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

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Date:   Wed, 3 Mar 2021 15:29:37 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Linus Walleij' <linus.walleij@...aro.org>,
        Will Deacon <will@...nel.org>
CC:     Jian Cai <jiancai@...gle.com>,
        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>,
        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>, 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" 
        <linux-security-module@...r.kernel.org>
Subject: RE: [PATCH v4] ARM: Implement SLS mitigation

From: Linus Walleij
> Sent: 03 March 2021 15:19
> 
> On Tue, Feb 23, 2021 at 11:05 AM Will Deacon <will@...nel.org> wrote:
> > On Mon, Feb 22, 2021 at 01:50:06PM -0800, Jian Cai wrote:
> > > 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.
> 
> I wrote some suggestions on follow-ups in my reply, and I can
> help out doing some of the patches, I think.
> 
> Since ARM32 RET is mov pc, <>
> git grep 'mov.*pc,' | wc -l gives 93 sites in arch/arm.
> I suppose these need to come out:
> 
> mov pc, lr
> dsb(nsh);
> isb();

Won't that go horribly wrong for conditional returns?

	David

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