On 05/02/18 10:01, Christoffer Dall wrote:

On Thu, Feb 01, 2018 at 11:07:32AM +0000, Marc Zyngier wrote:

quoted

This small series implements some basic BP hardening by invalidating
the BTB on 32bit ARM CPUs that are known to be susceptible to aliasing
attacks (Spectre variant 2). It doesn't help non-ARM 32bit CPUs, nor
32bit kernels that run on 64bit capable CPUs. This series doesn't
mitigate Spectre variant 1 either.

These patches are closely modelled against what we do on arm64,
although simpler as we can rely on an architected instruction to
perform the invalidation. The notable exception is Cortex-A15, where
BTB invalidation behaves like a NOP, and the only way to shoot the
predictor down is to invalidate the icache *and* to have ACTLR[0] set
to 1 (which is a secure-only operation).

The first patch reuses the Cortex-A8 BTB invalidation in switch_mm and
generalises it to be used on all affected CPUs. The second perform the
same invalidation on prefetch abort outside of the userspace
range. The third one nukes it on guest exit, and results in some major
surgery as we cannot take a branch from the vectors (that, and Thumb2
being a massive pain).

Patches 4 to 6 are doing a similar thing for Cortex-A15, with the
aforementioned ICIALLU.

To sum up the requirements:

- Cortex-A15 need to have ACTLR.IBE (bit 0) set to 1 from secure
  mode. Cortex-A8 also needs to have ACTLR.IBE (bit 6) set, overlaping
  with ARM_ERRATA_430973 which also requires it.
- Cortex-A9, A12 and A17 do not require any extra configuration.

Note 1: Contrary to the initial version, this new series relies on
the arm64/kpti branch (I reuse the per-CPU vector hook for KVM).

Note 2: M-class CPUs are not affected and for R-class cores, the
mitigation doesn't make much sense since we do not enforce user/kernel
isolation.

[Christoffer: since the patches have significantly changed since v3,
I've dropped your RB tags]

Except for the question on patch 4, you can add my tag back to the
series.

I particularly enjoyed the cute xor hack in patch 3.

I feel that I've written too much of that kind of hacks lately...

Reviewed-by: Christoffer Dall <redacted>

Thanks!

	M.
-- 
Jazz is not dead. It just smells funny...