linux-kernel - Re: [RFC PATCH v5 00/18] pkeys-based page table hardening

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Message-ID: <9e1b474e-b822-469a-990f-b27389a7f0e0@arm.com>
Date: Wed, 1 Oct 2025 14:22:12 +0200
From: Kevin Brodsky <kevin.brodsky@....com>
To: Will Deacon <will@...nel.org>
Cc: Yang Shi <yang@...amperecomputing.com>, linux-hardening@...r.kernel.org,
 Rick Edgecombe <rick.p.edgecombe@...el.com>, linux-kernel@...r.kernel.org,
 Andrew Morton <akpm@...ux-foundation.org>, Andy Lutomirski
 <luto@...nel.org>, Catalin Marinas <catalin.marinas@....com>,
 Dave Hansen <dave.hansen@...ux.intel.com>,
 David Hildenbrand <david@...hat.com>, Ira Weiny <ira.weiny@...el.com>,
 Jann Horn <jannh@...gle.com>, Jeff Xu <jeffxu@...omium.org>,
 Joey Gouly <joey.gouly@....com>, Kees Cook <kees@...nel.org>,
 Linus Walleij <linus.walleij@...aro.org>,
 Lorenzo Stoakes <lorenzo.stoakes@...cle.com>, Marc Zyngier <maz@...nel.org>,
 Mark Brown <broonie@...nel.org>, Matthew Wilcox <willy@...radead.org>,
 Maxwell Bland <mbland@...orola.com>, "Mike Rapoport (IBM)"
 <rppt@...nel.org>, Peter Zijlstra <peterz@...radead.org>,
 Pierre Langlois <pierre.langlois@....com>,
 Quentin Perret <qperret@...gle.com>, Ryan Roberts <ryan.roberts@....com>,
 Thomas Gleixner <tglx@...utronix.de>, Vlastimil Babka <vbabka@...e.cz>,
 linux-arm-kernel@...ts.infradead.org, linux-mm@...ck.org, x86@...nel.org
Subject: Re: [RFC PATCH v5 00/18] pkeys-based page table hardening

On 18/09/2025 16:57, Will Deacon wrote:
> On Thu, Sep 18, 2025 at 04:15:52PM +0200, Kevin Brodsky wrote:
>> [...]
>>
>> For now, my plan is to stick to solution 3 from [2], i.e. force the
>> linear map to be PTE-mapped. This is easily done on arm64 as it is the
>> default, and is required for rodata=full, unless [1] is applied and the
>> system supports BBML2_NOABORT. See [1] for the potential performance
>> improvements we'd be missing out on (~5% ballpark). I'm not quite sure
>> what the picture looks like on x86 - it may well be more significant as
>> Rick suggested.
> Just as a data point, but forcing the linear map down to 4k would likely
> prevent us from being able to enable this on Android. We've measured a
> considerable (double digit %) increase in CPU power consumption for some
> real-life camera workloads when mapping the linear map at 4k granularity
> thanks to the additional memory traffic from the PTW.

Good to know!

> At some point, KFENCE required 4k granularity for the linear map, but we
> fixed it. rodata=full requires 4k granularity, but there are patches to
> fix that too. So I think we should avoid making the same mistake from
> the start for this series.

Understood, makes sense. I'll be looking into implementing the custom
PTP allocator I suggested above then.

- Kevin