[PATCH v2 15/15] efi/arm64: ignore dtb= when UEFI SecureBoot is enabled

[Leif Lindholm]

From: Ard Biesheuvel <ard.biesheuvel at linaro.org>

Loading unauthenticated FDT blobs directly from storage is a security hazard,
so this should only be allowed when running with UEFI Secure Boot disabled.

Signed-off-by: Ard Biesheuvel <ard.biesheuvel at linaro.org>
Signed-off-by: Leif Lindholm <leif.lindholm at linaro.org>
---
 drivers/firmware/efi/arm-stub.c        |   23 ++++++++++++++---------
 drivers/firmware/efi/efi-stub-helper.c |   24 ++++++++++++++++++++++++
 2 files changed, 38 insertions(+), 9 deletions(-)

diff --git a/drivers/firmware/efi/arm-stub.c b/drivers/firmware/efi/arm-stub.c
index b505fde..733723c 100644
--- a/drivers/firmware/efi/arm-stub.c
+++ b/drivers/firmware/efi/arm-stub.c
@@ -41,7 +41,7 @@ unsigned long efi_entry(void *handle, efi_system_table_t *sys_table,
 	/* addr/point and size pairs for memory management*/
 	unsigned long initrd_addr;
 	u64 initrd_size = 0;
-	unsigned long fdt_addr;  /* Original DTB */
+	unsigned long fdt_addr = 0;  /* Original DTB */
 	u64 fdt_size = 0;  /* We don't get size from configuration table */
 	char *cmdline_ptr = NULL;
 	int cmdline_size = 0;
@@ -93,15 +93,20 @@ unsigned long efi_entry(void *handle, efi_system_table_t *sys_table,
 		goto fail_free_image;
 	}
 
-	/* Load a device tree from the configuration table, if present. */
-	fdt_addr = (uintptr_t)get_fdt(sys_table);
+	/*
+	 * Unauthenticated device tree data is a security hazard, so
+	 * ignore 'dtb=' unless UEFI Secure Boot is disabled.
+	 */
+	if (efi_secureboot_enabled(sys_table))
+		pr_efi(sys_table, "UEFI Secure Boot is enabled.\n");
+	else
+		handle_cmdline_files(sys_table, image, cmdline_ptr, "dtb=",
+				     ~0UL, (unsigned long *)&fdt_addr,
+				     (unsigned long *)&fdt_size);
 	if (!fdt_addr) {
-		status = handle_cmdline_files(sys_table, image, cmdline_ptr,
-					      "dtb=",
-					      ~0UL, (unsigned long *)&fdt_addr,
-					      (unsigned long *)&fdt_size);
-
-		if (status != EFI_SUCCESS) {
+		/* Look for a device tree configuration table entry. */
+		fdt_addr = (uintptr_t)get_fdt(sys_table);
+		if (!fdt_addr) {
 			pr_efi_err(sys_table, "Failed to load device tree!\n");
 			goto fail_free_cmdline;
 		}
diff --git a/drivers/firmware/efi/efi-stub-helper.c b/drivers/firmware/efi/efi-stub-helper.c
index 4a9986b..f2014f2 100644
--- a/drivers/firmware/efi/efi-stub-helper.c
+++ b/drivers/firmware/efi/efi-stub-helper.c
@@ -704,3 +704,27 @@ static char *efi_convert_cmdline(efi_system_table_t *sys_table_arg,
 	*cmd_line_len = options_bytes;
 	return (char *)cmdline_addr;
 }
+
+static int __init efi_secureboot_enabled(efi_system_table_t *sys_table_arg)
+{
+	static efi_guid_t const var_guid __initconst = EFI_GLOBAL_VARIABLE_GUID;
+	static efi_char16_t const var_name[] __initconst = {
+		'S', 'e', 'c', 'u', 'r', 'e', 'B', 'o', 'o', 't', 0 };
+
+	efi_get_variable_t *f_getvar = sys_table_arg->runtime->get_variable;
+	unsigned long size = sizeof(u8);
+	efi_status_t status;
+	u8 val;
+
+	status = efi_call_phys5(f_getvar, (efi_char16_t *)var_name,
+				(efi_guid_t *)&var_guid, NULL, &size, &val);
+
+	switch (status) {
+	case EFI_SUCCESS:
+		return val;
+	case EFI_NOT_FOUND:
+		return 0;
+	default:
+		return 1;
+	}
+}
-- 
1.7.10.4