linux-kernel - Re: [tip: x86/boot] x86/boot: Disable jump tables in PIC code

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Message-ID: <CAMj1kXFpE=P0_a3fTAnb7qQmXLt19dCtuEcd5U8xwYzAcO=ufQ@mail.gmail.com>
Date: Wed, 23 Apr 2025 17:01:42 +0200
From: Ard Biesheuvel <ardb@...nel.org>
To: Josh Poimboeuf <jpoimboe@...nel.org>
Cc: tip-bot2 for Ard Biesheuvel <tip-bot2@...utronix.de>, linux-tip-commits@...r.kernel.org, 
	Ingo Molnar <mingo@...nel.org>, Peter Zijlstra <peterz@...radead.org>, linux-efi@...r.kernel.org, 
	x86@...nel.org, linux-kernel@...r.kernel.org
Subject: Re: [tip: x86/boot] x86/boot: Disable jump tables in PIC code

On Wed, 23 Apr 2025 at 16:55, Josh Poimboeuf <jpoimboe@...nel.org> wrote:
>
> On Wed, Apr 23, 2025 at 07:48:01AM +0000, tip-bot2 for Ard Biesheuvel wrote:
> > The following commit has been merged into the x86/boot branch of tip:
> >
> > Commit-ID:     121c335b36e02d6aefb72501186e060474fdf33c
> > Gitweb:        https://git.kernel.org/tip/121c335b36e02d6aefb72501186e060474fdf33c
> > Author:        Ard Biesheuvel <ardb@...nel.org>
> > AuthorDate:    Tue, 22 Apr 2025 23:05:11 +02:00
> > Committer:     Ingo Molnar <mingo@...nel.org>
> > CommitterDate: Wed, 23 Apr 2025 09:30:57 +02:00
> >
> > x86/boot: Disable jump tables in PIC code
> >
> > objtool already struggles to identify jump tables correctly in non-PIC
> > code, where the idiom is something like
> >
> >   jmpq  *table(,%idx,8)
> >
> > and the table is a list of absolute addresses of jump targets.
> >
> > When using -fPIC, both the table reference as well as the jump targets
> > are emitted in a RIP-relative manner, resulting in something like
> >
> >   leaq    table(%rip), %tbl
> >   movslq  (%tbl,%idx,4), %offset
> >   addq    %offset, %tbl
> >   jmpq    *%tbl
> >
> > and the table is a list of offsets of the jump targets relative to the
> > start of the entire table.
> >
> > Considering that this sequence of instructions can be interleaved with
> > other instructions that have nothing to do with the jump table in
> > question, it is extremely difficult to infer the control flow by
> > deriving the jump targets from the indirect jump, the location of the
> > table and the relative offsets it contains.
> >
> > So let's not bother and disable jump tables for code built with -fPIC
> > under arch/x86/boot/startup.
>
> Hm, does objtool even run on boot code?
>

This is about startup code, not boot code. This is code that is part
of vmlinux, but runs from a different mapping of memory than the one
the linker assumes, and so it needs to be built with -fPIC to
discourage the compiler and linker from inserting symbol references
via the kernel virtual mapping, which may not be up yet when this code
runs.