Even though our EFI zboot decompressor is pedantically spec compliant
and idiomatic for EFI image loaders, calling LoadImage() and
StartImage() for the nested image is a bit of a burden. Not only does it
create workflow issues for the distros (as both the inner and outer
PE/COFF images need to be signed for secure boot), it also copies the
image around in memory numerous times:
- first, the image is decompressed into a buffer;
- the buffer is consumed by LoadImage(), which copies the sections into
a newly allocated memory region to hold the executable image;
- once the EFI stub is invoked by StartImage(), it will also move the
image in memory in case of KASLR, mirrored memory or if the image must
execute from a certain a priori defined address.
There are only two EFI spec compliant ways to load code into memory and
execute it:
- use LoadImage() and StartImage(),
- call ExitBootServices() and take ownership of the entire system, after
which anything goes.
Given that the EFI zboot decompressor always invokes the EFI stub, and
given that both are built from the same set of objects, let's merge the
two, so that we can avoid LoadImage()/StartImage but still load our
image into memory without breaking the above rules.
This also means we can decompress the image directly into its final
location, which could be randomized or meet other platform specific
constraints that LoadImage() does not know how to adhere to. It also
means that, even if the encapsulated image still has the EFI stub
incorporated as well, it does not need to be signed for secure boot when
wrapping it in the EFI zboot decompressor.
In the future, we might decide to retire the EFI stub attached to the
decompressed image, but for the time being, they can happily coexist.
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Split the efi_printk() routine into its own source file, and provide
local implementations of strlen() and strnlen() so that the standalone
zboot app can efi_err and efi_info etc.
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
We will no longer be able to call into the kernel image once we merge
the decompressor with the EFI stub, so we need our own implementation of
memcmp(). Let's add the one from lib/string.c and simplify it.
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
LoadImage() is supposed to install an instance of the protocol
EFI_LOADED_IMAGE_DEVICE_PATH_PROTOCOL onto the loaded image's handle so
that the program can figure out where it was loaded from. The reference
implementation even does this (with a NULL protocol pointer) if the call
to LoadImage() used the source buffer and size arguments, and passed
NULL for the image device path. Hand rolled implementations of LoadImage
may behave differently, though, and so it is better to tolerate
situations where the protocol is missing. And actually, concatenating an
Offset() node to a NULL device path (as we do currently) is not great
either.
So in cases where the protocol is absent, or when it points to NULL,
construct a MemoryMapped() device node as the base node that describes
the parent image's footprint in memory.
Cc: Daan De Meyer <daandemeyer@fb.com>
Cc: Jeremy Linton <jeremy.linton@arm.com>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
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>
