[PATCH v2 2/2] efi: Put Linux specific magic number in the DOS header

[Ard Biesheuvel]

GRUB currently relies on the magic number in the image header of ARM and
arm64 EFI kernel images to decide whether or not the image in question
is a bootable kernel.

However, the purpose of the magic number is to identify the image as one
that implements the bare metal boot protocol, and so GRUB, which only
does EFI boot, can only boot images that could potentially be booted in
a non-EFI manner as well.

This is problematic for the new zboot decompressor image format, as it
can only boot in EFI mode, and must therefore not use the bare metal
boot magic number in its header.

For this reason, the strict magic number was dropped from GRUB, to
permit essentially any kind of EFI executable to be booted via the
'linux' command, blurring the line between the linux loader and the
chainloader.

So let's use the same field in the DOS header that RISC-V and arm64
already use for their 'bare metal' magic numbers to store a 'generic
Linux kernel' magic number, which can be used to identify bootable
kernel images in PE format which don't necessarily implement a bare
metal boot protocol in the same binary. Note that, in the context of
EFI, the MSDOS header is only described in terms of the fields that it
shares with the hybrid PE/COFF image format, (i.e., the magic number at
offset #0 and the PE header offset at byte offset #0x3c). Since we aim
for compatibility with EFI only, and not with MS-DOS or MS-Windows, we
can use the remaining space in the MS-DOS header however we want.

Let's set the generic magic number for x86 images as well: existing
bootloaders already have their own methods to identify x86 Linux images
that can be booted in a non-EFI manner, and having the magic number in
place there will ease any future transitions in loader implementations
to merge the x86 and non-x86 EFI boot paths.

Note that 32-bit ARM already uses the same location in the header for a
different purpose, but the ARM support is already widely implemented and
the EFI zboot decompressor is not available on ARM anyway, so we just
disregard it here.

Cc: Huacai Chen <chenhuacai@kernel.org>
Cc: Atish Patra <atishp@rivosinc.com>
Cc: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Cc: Daniel Kiper <daniel.kiper@oracle.com>
Cc: Leif Lindholm <quic_llindhol@quicinc.com>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
---
 arch/loongarch/kernel/head.S                | 3 ++-
 arch/x86/boot/header.S                      | 3 ++-
 drivers/firmware/efi/libstub/zboot-header.S | 3 ++-
 include/linux/pe.h                          | 7 +++++++
 4 files changed, 13 insertions(+), 3 deletions(-)

diff --git a/arch/loongarch/kernel/head.S b/arch/loongarch/kernel/head.S
index 84970e2666588963..caa74439700eee93 100644
--- a/arch/loongarch/kernel/head.S
+++ b/arch/loongarch/kernel/head.S
@@ -25,7 +25,8 @@ _head:
        .dword  kernel_entry            /* Kernel entry point */
        .dword  _end - _text            /* Kernel image effective size */
        .quad   0                       /* Kernel image load offset from start 
of RAM */
-       .org    0x3c                    /* 0x20 ~ 0x3b reserved */
+       .org    0x38                    /* 0x20 ~ 0x38 reserved */
+       .long   LINUX_PE_MAGIC
        .long   pe_header - _head       /* Offset to the PE header */
 
 pe_header:
diff --git a/arch/x86/boot/header.S b/arch/x86/boot/header.S
index f912d777013052ea..be8f78a7ee325475 100644
--- a/arch/x86/boot/header.S
+++ b/arch/x86/boot/header.S
@@ -80,10 +80,11 @@ bs_die:
        ljmp    $0xf000,$0xfff0
 
 #ifdef CONFIG_EFI_STUB
-       .org    0x3c
+       .org    0x38
        #
        # Offset to the PE header.
        #
+       .long   LINUX_PE_MAGIC
        .long   pe_header
 #endif /* CONFIG_EFI_STUB */
 
diff --git a/drivers/firmware/efi/libstub/zboot-header.S 
b/drivers/firmware/efi/libstub/zboot-header.S
index bc2d7750d7f14174..ec4525d40e0cf6d6 100644
--- a/drivers/firmware/efi/libstub/zboot-header.S
+++ b/drivers/firmware/efi/libstub/zboot-header.S
@@ -20,7 +20,8 @@ __efistub_efi_zboot_header:
        .long           __efistub__gzdata_size - 12             // payload size
        .long           0, 0                                    // reserved
        .asciz          COMP_TYPE                               // compression 
type
-       .org            .Ldoshdr + 0x3c
+       .org            .Ldoshdr + 0x38
+       .long           LINUX_PE_MAGIC
        .long           .Lpehdr - .Ldoshdr                      // PE header 
offset
 
 .Lpehdr:
diff --git a/include/linux/pe.h b/include/linux/pe.h
index 056a1762de904fc1..1db4c944efd78f51 100644
--- a/include/linux/pe.h
+++ b/include/linux/pe.h
@@ -31,6 +31,13 @@
 #define LINUX_EFISTUB_MAJOR_VERSION            0x1
 #define LINUX_EFISTUB_MINOR_VERSION            0x1
 
+/*
+ * LINUX_PE_MAGIC appears at offset 0x38 into the MSDOS header of EFI bootable
+ * Linux kernel images that target the architecture as specified by the PE/COFF
+ * header machine type field.
+ */
+#define LINUX_PE_MAGIC 0x818223cd
+
 #define MZ_MAGIC       0x5a4d  /* "MZ" */
 
 #define PE_MAGIC               0x00004550      /* "PE\0\0" */
-- 
2.35.1