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Date: Wed, 30 Oct 2019 20:28:17 +0100
From: Christophe Leroy <>
To: Kees Cook <>
Cc: Russell Currey <>,,,,,,,
Subject: Re: [PATCH v5 0/5] Implement STRICT_MODULE_RWX for powerpc

Le 30/10/2019 à 19:30, Kees Cook a écrit :
> On Wed, Oct 30, 2019 at 09:58:19AM +0100, Christophe Leroy wrote:
>> Le 30/10/2019 à 08:31, Russell Currey a écrit :
>>> v4 cover letter:
>>> v3 cover letter:
>>> Changes since v4:
>>> 	[1/5]: Addressed review comments from Michael Ellerman (thanks!)
>>> 	[4/5]: make ARCH_HAS_STRICT_MODULE_RWX depend on
>>> 	       ARCH_HAS_STRICT_KERNEL_RWX to simplify things and avoid
>>> 	       STRICT_MODULE_RWX being *on by default* in cases where
>>> 	       STRICT_KERNEL_RWX is *unavailable*
>>> 	[5/5]: split skiroot_defconfig changes out into its own patch
>>> The whole Kconfig situation is really weird and confusing, I believe the
>>> correct resolution is to change arch/Kconfig but the consequences are so
>>> minor that I don't think it's worth it, especially given that I expect
>>> powerpc to have mandatory strict RWX Soon(tm).
>> I'm not such strict RWX can be made mandatory due to the impact it has on
>> some subarches:
>> - On the 8xx, unless all areas are 8Mbytes aligned, there is a significant
>> overhead on TLB misses. And Aligning everthing to 8M is a waste of RAM which
>> is not acceptable on systems having very few RAM.
>> - On hash book3s32, we are able to map the kernel BATs. With a few alignment
>> constraints, we are able to provide STRICT_KERNEL_RWX. But we are unable to
>> provide exec protection on page granularity. Only on 256Mbytes segments. So
>> for modules, we have to have the vmspace X. It is also not possible to have
>> a kernel area RO. Only user areas can be made RO.
> As I understand it, the idea was for it to be mandatory (or at least
> default-on) only for the subarches where it wasn't totally insane to
> accomplish. :) (I'm not familiar with all the details on the subarchs,
> but it sounded like the more modern systems would be the targets for
> this?)

Yes I guess so.

I was just afraid by "I expect powerpc to have mandatory strict RWX"

By the way, we have an open issue #223 namely 'Make strict kernel RWX 
the default on 64-bit', so no worry as 32-bit is aside for now.


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