linux-kernel - Re: [patch V4 2/8] mm/highmem: Provide CONFIG_DEBUG_KMAP_LOCAL_FORCE

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Date:   Thu, 19 Nov 2020 09:19:46 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Mel Gorman <mgorman@...e.de>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        Christoph Hellwig <hch@....de>,
        Matthew Wilcox <willy@...radead.org>,
        Daniel Vetter <daniel@...ll.ch>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linux-MM <linux-mm@...ck.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>,
        Daniel Bristot de Oliveira <bristot@...hat.com>
Subject: Re: [patch V4 2/8] mm/highmem: Provide CONFIG_DEBUG_KMAP_LOCAL_FORCE_MAP

On Thu, Nov 19, 2020 at 12:46 AM Mel Gorman <mgorman@...e.de> wrote:
>
> And not start thinking it as a security hardening option.

It's probably the reverse of a hardening option, since it will cause
mapping of stuff in known and controllable virtual addresses.

Although any kmap'able page is likely to already be something you can
read anyway (ie page cache long after security checks have been done).
So it probably really doesn't matter either way.

The only thing that is certain is that it's going to slow down
important code-paths.

             Linus