efi/zboot: arm64: Inject kernel code size symbol into the zboot payload

The EFI zboot code is not built as part of the kernel proper, like the
ordinary EFI stub, but still needs access to symbols that are defined
only internally in the kernel, and are left unexposed deliberately to
avoid creating ABI inadvertently that we're stuck with later.

So capture the kernel code size of the kernel image, and inject it as an
ELF symbol into the object that contains the compressed payload, where
it will be accessible to zboot code that needs it.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Acked-by: Mark Rutland <mark.rutland@arm.com>

efi: libstub: Fix incorrect payload size in zboot header

The linker script symbol definition that captures the size of the
compressed payload inside the zboot decompressor (which is exposed via
the image header) refers to '.' for the end of the region, which does
not give the correct result as the expression is not placed at the end
of the payload. So use the symbol name explicitly.

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

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>