linux-kernel - Re: [PATCH v4] ARM: Implement SLS mitigation

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Date:   Tue, 23 Feb 2021 10:04:53 +0000
From:   Will Deacon <will@...nel.org>
To:     Jian Cai <jiancai@...gle.com>
Cc:     Nick Desaulniers <ndesaulniers@...gle.com>,
        Manoj Gupta <manojgupta@...gle.com>,
        Luis Lozano <llozano@...gle.com>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Nathan Chancellor <nathan@...nel.org>,
        David Laight <David.Laight@...lab.com>,
        Russell King <linux@...linux.org.uk>,
        Catalin Marinas <catalin.marinas@....com>,
        James Morris <jmorris@...ei.org>,
        "Serge E. Hallyn" <serge@...lyn.com>,
        Arnd Bergmann <arnd@...db.de>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        Ard Biesheuvel <ardb@...nel.org>,
        Andreas Färber <afaerber@...e.de>,
        Ingo Molnar <mingo@...nel.org>,
        Linus Walleij <linus.walleij@...aro.org>,
        Marc Zyngier <maz@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mike Rapoport <rppt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        David Brazdil <dbrazdil@...gle.com>,
        James Morse <james.morse@....com>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-security-module@...r.kernel.org
Subject: Re: [PATCH v4] ARM: Implement SLS mitigation

On Mon, Feb 22, 2021 at 01:50:06PM -0800, Jian Cai wrote:
> Please see my comments inlined below.
> 
> Thanks,
> Jian
> 
> On Mon, Feb 22, 2021 at 3:58 AM Will Deacon <will@...nel.org> 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:
> > > https://reviews.llvm.org/D93221
> > > https://reviews.llvm.org/D81404
> > > https://developer.arm.com/support/arm-security-updates/speculative-processor-vulnerability/downloads/straight-line-speculation
> > > https://developer.arm.com/support/arm-security-updates/speculative-processor-vulnerability/frequently-asked-questions#SLS2
> > >
> > > Suggested-by: Manoj Gupta <manojgupta@...gle.com>
> > > Suggested-by: Nick Desaulniers <ndesaulniers@...gle.com>
> > > Suggested-by: Nathan Chancellor  <nathan@...nel.org>
> > > Suggested-by: David Laight <David.Laight@...lab.com>
> > > Suggested-by: Will Deacon <will@...nel.org>
> > > Reviewed-by: Nathan Chancellor <nathan@...nel.org>
> > > Signed-off-by: Jian Cai <jiancai@...gle.com>
> > > ---
> >
> > Please can you reply to my previous questions?
> >
> > https://lore.kernel.org/linux-arm-kernel/20210217094859.GA3706@willie-the-truck/
> >
> > (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.

I'm not sure I see the point; either it's needed or its not. I wonder if
there's a plan to fix this in future CPUs (another question for the Arm
folks).

> > 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
> used.

If the mitigation is required, I'm not sure I see a lot of point in only
doing a half-baked job of it. It feels a bit like a box-ticking exercise,
in which case any overhead is too much.

Will