Re: [PATCH 0/6] efi/x86: Avoid legacy decompressor during EFI boot

[Tom Lendacky <thomas.lendacky@amd.com>]

On 4/24/23 11:57, Ard Biesheuvel wrote:
> This series is conceptually a combination of Evgeny's series [0] and
> mine [1], both of which attempt to make the early decompressor code more
> amenable to executing in the EFI environment with stricter handling of
> memory permissions.
> 
> My series [1] implemented zboot for x86, by getting rid of the entire
> x86 decompressor, and replacing it with existing EFI code that does the
> same but in a generic way. The downside of this is that only EFI boot is
> supported, making it unviable for distros, which need to support BIOS
> boot and hybrid EFI boot modes that omit the EFI stub.
> 
> Evgeny's series [0] adapted the entire decompressor code flow to allow
> it to execute in the EFI context as well as the bare metal context, and
> this involves changes to the 1:1 mapping code and the page fault
> handlers etc, none of which are really needed when doing EFI boot in the
> first place.
> 
> So this series attempts to occupy the middle ground here: it makes
> minimal changes to the existing decompressor so some of it can be called
> from the EFI stub. Then, it reimplements the EFI boot flow to decompress
> the kernel and boot it directly, without relying on the trampoline code,
> page table code or page fault handling code. This allows us to get rid
> of quite a bit of unsavory EFI stub code, and replace it with two clear
> invocations of the EFI firmware APIs to clear NX restrictions from
> allocations that have been populated with executable code.
> 
> The only code that is being reused is the decompression library itself,
> along with the minimal ELF parsing that is required to copy the ELF
> segments in place, and the relocation processing that fixes up absolute
> symbol references to refer to the correct virtual addresses.
> 
> Note that some of Evgeny's changes to clean up the PE/COFF header
> generation will still be needed, but I've omitted those here for
> brevity.

I tried booting an SEV and an SEV-ES guest using this and both failed to boot:

EFI stub: WARNING: Decompression failed: Out of memory while allocating 
z_stream

I'll have to take a closer look as to why, but it might be a couple of 
days before I can get to it.

Thanks,
Tom

> 
> Cc: Evgeniy Baskov <baskov@ispras.ru>
> Cc: Borislav Petkov <bp@alien8.de>
> Cc: Andy Lutomirski <luto@kernel.org>
> Cc: Dave Hansen <dave.hansen@linux.intel.com>
> Cc: Ingo Molnar <mingo@redhat.com>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Alexey Khoroshilov <khoroshilov@ispras.ru>
> Cc: Peter Jones <pjones@redhat.com>
> Cc: Gerd Hoffmann <kraxel@redhat.com>
> Cc: Dave Young <dyoung@redhat.com>
> Cc: Mario Limonciello <mario.limonciello@amd.com>
> Cc: Kees Cook <keescook@chromium.org>
> Cc: Tom Lendacky <thomas.lendacky@amd.com>
> Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> Cc: Linus Torvalds <torvalds@linux-foundation.org>
> 
> [0] https://lore.kernel.org/all/cover.1678785672.git.baskov@ispras.ru/
> [1] https://lore.kernel.org/all/20230416120729.2470762-1-ardb@kernel.org/
> 
> Ard Biesheuvel (6):
>    x86: decompressor: Move global symbol references to C code
>    x86: decompressor: Factor out kernel decompression and relocation
>    x86: efistub: Obtain ACPI RSDP address while running in the stub
>    x86: efistub: Perform 4/5 level paging switch from the stub
>    x86: efistub: Prefer EFI memory attributes protocol over DXE services
>    x86: efistub: Avoid legacy decompressor when doing EFI boot
> 
>   arch/x86/boot/compressed/efi_mixed.S           |  55 ---
>   arch/x86/boot/compressed/head_32.S             |  24 --
>   arch/x86/boot/compressed/head_64.S             |  39 +--
>   arch/x86/boot/compressed/misc.c                |  44 ++-
>   arch/x86/include/asm/efi.h                     |   2 +
>   drivers/firmware/efi/libstub/efi-stub-helper.c |   4 +
>   drivers/firmware/efi/libstub/x86-stub.c        | 360 +++++++++++++-------
>   7 files changed, 279 insertions(+), 249 deletions(-)
>