[UPSTREAM][RFC PATCH] integrity: Load mokx certs from the EFI MOK config table

[Dimitri John Ledkov]

Refactor load_moklist_certs() to load either MokListRT into db, or
MokListXRT into dbx. Call load_moklist_certs() twice - first to load
mokx certs into dbx, then mok certs into db.

This thus now attempts to load mokx certs via the EFI MOKvar config
table first, and if that fails, via the EFI variable. Previously mokx
certs were only loaded via the EFI variable. Which fails when
MokListXRT is large and instead of MokListXRT is only available as
MokListXRT{1,2,3}. This is the case with Ubuntu's 15.4 based
shim. This patch is required to address CVE-2020-26541 when
certificates are revoked via MokListXRT.

Fixes: ebd9c2ae369a45bdd9f8615484db09be58fc242b

Signed-off-by: Dimitri John Ledkov <dimitri.ledkov at canonical.com>
---
 @Ubuntu kernel-team / security-team

 Presubmitting this patch internally, to see if it's ok to submit
 upstream. Intention is to send this to keyrings@
 linux-security-module@ linux-kernel@ Eric Snowberg etc that have
 introduced the referenced feature into v5.13-rc1 which on ubuntu is
 buggy and doesn't address the CVE in question.

 security/integrity/platform_certs/load_uefi.c | 73 ++++++++++---------
 1 file changed, 39 insertions(+), 34 deletions(-)

diff --git a/security/integrity/platform_certs/load_uefi.c b/security/integrity/platform_certs/load_uefi.c
index f290f78c3f301..e1f4ebb71abe0 100644
--- a/security/integrity/platform_certs/load_uefi.c
+++ b/security/integrity/platform_certs/load_uefi.c
@@ -66,17 +66,18 @@ static __init void *get_cert_list(efi_char16_t *name, efi_guid_t *guid,
 }
 
 /*
- * load_moklist_certs() - Load MokList certs
+ * load_moklist_certs() - Load Mok(X)List certs
+ * @load_db: Load MokListRT into db when true; MokListXRT into dbx when false
  *
- * Load the certs contained in the UEFI MokListRT database into the
- * platform trusted keyring.
+ * Load the certs contained in the UEFI MokList(X)RT database into the
+ * platform trusted/denied keyring.
  *
  * This routine checks the EFI MOK config table first. If and only if
- * that fails, this routine uses the MokListRT ordinary UEFI variable.
+ * that fails, this routine uses the MokList(X)RT ordinary UEFI variable.
  *
  * Return:	Status
  */
-static int __init load_moklist_certs(void)
+static int __init load_moklist_certs(const bool load_db)
 {
 	struct efi_mokvar_table_entry *mokvar_entry;
 	efi_guid_t mok_var = EFI_SHIM_LOCK_GUID;
@@ -84,41 +85,54 @@ static int __init load_moklist_certs(void)
 	unsigned long moksize;
 	efi_status_t status;
 	int rc;
+	char *mokvar_name = "MokListRT";
+	efi_char16_t *efivar_name = L"MokListRT";
+	char *parse_mokvar_name = "UEFI:MokListRT (MOKvar table)";
+	char *parse_efivar_name = "UEFI:MokListRT";
+	efi_element_handler_t (*get_handler_for_guid)(const efi_guid_t *) = get_handler_for_db;
+
+	if (!load_db) {
+		mokvar_name = "MokListXRT";
+		efivar_name = L"MokListXRT";
+		parse_mokvar_name = "UEFI:MokListXRT (MOKvar table)";
+		parse_efivar_name = "UEFI:MokListXRT";
+		get_handler_for_guid = get_handler_for_dbx;
+	}
 
 	/* First try to load certs from the EFI MOKvar config table.
 	 * It's not an error if the MOKvar config table doesn't exist
 	 * or the MokListRT entry is not found in it.
 	 */
-	mokvar_entry = efi_mokvar_entry_find("MokListRT");
+	mokvar_entry = efi_mokvar_entry_find(mokvar_name);
 	if (mokvar_entry) {
-		rc = parse_efi_signature_list("UEFI:MokListRT (MOKvar table)",
+		rc = parse_efi_signature_list(parse_mokvar_name,
 					      mokvar_entry->data,
 					      mokvar_entry->data_size,
-					      get_handler_for_db);
+					      get_handler_for_guid);
 		/* All done if that worked. */
 		if (!rc)
 			return rc;
 
-		pr_err("Couldn't parse MokListRT signatures from EFI MOKvar config table: %d\n",
-		       rc);
+		pr_err("Couldn't parse %s signatures from EFI MOKvar config table: %d\n",
+		       mokvar_name, rc);
 	}
 
 	/* Get MokListRT. It might not exist, so it isn't an error
 	 * if we can't get it.
 	 */
-	mok = get_cert_list(L"MokListRT", &mok_var, &moksize, &status);
+	mok = get_cert_list(efivar_name, &mok_var, &moksize, &status);
 	if (mok) {
-		rc = parse_efi_signature_list("UEFI:MokListRT",
-					      mok, moksize, get_handler_for_db);
+		rc = parse_efi_signature_list(parse_efivar_name,
+					      mok, moksize, get_handler_for_guid);
 		kfree(mok);
 		if (rc)
-			pr_err("Couldn't parse MokListRT signatures: %d\n", rc);
+			pr_err("Couldn't parse %s signatures: %d\n", mokvar_name, rc);
 		return rc;
 	}
 	if (status == EFI_NOT_FOUND)
-		pr_debug("MokListRT variable wasn't found\n");
+		pr_debug("%s variable wasn't found\n", mokvar_name);
 	else
-		pr_info("Couldn't get UEFI MokListRT\n");
+		pr_info("Couldn't get UEFI %s\n", mokvar_name);
 	return 0;
 }
 
@@ -132,9 +146,8 @@ static int __init load_moklist_certs(void)
 static int __init load_uefi_certs(void)
 {
 	efi_guid_t secure_var = EFI_IMAGE_SECURITY_DATABASE_GUID;
-	efi_guid_t mok_var = EFI_SHIM_LOCK_GUID;
-	void *db = NULL, *dbx = NULL, *mokx = NULL;
-	unsigned long dbsize = 0, dbxsize = 0, mokxsize = 0;
+	void *db = NULL, *dbx = NULL;
+	unsigned long dbsize = 0, dbxsize = 0;
 	efi_status_t status;
 	int rc = 0;
 
@@ -176,23 +189,15 @@ static int __init load_uefi_certs(void)
 		kfree(dbx);
 	}
 
-	mokx = get_cert_list(L"MokListXRT", &mok_var, &mokxsize, &status);
-	if (!mokx) {
-		if (status == EFI_NOT_FOUND)
-			pr_debug("mokx variable wasn't found\n");
-		else
-			pr_info("Couldn't get mokx list\n");
-	} else {
-		rc = parse_efi_signature_list("UEFI:MokListXRT",
-					      mokx, mokxsize,
-					      get_handler_for_dbx);
-		if (rc)
-			pr_err("Couldn't parse mokx signatures %d\n", rc);
-		kfree(mokx);
-	}
+	/* Load the MokListXRT certs */
+	rc = load_moklist_certs(false);
+	if (rc)
+		pr_err("Couldn't parse mokx signatures: %d\n", rc);
 
 	/* Load the MokListRT certs */
-	rc = load_moklist_certs();
+	rc = load_moklist_certs(true);
+	if (rc)
+		pr_err("Couldn't parse mok signatures: %d\n", rc);
 
 	return rc;
 }
-- 
2.27.0