Subject: Re: [PATCH v2 2/2] arm64: Don't use KPTI where we have E0PD

From: Mark Brown <[email-masked]>
Date: Thu, 15 Aug 2019 19:00:30 +0100

On Thu, Aug 15, 2019 at 05:35:42PM +0100, Will Deacon wrote:

> I'm still unsure as to how this works with the kaslr check in
> kpti_install_ng_mappings(). Imagine you have a big.LITTLE system using
> kaslr where the boot CPU has E0PD but the secondary CPU doesn't, and
> requires kpti.

Yes, in fact that is my default big.LITTLE test case.

> In this case, I think we'll:

> 	1. Start off with global mappings installed by the boot CPU
> 	2. Detect KPTI as being required on the secondary CPU
> 	3. Avoid rewriting the page tables because kaslr_offset > 0

> At this point, we've got exposed global mappings on the secondary CPU.

Right, yes.  It'd be enormously helpful if KASLR were a bit more visible
in the boot logs or something since I yet again managed to do that bit
of my testing without KASLR actually taking effect :/

> Thinking about this further, I think we can simply move all of the
> 'kaslr_offset() > 0' checks used by the kpti code (i.e. in
> arm64_kernel_unmapped_at_el0(), kpti_install_ng_mappings() and
> unmap_kernel_at_el0()) into a helper function which does the check for
> E0PD as well. Perhaps 'kaslr_requires_kpti()' ?

> I think that should simplify your patch as well. What do you think?

Dunno about simplifying the patch particularly, looks very similar but
in any case it does appear to solve the problem - thanks.
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