Re: [PATCH v5] ARM: Implement SLS mitigation

[Linus Walleij <linus.walleij@linaro.org>]

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