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 <redacted>
Suggested-by: Nick Desaulniers <ndesaulniers@google.com>
Suggested-by: Nathan Chancellor <nathan@kernel.org>
Suggested-by: David Laight <redacted>
Suggested-by: Will Deacon <will@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