[PATCH v8 00/23] efi/x86: Avoid bare metal decompressor during EFI boot

From: Ard Biesheuvel
Date: Wed Aug 02 2023 - 11:49:02 EST

Update the x86 boot path to avoid the bare metal decompressor when
booting via the EFI stub. The bare metal decompressor inherits the
loader's 1:1 mapping of DRAM when entering in 64-bit mode, and assumes
that all of it is mapped read/write/execute, which will no longer be the
case on systems built to comply with recently tightened logo
requirements (*).

Changes since v7 [10]:
- Rebase onto suggested x86/kexec fix [11] that will most likely land
- Drop only vaguely relevant i386 head_32.S patch
- Restructure the 32-bit trampoline changes slightly into steps that are
clearer and easier to follow.
- Fix wording and placement of comments according to suggestions by

Changes since v6 [9]:
- add new patch to fix our current reliance on 64-bit GPRs retaining
their full width contents across the switch into 32-bit protected
mode (with fixes: tag, may need to go to -stable);
- preserve the top half of RSP explicitly, and preserve all callee save
registers on the stack across the mode switch; this fixes a reported
issue on Ice Lake with kexec (which loads the kernel above 4G)

Changes since v5 [8]:
- reintroduce patch removing redundant RSI pushes and pops from
- avoid bare constant 0x200 for the offset of startup_64() in the
- rejig SEV/SNP logic in patch #20 once again, to ensure that CPUID
calls and VM exits only occur when the active configuration permits
- improve/clarify some code comments and commit logs
- rebase onto v6.5-rc1

Changes since v4 [7]:
- avoid CPUID calls after protocol negotiation but before configuring
exception handling;
- drop patch removing redundant RSI pushes and pops from
- rebase onto -tip x86/cc - the conflicts are mostly trivial and
restricted to the last 4 patches in the series, so applying this onto
a separate topic branch should be straight-forward as well.

Changes since v3 [6]:
- trivial rebase onto Kirill's unaccepted memory series v13
- test SNP feature mask while running in the EFI boot services, and fail
gracefully on a mismatch
- perform only the SEV init after ExitBootServices()

Changes since v2 [4]:
- update prose style to comply with -tip guidelines
- rebased onto Kirill's unaccepted memory series [3]
- add Kirill's ack to 4/5-level paging changes
- perform SEV init and SNP feature check after ExitBootServices(), to
avoid corrupting the firmware's own SEV state
- split out preparatory refactor of handover entry code and BSS clearing
(patches #1 to #4)

Changes since v1 [2]:
- streamline existing 4/5 level switching code and call it directly from
the EFI stub - this is covered by the first 9 patches, which can be
applied in isolation, if desired;
- deal with SEV/SNP init explicitly;
- clear BSS when booting via the 'handover protocol'
- switch to kernel CS before calling SEV init code in kernel proper.

---- v1 cover letter follows ----

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
allocation 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

(*) IMHO the following developments are likely to occur:
- the Windows boot chain (along with 3rd party drivers) is cleaned up so
that it never relies on memory being writable and executable at the
same time when running under the EFI boot services;
- the EFI reference implementation gets updated to map all memory NX by
default, and to require read-only permissions for executable mappings;
- BIOS vendors incorporate these changes into their codebases, and
deploy it more widely than just the 'secure' SKUs;
- OEMs only care about the Windows sticker [5], so they only boot test
Windows, which works fine in this more restricted context;
- Linux boot no longer works reliably on new hardware built for Windows
unless we clean up our boot chain as well.

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>
Cc: Joerg Roedel <jroedel@xxxxxxx>

[0] https://lore.kernel.org/all/cover.1678785672.git.baskov@xxxxxxxxx/
[1] https://lore.kernel.org/all/20230416120729.2470762-1-ardb@xxxxxxxxxx/
[2] https://lore.kernel.org/all/20230424165726.2245548-1-ardb@xxxxxxxxxx/
[3] https://lore.kernel.org/all/20230518231434.26080-1-kirill.shutemov@xxxxxxxxxxxxxxx/
[4] https://lore.kernel.org/all/20230508070330.582131-1-ardb@xxxxxxxxxx/
[5] https://techcommunity.microsoft.com/t5/hardware-dev-center/new-uefi-ca-memory-mitigation-requirements-for-signing/ba-p/3608714
[6] https://lore.kernel.org/all/20230522071415.501717-1-ardb@xxxxxxxxxx/
[7] https://lore.kernel.org/all/20230602101313.3557775-1-ardb@xxxxxxxxxx/
[8] https://lore.kernel.org/all/20230607072342.4054036-1-ardb@xxxxxxxxxx/
[9] https://lore.kernel.org/all/20230711091453.2543622-1-ardb@xxxxxxxxxx/
[10] https://lore.kernel.org/all/20230728090916.1538550-1-ardb@xxxxxxxxxx/
[11] https://lore.kernel.org/all/20230802093927.GAZMokT57anC5jBISK@fat_crate.local/

Ard Biesheuvel (23):
x86/decompressor: Don't rely on upper 32 bits of GPRs being preserved
x86/head_64: Store boot_params pointer in callee save register
x86/efistub: Branch straight to kernel entry point from C code
x86/efistub: Simplify and clean up handover entry code
x86/decompressor: Avoid magic offsets for EFI handover entrypoint
x86/efistub: Clear BSS in EFI handover protocol entrypoint
x86/decompressor: Store boot_params pointer in callee save register
x86/decompressor: Assign paging related global variables earlier
x86/decompressor: Call trampoline as a normal function
x86/decompressor: Use standard calling convention for trampoline
x86/decompressor: Avoid the need for a stack in the 32-bit trampoline
x86/decompressor: Call trampoline directly from C code
x86/decompressor: Only call the trampoline when changing paging levels
x86/decompressor: Pass pgtable address to trampoline directly
x86/decompressor: Merge trampoline cleanup with switching code
x86/efistub: Perform 4/5 level paging switch from the stub
x86/efistub: Prefer EFI memory attributes protocol over DXE services
decompress: Use 8 byte alignment
x86/decompressor: Move global symbol references to C code
x86/decompressor: Factor out kernel decompression and relocation
efi/libstub: Add limit argument to efi_random_alloc()
x86/efistub: Perform SNP feature test while running in the firmware
x86/efistub: Avoid legacy decompressor when doing EFI boot

Documentation/arch/x86/boot.rst | 2 +-
arch/x86/boot/compressed/Makefile | 5 +
arch/x86/boot/compressed/efi_mixed.S | 107 +++-----
arch/x86/boot/compressed/head_32.S | 32 ---
arch/x86/boot/compressed/head_64.S | 280 ++++++-------------
arch/x86/boot/compressed/misc.c | 44 ++-
arch/x86/boot/compressed/misc.h | 2 -
arch/x86/boot/compressed/pgtable.h | 10 +-
arch/x86/boot/compressed/pgtable_64.c | 87 +++---
arch/x86/boot/compressed/sev.c | 112 ++++----
arch/x86/include/asm/boot.h | 8 +
arch/x86/include/asm/efi.h | 7 +-
arch/x86/include/asm/sev.h | 6 +
arch/x86/kernel/head_64.S | 31 +--
drivers/firmware/efi/libstub/Makefile | 1 +
drivers/firmware/efi/libstub/arm64-stub.c | 2 +-
drivers/firmware/efi/libstub/efi-stub-helper.c | 2 +
drivers/firmware/efi/libstub/efistub.h | 3 +-
drivers/firmware/efi/libstub/randomalloc.c | 10 +-
drivers/firmware/efi/libstub/x86-5lvl.c | 95 +++++++
drivers/firmware/efi/libstub/x86-stub.c | 285 +++++++++++---------
drivers/firmware/efi/libstub/x86-stub.h | 17 ++
drivers/firmware/efi/libstub/zboot.c | 2 +-
include/linux/decompress/mm.h | 2 +-
24 files changed, 588 insertions(+), 564 deletions(-)
create mode 100644 drivers/firmware/efi/libstub/x86-5lvl.c
create mode 100644 drivers/firmware/efi/libstub/x86-stub.h