[PATCH 21/22] arm: efistub: ignore dtb= when UEFI SecureBoot is enabled

[Leif Lindholm]

From: Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx>

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@xxxxxxxxxx>
Signed-off-by: Leif Lindholm <leif.lindholm@xxxxxxxxxx>
---
 drivers/firmware/efi/arm-stub.c        |    4 +++-
 drivers/firmware/efi/efi-stub-helper.c |   24 ++++++++++++++++++++++++
 2 files changed, 27 insertions(+), 1 deletion(-)

diff --git a/drivers/firmware/efi/arm-stub.c b/drivers/firmware/efi/arm-stub.c
index b505fde..c651082 100644
--- a/drivers/firmware/efi/arm-stub.c
+++ b/drivers/firmware/efi/arm-stub.c
@@ -95,7 +95,9 @@ unsigned long efi_entry(void *handle, efi_system_table_t *sys_table,
 
 	/* Load a device tree from the configuration table, if present. */
 	fdt_addr = (uintptr_t)get_fdt(sys_table);
-	if (!fdt_addr) {
+	if (efi_secureboot_enabled(sys_table))
+		pr_efi(sys_table, "UEFI Secure Boot is enabled, ignoring dtb= commandline option.\n");
+	else if (!fdt_addr) {
 		status = handle_cmdline_files(sys_table, image, cmdline_ptr,
 					      "dtb=",
 					      ~0UL, (unsigned long *)&fdt_addr,
diff --git a/drivers/firmware/efi/efi-stub-helper.c b/drivers/firmware/efi/efi-stub-helper.c
index 2ee69ea..6221be7 100644
--- a/drivers/firmware/efi/efi-stub-helper.c
+++ b/drivers/firmware/efi/efi-stub-helper.c
@@ -721,3 +721,27 @@ 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

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