Re: [PATCH v4] ARM: Implement SLS mitigation

[Will Deacon]

On Mon, Feb 22, 2021 at 01:50:06PM -0800, Jian Cai wrote:
> Please see my comments inlined below.
> 
> Thanks,
> Jian
> 
> On Mon, Feb 22, 2021 at 3:58 AM Will Deacon <will@xxxxxxxxxx> 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@xxxxxxxxxx>
> > > Suggested-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
> > > Suggested-by: Nathan Chancellor  <nathan@xxxxxxxxxx>
> > > Suggested-by: David Laight <David.Laight@xxxxxxxxxx>
> > > Suggested-by: Will Deacon <will@xxxxxxxxxx>
> > > Reviewed-by: Nathan Chancellor <nathan@xxxxxxxxxx>
> > > Signed-off-by: Jian Cai <jiancai@xxxxxxxxxx>
> > > ---
> >
> > 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.

I'm not sure I see the point; either it's needed or its not. I wonder if
there's a plan to fix this in future CPUs (another question for the Arm
folks).

> > 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.

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.

Will