[PATCH v5] ARM: Implement SLS mitigation

Jian Cai jiancai at google.com
Thu Mar 4 23:22:51 UTC 2021

On Wed, Mar 3, 2021 at 7:04 AM Linus Walleij <linus.walleij at linaro.org> wrote:
> On Tue, Feb 23, 2021 at 3:36 AM Jian Cai <jiancai at google.com> 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.
> I heard about compiler protection for this, so nice to see it happening!
> Would you happen to know if there is any plan to do the same for GCC?
> I know you folks at Google like LLVM, but if you know let us know.

I think gcc also has these options.

> > +config HARDEN_SLS_ALL
> > +       bool "enable SLS vulnerability hardening"
> I would go in and also edit arch/arm/mm/Kconfig under:
> Because if the user wants hardening for branch prediction
> in general then the user certainly wants this as well, if
> available. The help text for that option literally says:
>  "This config option will take CPU-specific actions to harden
>    the branch predictor against aliasing attacks and may rely on
>    specific instruction sequences or control bits being set by
>    the system firmware."
> Notice this only turns on for CPUs with CPU_SPECTRE
> defined which makes sense. Also it is default y which fulfils
> Will's request that it be turned on by default where
> applicable. Notably it will not be turned on for pre-v7 silicon
> which would be unhelpful as they don't suffer from
> these bugs.

Thanks for the suggestion. I will update the patch.

> Reading Kristofs compiler patch here:
> https://reviews.llvm.org/rG195f44278c4361a4a32377a98a1e3a15203d3647
> I take it that for affected CPUs we should also patch all assembly
> in the kernel containing a RET, BR or BLR with
> DSB SYS followed by ISB?
> I suppose we would also need to look for any mov PC, <>
> code...
> I guess we can invent a "SB" macro to mimic what Aarch64 is
> doing so the code is easy to read. (Thinking aloud.)
> Yours,
> Linus Walleij

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