linux-kernel - Re: [RFC PATCH 2/2] x86/sev: Disable jump tables in SEV startup code

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Message-ID: <CAMj1kXGyX1-g1mJNTyywQtDoR5Pqr+LHndum7eJPnVUNXAZJGA@mail.gmail.com>
Date: Mon, 27 Jan 2025 23:28:19 +0100
From: Ard Biesheuvel <ardb@...nel.org>
To: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Ard Biesheuvel <ardb+git@...gle.com>, linux-kernel@...r.kernel.org, x86@...nel.org, 
	Ingo Molnar <mingo@...nel.org>, Tom Lendacky <thomas.lendacky@....com>, 
	Nathan Chancellor <nathan@...nel.org>
Subject: Re: [RFC PATCH 2/2] x86/sev: Disable jump tables in SEV startup code

On Mon, 27 Jan 2025 at 23:15, Ard Biesheuvel <ardb@...nel.org> wrote:
>
> On Mon, 27 Jan 2025 at 18:10, Linus Torvalds
> <torvalds@...ux-foundation.org> wrote:
> >
> > On Mon, 27 Jan 2025 at 03:43, Ard Biesheuvel <ardb+git@...gle.com> wrote:
> > >
> > > +# jump tables are emitted using absolute references in non-PIC code
> > > +# so they cannot be used in the early SEV startup code
> > > +CFLAGS_core.o += -fno-jump-tables
> >
> > I can confirm that this looks like it fixes the problem for my
> > particular config.
> >
>
> Great.
>
> > Is the SEV code the only thing that needs this? And on the other hand,
> > isn't it just a _part_ of core.c that needs this? Maybe the "_head"
> > parts should be in a separate file?
> >
> > I'm looking at (for example) arch/x86/mm/mem_encrypt_identity.c and
> > arch/x86/kernel/head64.c, and it looks like it really should have that
> > -fno-jump-tables thing too.
> >
> > It just randomly may not have any switch tables or other things that
> > makes the compiler generate that code pattern.
> >
>
...
> The use case is also quite similar to the x86 one, as a matter of
> fact: the initial mapping of the arm64 kernel is also 1:1, and some
> non-trivial work is needed before the kernel's virtual mapping can be
> created, and doing all of that in C was becoming intractable.
>

Doing it in *assembler* was becoming intractable, in case that wasn't clear.