[PATCH v4 0/7] x86: Rid .head.text of all abs references

Ard Biesheuvel
2024-12-05T11:28:22Z

From: Ard Biesheuvel <ardb@kernel.org>

This series removes the last remaining absolute symbol references from
.head.text. Doing so is necessary because code in this section may be
called from a 1:1 mapping of memory, which deviates from the mapping
this code was linked and/or relocated to run at. This is not something
that the toolchains support: even PIC/PIE code is still assumed to
execute from the same mapping that it was relocated to run from by the
startup code or dynamic loader. This means we are basically on our own
here, and need to add measures to ensure the code works as expected in
this manner.

Given that the startup code needs to create the kernel virtual mapping
in the page tables, early references to some kernel virtual addresses
are valid even if they cannot be dereferenced yet. To avoid having to
make this distinction at build time, patches #2 and #3 replace such
valid references with RIP-relative references with an offset applied.

Patch #1 removes some absolute references from .head.text that don't
need to be there in the first place.

Changes since v3:
- add patch to disable UBSAN in .head.text C code
- rebase onto v6.13-rc1

Changes since v2:
- drop Xen changes, which have been merged in the meantime
- update patch #1 with feedback from Tom
- reorganize the .text section and emit .head.text into a separate
output section for easier diagnostics
- update the 'relocs' tool to reject absolute ELF relocations in
.head.text

Changes since v1/RFC:
- rename va_offset to p2v_offset
- take PA of _text in C code directly

Cc: Tom Lendacky <thomas.lendacky@amd.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Kees Cook <keescook@chromium.org>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Kevin Loughlin <kevinloughlin@google.com>

Ard Biesheuvel (7):
x86/sev: Avoid WARN()s and panic()s in early boot code
x86/boot/64: Determine VA/PA offset before entering C code
x86/boot/64: Avoid intentional absolute symbol references in
.head.text
x86/boot: Disable UBSAN in early boot code
x86/kernel: Move ENTRY_TEXT to the start of the image
x86/boot: Move .head.text into its own output section
x86/boot: Reject absolute references in .head.text

arch/x86/coco/sev/core.c | 15 +++-----
arch/x86/coco/sev/shared.c | 16 +++++----
arch/x86/include/asm/init.h | 2 +-
arch/x86/include/asm/setup.h | 2 +-
arch/x86/kernel/head64.c | 38 ++++++++++++--------
arch/x86/kernel/head_64.S | 12 +++++--
arch/x86/kernel/vmlinux.lds.S | 29 ++++++++-------
arch/x86/tools/relocs.c | 8 ++++-
8 files changed, 71 insertions(+), 51 deletions(-)

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2.47.0.338.g60cca15819-goog