linux-kernel - Re: [PATCH v5] ARM: Implement SLS mitigation

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Date:   Mon, 22 Mar 2021 12:45:45 +0100
From:   Linus Walleij <linus.walleij@...aro.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>,
        Will Deacon <will@...nel.org>,
        Russell King <rmk+kernel@...linux.org.uk>,
        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>,
        Krzysztof Kozlowski <krzk@...nel.org>,
        Marc Zyngier <maz@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        Andreas Färber <afaerber@...e.de>,
        Ard Biesheuvel <ardb@...nel.org>,
        Ingo Molnar <mingo@...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@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        linux-security-module@...r.kernel.org
Subject: Re: [PATCH v5] ARM: Implement SLS mitigation

On Wed, Mar 10, 2021 at 5:43 AM Jian Cai <jiancai@...gle.com> wrote:
> On Sat, Mar 6, 2021 at 4:25 AM Linus Walleij <linus.walleij@...aro.org> wrote:
> > On Fri, Mar 5, 2021 at 12:23 AM Jian Cai <jiancai@...gle.com> wrote:
> > > On Wed, Mar 3, 2021 at 7:04 AM Linus Walleij <linus.walleij@...aro.org> wrote:

> > > I think gcc also has these options.
> > > https://gcc.gnu.org/onlinedocs/gcc/AArch64-Options.html
> >
> > And how does that work with this part of your patch:
> >
> > +#define SLS_TEXT                                       \
> > +       ALIGN_FUNCTION();                              \
> > +       *(.text.__llvm_slsblr_thunk_*)
> >
> > This does not look compiler agnostic?
>
> You are right, GCC does generate different oraphan section names. I
> will address it in the next version of the patch. Also it seems only
> arm64 gcc supports -mharden-sls=* at this moment, arm32 gcc does not
> support it yet. I don't know if there is any plan to implement it for
> 32-bit gcc, but should we patch arm32 linker script preemptively,
> assuming the sections will be named with the same pattern like how
> clang does so the kernel would not fail to boot when the flag is
> implemented?

I think the best thing is to have something like this:
Implement a macro such as this in
include/linux/compiler-clang.h

#define SLS_TEXT_SECTION *(.text.__llvm_slsblr_thunk_*)

then the corresponding in include/linux/compiler-gcc.h
but here also add a

#define SLS_TEXT_SECTION #error "no compiler support"

if the compiler version does not have this.

I don't know the exact best approach sadly, as the patch
looks now it seems a bit fragile, I wonder if you get linker
warnings when this section is unused?

Yours,
Linus Walleij