L"db", &secure_var, &dbsize, &status);
 		if (!db) {
+			if (status == EFI_NOT_FOUND)
+				pr_debug("MODSIGN: db variable wasn't found\n");
+			else
 				pr_err("MODSIGN: Couldn't get UEFI db list\n");
 		} else {
 			rc = parse_efi_signature_list("UEFI:db",
@@ -96,8 +102,11 @@ static int __init load_uefi_certs(void)
 		}
 	}
 
-	mok = get_cert_list(L"MokListRT", &mok_var, &moksize);
+	mok = get_cert_list(L"MokListRT", &mok_var, &moksize, &status);
 	if (!mok) {
+		if (status == EFI_NOT_FOUND)
+			pr_debug("MokListRT variable wasn't found\n");
+		else
 			pr_info("Couldn't get UEFI MokListRT\n");
 	} else {
 		rc = parse_efi_signature_list("UEFI:MokListRT",
@@ -107,8 +116,11 @@ static int __init load_uefi_certs(void)
 		kfree(mok);
 	}
 
-	dbx = get_cert_list(L"dbx", &secure_var, &dbxsize);
+	dbx = get_cert_list(L"dbx", &secure_var, &dbxsize, &status);
 	if (!dbx) {
+		if (status == EFI_NOT_FOUND)
+			pr_debug("dbx variable wasn't found\n");
+		else
 			pr_info("Couldn't get UEFI dbx list\n");
 	} else {
 		rc = parse_efi_signature_list("UEFI:dbx",