linux-kernel - Re: [PATCH] x86/cmdline: Disable jump tables for cmdline.c

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Message-ID: <CAKwvOdm8qUpueOdO7e-rQotO5UynrgU2TZ9DEe=O4f9eU5EY0g@mail.gmail.com>
Date:   Mon, 21 Dec 2020 11:14:39 -0800
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Arvind Sankar <nivedita@...m.mit.edu>
Cc:     "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Tom Lendacky <thomas.lendacky@....com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] x86/cmdline: Disable jump tables for cmdline.c

On Wed, Sep 2, 2020 at 7:31 PM Arvind Sankar <nivedita@...m.mit.edu> wrote:
>
> When CONFIG_RETPOLINE is disabled, Clang uses a jump table for the
> switch statement in cmdline_find_option (jump tables are disabled when
> CONFIG_RETPOLINE is enabled). This function is called very early in boot
> from sme_enable() if CONFIG_AMD_MEM_ENCRYPT is enabled. At this time,

Hi Arvind, sorry I missed this when you first sent it.  I'm going
through and mass deleting my inbox (email bankruptcy) but noticed
this.  I couldn't reproduce jump tables in cmdline_find_option with
CONFIG_RETPOLINE disabled but CONFIG_AMD_MEM_ENCRYPT on today's
linux-next. Can you please confirm that this is still an issue? I will
reread the disassembly, but it looks like a bunch of cmp/test+jumps.

> the kernel is still executing out of the identity mapping, but the jump
> table will contain virtual addresses.
>
> Fix this by disabling jump tables for cmdline.c when AMD_MEM_ENCRYPT is
> enabled.
>
> Signed-off-by: Arvind Sankar <nivedita@...m.mit.edu>
> ---
>  arch/x86/lib/Makefile | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/arch/x86/lib/Makefile b/arch/x86/lib/Makefile
> index d46fff11f06f..aa067859a70b 100644
> --- a/arch/x86/lib/Makefile
> +++ b/arch/x86/lib/Makefile
> @@ -24,7 +24,7 @@ ifdef CONFIG_FUNCTION_TRACER
>  CFLAGS_REMOVE_cmdline.o = -pg
>  endif
>
> -CFLAGS_cmdline.o := -fno-stack-protector
> +CFLAGS_cmdline.o := -fno-stack-protector -fno-jump-tables
>  endif
>
>  inat_tables_script = $(srctree)/arch/x86/tools/gen-insn-attr-x86.awk
> --
> 2.26.2
>
> --
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-- 
Thanks,
~Nick Desaulniers