linux-kernel - Re: [PATCH v3 0/4] x86: Rid .head.text of all abs references

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Message-ID: <CAMj1kXEy0B3Jf_njyign-WuPup-hEB4a9C3dPk4xrZzH9mz4Ww@mail.gmail.com>
Date: Thu, 7 Nov 2024 17:12:27 +0100
From: Ard Biesheuvel <ardb@...nel.org>
To: Borislav Petkov <bp@...en8.de>
Cc: Ard Biesheuvel <ardb+git@...gle.com>, linux-kernel@...r.kernel.org, 
	Tom Lendacky <thomas.lendacky@....com>, Thomas Gleixner <tglx@...utronix.de>, 
	Ingo Molnar <mingo@...hat.com>, Dave Hansen <dave.hansen@...ux.intel.com>, 
	Andy Lutomirski <luto@...nel.org>, Arnd Bergmann <arnd@...db.de>, Kees Cook <keescook@...omium.org>, 
	Brian Gerst <brgerst@...il.com>
Subject: Re: [PATCH v3 0/4] x86: Rid .head.text of all abs references

On Thu, 7 Nov 2024 at 17:00, Borislav Petkov <bp@...en8.de> wrote:
>
> On Wed, Jun 05, 2024 at 12:16:11PM +0200, Ard Biesheuvel wrote:
> > From: Ard Biesheuvel <ardb@...nel.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 #3 and #4 replace such
> > valid references with RIP-relative references with an offset applied.
> >
> > Patches #1 and #2 remove some absolute references from .head.text that
> > don't need to be there in the first place.
> >
> > Changes since v2:
> > - Rebase onto v6.10-rc2
> > - Tweak commit log of patch #3
> >
> > Changes since v1/RFC:
> > - rename va_offset to p2v_offset
> > - take PA of _text in C code directly
> >
> > Cc: Tom Lendacky <thomas.lendacky@....com>
> > Cc: Thomas Gleixner <tglx@...utronix.de>
> > Cc: Ingo Molnar <mingo@...hat.com>
> > Cc: Borislav Petkov <bp@...en8.de>
> > Cc: Dave Hansen <dave.hansen@...ux.intel.com>
> > Cc: Andy Lutomirski <luto@...nel.org>
> > Cc: Arnd Bergmann <arnd@...db.de>
> > Cc: Kees Cook <keescook@...omium.org>
> > Cc: Brian Gerst <brgerst@...il.com>
> >
> > Ard Biesheuvel (4):
> >   x86/sev: Avoid WARN()s in early boot code
> >   x86/xen/pvh: Move startup code into .ref.text
> >   x86/boot/64: Determine VA/PA offset before entering C code
> >   x86/boot/64: Avoid intentional absolute symbol references in
> >     .head.text
>
> Those look forgotten in my mbox. Do we still want them to go somewhere?
>

I'll get back to this next cycle. Patch #2 has become redundant now,
and I need to incorporate some feedback from Tom into patch #1.

I imagine #3 and #4 should still apply, and they could be taken
independently, but there is no urgency.