[PATCH v6] ARM: Implement SLS mitigation

[Will Deacon]

On Thu, Mar 04, 2021 at 04:53:18PM -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>

I'm still reasonably opposed to this patch, so please don't add my
"Suggested-by" here as, if I were to suggest anything, it would be not
to apply this patch :)

I still don't see why SLS is worth a compiler mitigation which will affect
all CPUs that run the kernel binary, but Spectre-v1 is not. In other words,
the big thing missing from this is a justification as to why SLS is a
problem worth working around for general C code.

Will