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Date:   Mon, 22 Feb 2021 13:50:06 -0800
From:   Jian Cai <>
To:     Will Deacon <>
Cc:     Nick Desaulniers <>,
        Manoj Gupta <>,
        Luis Lozano <>,
        clang-built-linux <>,
        Nathan Chancellor <>,
        David Laight <>,
        Russell King <>,
        Catalin Marinas <>,
        James Morris <>,
        "Serge E. Hallyn" <>,
        Arnd Bergmann <>,
        Masahiro Yamada <>,
        Kees Cook <>,
        Ard Biesheuvel <>,
        Andreas Färber <>,
        Ingo Molnar <>,
        Linus Walleij <>,
        Marc Zyngier <>,
        Andrew Morton <>,
        Mike Rapoport <>,
        Mark Rutland <>,
        David Brazdil <>,
        James Morse <>,
        Linux ARM <>,
        Linux Kernel Mailing List <>,
Subject: Re: [PATCH v4] ARM: Implement SLS mitigation

Please see my comments inlined below.


On Mon, Feb 22, 2021 at 3:58 AM Will Deacon <> wrote:
> On Fri, Feb 19, 2021 at 03:08:13PM -0800, Jian Cai wrote:
> > This patch adds CONFIG_HARDEN_SLS_ALL that can be used to turn on
> > -mharden-sls=all, which mitigates the straight-line speculation
> > vulnerability, speculative execution of the instruction following some
> > unconditional jumps. Notice -mharden-sls= has other options as below,
> > and this config turns on the strongest option.
> >
> > all: enable all mitigations against Straight Line Speculation that are implemented.
> > none: disable all mitigations against Straight Line Speculation.
> > retbr: enable the mitigation against Straight Line Speculation for RET and BR instructions.
> > blr: enable the mitigation against Straight Line Speculation for BLR instructions.
> >
> > Links:
> >
> >
> >
> >
> >
> > Suggested-by: Manoj Gupta <>
> > Suggested-by: Nick Desaulniers <>
> > Suggested-by: Nathan Chancellor  <>
> > Suggested-by: David Laight <>
> > Suggested-by: Will Deacon <>
> > Reviewed-by: Nathan Chancellor <>
> > Signed-off-by: Jian Cai <>
> > ---
> Please can you reply to my previous questions?
> (apologies if you did, but I don't see them in the archive or my inbox)

I should have clarified the suggested-by tag was in regard to the
Kconfig text change. Regarding your earlier questions, please see my
comments below.

> So I think that either we enable this unconditionally, or we don't enable it
> at all (and people can hack their CFLAGS themselves if they want to).

Not sure if this answers your question but this config should provide
a way for people to turn on the mitigation at their own risk.

> It would be helpful for one of the Arm folks to chime in, as I'm yet to see any
> evidence that this is actually exploitable. Is it any worse that Spectre-v1,
> where we _don't_ have a compiler mitigation?

> Finally, do we have to worry about our assembly code?

I am not sure if there are any plans to protect assembly code and I
will leave it to the Arm folks since they know a whole lot better. But
even without that part, we should still have better protection,
especially when overhead does not look too bad: I did some preliminary
experiments on ChromeOS, code size of vmlinux increased 3%, and there
were no noticeable changes to run-time performance of the benchmarks I

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