efi/libstub: implement generic EFI zboot

Implement a minimal EFI app that decompresses the real kernel image and
launches it using the firmware's LoadImage and StartImage boot services.
This removes the need for any arch-specific hacks.

Note that on systems that have UEFI secure boot policies enabled,
LoadImage/StartImage require images to be signed, or their hashes known
a priori, in order to be permitted to boot.

There are various possible strategies to work around this requirement,
but they all rely either on overriding internal PI/DXE protocols (which
are not part of the EFI spec) or omitting the firmware provided
LoadImage() and StartImage() boot services, which is also undesirable,
given that they encapsulate platform specific policies related to secure
boot and measured boot, but also related to memory permissions (whether
or not and which types of heap allocations have both write and execute
permissions.)

The only generic and truly portable way around this is to simply sign
both the inner and the outer image with the same key/cert pair, so this
is what is implemented here.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>

1 files changed, 41 insertions, 0 deletions

diff --git a/drivers/firmware/efi/Kconfig b/drivers/firmware/efi/Kconfig
index cbf1c55dc224..5b79a4a4a88d 100644
--- a/drivers/firmware/efi/Kconfig
+++ b/drivers/firmware/efi/Kconfig
@@ -105,6 +105,47 @@ config EFI_RUNTIME_WRAPPERS
 config EFI_GENERIC_STUB
 	bool
 
+config EFI_ZBOOT
+	bool "Enable the generic EFI decompressor"
+	depends on EFI_GENERIC_STUB && !ARM
+	select HAVE_KERNEL_GZIP
+	select HAVE_KERNEL_LZ4
+	select HAVE_KERNEL_LZMA
+	select HAVE_KERNEL_LZO
+	select HAVE_KERNEL_XZ
+	select HAVE_KERNEL_ZSTD
+	help
+	  Create the bootable image as an EFI application that carries the
+	  actual kernel image in compressed form, and decompresses it into
+	  memory before executing it via LoadImage/StartImage EFI boot service
+	  calls. For compatibility with non-EFI loaders, the payload can be
+	  decompressed and executed by the loader as well, provided that the
+	  loader implements the decompression algorithm and that non-EFI boot
+	  is supported by the encapsulated image. (The compression algorithm
+	  used is described in the zboot image header)
+
+config EFI_ZBOOT_SIGNED
+	def_bool y
+	depends on EFI_ZBOOT_SIGNING_CERT != ""
+	depends on EFI_ZBOOT_SIGNING_KEY != ""
+
+config EFI_ZBOOT_SIGNING
+	bool "Sign the EFI decompressor for UEFI secure boot"
+	depends on EFI_ZBOOT
+	help
+	  Use the 'sbsign' command line tool (which must exist on the host
+	  path) to sign both the EFI decompressor PE/COFF image, as well as the
+	  encapsulated PE/COFF image, which is subsequently compressed and
+	  wrapped by the former image.
+
+config EFI_ZBOOT_SIGNING_CERT
+	string "Certificate to use for signing the compressed EFI boot image"
+	depends on EFI_ZBOOT_SIGNING
+
+config EFI_ZBOOT_SIGNING_KEY
+	string "Private key to use for signing the compressed EFI boot image"
+	depends on EFI_ZBOOT_SIGNING
+
 config EFI_ARMSTUB_DTB_LOADER
 	bool "Enable the DTB loader"
 	depends on EFI_GENERIC_STUB && !RISCV && !LOONGARCH