[PATCH v2] ARM: Implement Clang's SLS mitigation

[David Laight]

From: Will Deacon
> Sent: 17 February 2021 09:49
> 
> On Fri, Feb 12, 2021 at 11:52:53AM -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.
> 
> What exactly does this mitigation do? This should be documented somewhere,
> maybe in the Kconfig text?

I looked it up, it adds some fairly heavy serialising instructions
after the unconditional jump.
For BLR (call indirect) it has to use a BL (call) to an indirect jump.

I don't know if the execution of the serialising instructions
gets aborted.
If not you could end up with unexpected delays - like those on
some x86 cpu when they speculatively executed trig functions.

It all seems pretty broken though.
I'd expect the branch prediction unit to speculate at the jump
target for 'predicted taken' conditional jumps.
So you'd really expect unconditional jumps to behave the same way.
BLR ought to be using the branch target buffer (BTB).

(It isn't actually 100% clear that some processors don't use the BTB
for non-indirect jumps though....)

	David

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