Add configuration option to boot an alternative 2nd stage

Add the ability for shim to load an optional configuration file.  This
new file is called "options.csv".  The configuration file is completely
optional. If used, it is located in the same directory as the booted
shim.  The "options.csv" file currently allows a single entry.  Other
options could be added to it in the future.  The first and only entry in
the file is the name of the secondary boot loader shim will load. The
"options.csv" file is in Unicode LE format.

This allows a signed shim to directly load a UKI without the need to
rename it to grub. Shim's transitive trust is maintained. If the
alternative 2nd stage can not be verified, it will not boot.

Signed-off-by: Eric Snowberg <eric.snowberg@oracle.com>

Makefile

@@ -38,7 +38,7 @@ CFLAGS += -DENABLE_SHIM_CERT
 else
 TARGETS += $(MMNAME) $(FBNAME)
 endif
-OBJS	= shim.o globals.o mok.o netboot.o cert.o dp.o replacements.o tpm.o version.o errlog.o sbat.o sbat_data.o sbat_var.o pe.o pe-relocate.o httpboot.o csv.o load-options.o
+OBJS	= shim.o globals.o mok.o netboot.o cert.o dp.o replacements.o tpm.o version.o errlog.o sbat.o sbat_data.o sbat_var.o pe.o pe-relocate.o httpboot.o csv.o load-options.o utils.o
 KEYS	= shim_cert.h ocsp.* ca.* shim.crt shim.csr shim.p12 shim.pem shim.key shim.cer
 ORIG_SOURCES	= shim.c globals.c mok.c netboot.c dp.c replacements.c tpm.c errlog.c sbat.c pe.c pe-relocate.c httpboot.c shim.h version.h $(wildcard include/*.h) cert.S sbat_var.S
 MOK_OBJS = MokManager.o PasswordCrypt.o crypt_blowfish.o errlog.o sbat_data.o globals.o dp.o

include/load-options.h

@@ -13,6 +13,7 @@ EFI_STATUS generate_path_from_image_path(EFI_LOADED_IMAGE *li,
 EFI_STATUS parse_load_options(EFI_LOADED_IMAGE *li);
 
 extern CHAR16 *second_stage;
+extern CHAR16 *optional_second_stage;
 extern void *load_options;
 extern UINT32 load_options_size;
 

load-options.c

@@ -6,6 +6,7 @@
 #include "shim.h"
 
 CHAR16 *second_stage;
+CHAR16 *optional_second_stage = NULL;
 void *load_options;
 UINT32 load_options_size;
 

shim.c

@@ -1256,7 +1256,7 @@ EFI_STATUS set_second_stage (EFI_HANDLE image_handle)
 	EFI_STATUS efi_status;
 	EFI_LOADED_IMAGE *li = NULL;
 
-	second_stage = DEFAULT_LOADER;
+	second_stage = (optional_second_stage) ? optional_second_stage : DEFAULT_LOADER;
 	load_options = NULL;
 	load_options_size = 0;
 
@@ -1668,6 +1668,85 @@ load_unbundled_trust(EFI_HANDLE image_handle)
 	return efi_status;
 }
 
+/* Read optional options file */
+EFI_STATUS
+load_shim_options(EFI_HANDLE image_handle)
+{
+	EFI_STATUS efi_status;
+	EFI_HANDLE device;
+	EFI_LOADED_IMAGE *li = NULL;
+	EFI_FILE_IO_INTERFACE *drive;
+	EFI_FILE *root;
+	EFI_FILE_HANDLE ofile;
+	CHAR16 *PathName = NULL;
+	CHAR16 *buffer;
+	UINTN comma0;
+	UINT64 bs;
+
+	efi_status = gBS->HandleProtocol(image_handle, &EFI_LOADED_IMAGE_GUID,
+					 (void **)&li);
+	if (EFI_ERROR(efi_status)) {
+		perror(L"Unable to init protocol\n");
+		return efi_status;
+	}
+
+	efi_status = generate_path_from_image_path(li, L"options.csv", &PathName);
+	if (EFI_ERROR(efi_status))
+		goto done;
+
+	device = li->DeviceHandle;
+
+	efi_status = BS->HandleProtocol(device, &EFI_SIMPLE_FILE_SYSTEM_GUID,
+					(void **) &drive);
+	if (EFI_ERROR(efi_status))
+		goto done;
+
+	efi_status = drive->OpenVolume(drive, &root);
+	if (EFI_ERROR(efi_status)) {
+		perror(L"Failed to open fs: %r\n", efi_status);
+		goto done;
+	}
+
+	efi_status = root->Open(root, &ofile, PathName, EFI_FILE_READ_ONLY, 0);
+	if (EFI_ERROR(efi_status)) {
+		if (efi_status != EFI_NOT_FOUND)
+			perror(L"Failed to open %s - %r\n", PathName, efi_status);
+		goto done;
+	}
+
+	efi_status = read_file(ofile, PathName, &buffer, &bs);
+	if (EFI_ERROR(efi_status)) {
+		perror(L"Failed to read file\n");
+		goto done;
+	}
+
+	/*
+	 * This file may or may not start with the Unicode byte order marker.
+	 * Since UEFI is defined as LE, this file must also be LE.
+	 * If we find the LE byte order marker, just skip its.
+	 */
+	if (*buffer == 0xfeff)
+		buffer++;
+
+	comma0 = StrCSpn(buffer, L",");
+	if (comma0 == 0) {
+		perror(L"Invalid csv file\n");
+		goto done;
+	}
+
+	/*
+	 * Currently the options.csv file allows one entry for the optional
+	 * secondary boot stage, anything afterwards is skipped.
+	 */
+	buffer[comma0] = L'\0';
+	console_print(L"Optional 2nd stage: %s\n", buffer);
+	optional_second_stage=buffer;
+
+done:
+	FreePool(PathName);
+	return EFI_SUCCESS;
+}
+
 EFI_STATUS
 shim_init(void)
 {
@@ -1952,6 +2031,8 @@ efi_main (EFI_HANDLE passed_image_handle, EFI_SYSTEM_TABLE *passed_systab)
 	 */
 	(void) del_variable(SHIM_RETAIN_PROTOCOL_VAR_NAME, SHIM_LOCK_GUID);
 
+	load_shim_options(image_handle);
+
 	efi_status = shim_init();
 	if (EFI_ERROR(efi_status)) {
 		msg = SHIM_INIT;