[PATCH v4] ARM: Implement SLS mitigation

[Jian Cai]

Please see my comments inlined below.

Thanks,
Jian

On Mon, Feb 22, 2021 at 3:58 AM Will Deacon <will at kernel.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 at google.com>
> > Suggested-by: Nick Desaulniers <ndesaulniers at google.com>
> > Suggested-by: Nathan Chancellor  <nathan at kernel.org>
> > Suggested-by: David Laight <David.Laight at aculab.com>
> > Suggested-by: Will Deacon <will at kernel.org>
> > Reviewed-by: Nathan Chancellor <nathan at kernel.org>
> > Signed-off-by: Jian Cai <jiancai at google.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.

> 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.