From b9adb76886d4ad07b5f700c293ffb2369d100177 Mon Sep 17 00:00:00 2001
From: Pete Batard <pete@akeo.ie>
Date: Tue, 5 Sep 2023 17:18:37 +0100
Subject: [PATCH] add Windows bootloader detection to better report on the
 reasons for a Windows boot failure

* Unfortunately, Microsoft designed their Windows UEFI bootloaders to not halt and report an
  explicit error when a boot issue arises, but instead *silently* hand over to the next
  available boot device (which is accomplished by having the UEFI bootloader return error
  code EFI_NO_MAPPING).
* This leaves many users very confused as to why their Windows installation media will not
  boot, even more so in light of the BlackLotus Microsoft bootloader revocation, where users
  will get EFI_NO_MAPPING silently being returned when the UEFI lock is enabled and they
  attempt to boot using a pre-2023.05 Windows bootloader.
* Because of this, we attempt to detect Windows bootloaders (which are all compiled from
  binaries containing a 'bootmgr.dll' string) and report a more explicit error and potential
  cause when a Windows UEFI bootloader returns EFI_NO_MAPPING.
---
 boot.c | 36 +++++++++++++++++++++++++++++++++---
 1 file changed, 33 insertions(+), 3 deletions(-)

diff --git a/boot.c b/boot.c
index 4f1843f..272210b 100644
--- a/boot.c
+++ b/boot.c
@@ -210,10 +210,14 @@ EFI_STATUS EFIAPI efi_main(EFI_HANDLE BaseImageHandle, EFI_SYSTEM_TABLE *SystemT
 	EFI_FILE_SYSTEM_VOLUME_LABEL* VolumeInfo;
 	EFI_FILE_HANDLE Root;
 	EFI_BLOCK_IO_PROTOCOL *BlockIo;
+	// We'll search for "bootmgr.dll" in UEFI bootloaders to identify Windows
+	// bootloaders, but we don't want to match our own bootloader in the process.
+	// So we use a modifiable string buffer where the first character is not set.
+	CHAR8 BootMgrName[] = "_ootmgr.dll", BootMgrNameFirstLetter = 'b';
 	CHAR8* Buffer;
 	INTN SecureBootStatus;
 	UINTN Index, FsType = 0, Try, Event, HandleCount = 0, Size;
-	BOOLEAN SameDevice;
+	BOOLEAN SameDevice, WindowsBootMgr = FALSE;
 
 #if defined(_GNU_EFI)
 	InitializeLib(BaseImageHandle, SystemTable);
@@ -451,9 +455,35 @@ EFI_STATUS EFIAPI efi_main(EFI_HANDLE BaseImageHandle, EFI_SYSTEM_TABLE *SystemT
 		goto out;
 	}
 
+	// Look for a "bootmgr.dll" string in the loaded image to identify a Windows bootloader.
+	BootMgrName[0] = BootMgrNameFirstLetter;
+	Status = gBS->OpenProtocol(ImageHandle, &gEfiLoadedImageProtocolGuid,
+		(VOID**)&LoadedImage, MainImageHandle, NULL, EFI_OPEN_PROTOCOL_GET_PROTOCOL);
+	if (EFI_ERROR(Status)) {
+		PrintWarning(L"  Unable to inspect loaded executable");
+	} else for (Index = 0x40; Index < LoadedImage->ImageSize - sizeof(BootMgrName); Index++) {
+		if (CompareMem((CHAR8*)((UINTN)LoadedImage->ImageBase + Index),
+			BootMgrName, sizeof(BootMgrName)) == 0) {
+			WindowsBootMgr = TRUE;
+			PrintInfo(L"Starting Microsoft Windows bootmgr...");
+			break;
+		}
+	}
+
 	Status = gBS->StartImage(ImageHandle, NULL, NULL);
-	if (EFI_ERROR(Status))
-		PrintError(L"  Start failure");
+	if (EFI_ERROR(Status)) {
+		// Windows bootmgr simply returns EFI_NO_MAPPING on any internal error or security
+		// violation, instead of halting and explicitly reporting the issue, leaving many
+		// users extremely confused as to why their media did not boot. This can happen, for
+		// instance, if the machine has had the BlackLotus UEFI lock enabled and the user
+		// attempts to boot a pre 2023.05 version of the Windows installers.
+		// We therefore take it upon ourselves to report what Windows bootmgr will not report.
+		if (Status == EFI_NO_MAPPING && WindowsBootMgr) {
+			SetText(TEXT_RED); Print(L"[FAIL]"); DefText();
+			Print(L"   Windows bootmgr encountered a security validation or internal error\n");
+		} else
+			PrintError(L"  Start failure");
+	}
 
 out:
 	SafeFree(ParentDevicePath);