Re: [RFC PATCH 0/3] efi: Implement generic zboot support

[Andy Lutomirski]

On Sun, Apr 16, 2023, at 5:07 AM, Ard Biesheuvel wrote:
> This series is a proof-of-concept that implements support for the EFI
> zboot decompressor for x86. It replaces the ordinary decompressor, and
> instead, performs the decompression, KASLR randomization and the 4/5
> level paging switch while running in the execution context of EFI.

I like the concept.  A couple high-level questions, since I haven’t dug into the code:

Could zboot and bzImage be built into the same kernel image?  That would get this into distros, and eventually someone could modify the legacy path to switch to long mode and invoke zboot (because zboot surely doesn’t need actual UEFI — just a sensible environment like what UEFI provides.)

Does zboot set up BSS correctly?  I once went down a rabbit hole trying to get the old decompressor to jump into the kernel with BSS already usable and zeroed, and the result was an incredible mess — IIRC the decompressor does some in-place shenanigans that looked incompatible with handling BSS without a rewrite. And so we clear BSS in C after jumping to the kernel, which is gross.

—Andy

>
> This simplifies things substantially, and makes it straight-forward to
> abide by stricter future requirements related to the use of writable and
> executable memory under EFI, which will come into effect on x86 systems
> that are certified as being 'more secure', and ship with an even shinier
> Windows sticker.
>
> This is an alternative approach to the work being proposed by Evgeny [0]
> that makes rather radical changes to the existing decompressor, which
> has accumulated too many features already, e.g., related to confidential
> compute etc.
>
> EFI zboot images can be booted in two ways:
> - by EFI firmware, which loads and starts it as an ordinary EFI
>   application, just like the existing EFI stub (with which it shares
>   most of its code);
> - by a non-EFI loader that parses the image header for the compression
>   metadata, and decompresses the image into memory and boots it.
>
> Realistically, the second option is unlikely to ever be used on x86,
> given that it already has its existing bzImage, but the first option is
> a good choice for distros that target EFI boot only (and some distros
> switched to this format already for arm64). The fact that EFI zboot is
> implemented in the same way on arm64, RISC-V, LoongArch and [shortly]
> ARM helps with maintenance, not only of the kernel itself, but also the
> tooling around it relating to kexec, code signing, deployment, etc.
>
> Series can be pulled from [1], which contains some prerequisite patches
> that are only tangentially related.
>
> [0] https://lore.kernel.org/all/cover.1678785672.git.baskov@xxxxxxxxx/
> [1] 
> https://git.kernel.org/pub/scm/linux/kernel/git/ardb/linux.git/log/?h=efi-x86-zboot
>
> Cc: Evgeniy Baskov <baskov@xxxxxxxxx>
> Cc: Borislav Petkov <bp@xxxxxxxxx>
> Cc: Andy Lutomirski <luto@xxxxxxxxxx>
> Cc: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Cc: Alexey Khoroshilov <khoroshilov@xxxxxxxxx>
> Cc: Peter Jones <pjones@xxxxxxxxxx>
> Cc: Gerd Hoffmann <kraxel@xxxxxxxxxx>
> Cc: Dave Young <dyoung@xxxxxxxxxx>
> Cc: Mario Limonciello <mario.limonciello@xxxxxxx>
> Cc: Kees Cook <keescook@xxxxxxxxxxxx>
> Cc: Tom Lendacky <thomas.lendacky@xxxxxxx>
> Cc: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
> Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
>
> Ard Biesheuvel (3):
>   efi/libstub: x86: Split off pieces shared with zboot
>   efi/zboot: x86: Implement EFI zboot support
>   efi/zboot: x86: Clear NX restrictions on populated code regions
>
>  arch/x86/Makefile                              |  18 +-
>  arch/x86/include/asm/efi.h                     |  10 +
>  arch/x86/kernel/head_64.S                      |  15 +
>  arch/x86/zboot/Makefile                        |  29 +
>  drivers/firmware/efi/Kconfig                   |   2 +-
>  drivers/firmware/efi/libstub/Makefile          |  15 +-
>  drivers/firmware/efi/libstub/Makefile.zboot    |   2 +-
>  drivers/firmware/efi/libstub/efi-stub-helper.c |   3 +
>  drivers/firmware/efi/libstub/x86-stub.c        | 592 +------------------
>  drivers/firmware/efi/libstub/x86-zboot.c       | 322 ++++++++++
>  drivers/firmware/efi/libstub/x86.c             | 612 ++++++++++++++++++++
>  drivers/firmware/efi/libstub/zboot.c           |   3 +-
>  drivers/firmware/efi/libstub/zboot.lds         |   5 +
>  13 files changed, 1031 insertions(+), 597 deletions(-)
>  create mode 100644 arch/x86/zboot/Makefile
>  create mode 100644 drivers/firmware/efi/libstub/x86-zboot.c
>  create mode 100644 drivers/firmware/efi/libstub/x86.c
>
> -- 
> 2.39.2