linux-kernel - Re: [PATCH 11/14] MIPS: memblock: Print out kernel virtual mem layout

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Message-ID: <20180119142712.GA3101@mobilestation>
Date:   Fri, 19 Jan 2018 17:27:12 +0300
From:   Serge Semin <fancer.lancer@...il.com>
To:     Matt Redfearn <matt.redfearn@...s.com>
Cc:     Florian Fainelli <f.fainelli@...il.com>, ralf@...ux-mips.org,
        miodrag.dinic@...s.com, jhogan@...nel.org, goran.ferenc@...s.com,
        david.daney@...ium.com, paul.gortmaker@...driver.com,
        paul.burton@...s.com, alex.belits@...ium.com,
        Steven.Hill@...ium.com, alexander.sverdlin@...ia.com,
        kumba@...too.org, marcin.nowakowski@...s.com, James.hogan@...s.com,
        Peter.Wotton@...s.com, Sergey.Semin@...latforms.ru,
        linux-mips@...ux-mips.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 11/14] MIPS: memblock: Print out kernel virtual mem layout

On Fri, Jan 19, 2018 at 07:59:43AM +0000, Matt Redfearn <matt.redfearn@...s.com> wrote:

Hello Matt,

> Hi Serge,
> 
> 
> 
> On 18/01/18 20:18, Serge Semin wrote:
> >On Thu, Jan 18, 2018 at 12:03:03PM -0800, Florian Fainelli <f.fainelli@...il.com> wrote:
> >>On 01/17/2018 02:23 PM, Serge Semin wrote:
> >>>It is useful to have the kernel virtual memory layout printed
> >>>at boot time so to have the full information about the booted
> >>>kernel. In some cases it might be unsafe to have virtual
> >>>addresses freely visible in logs, so the %pK format is used if
> >>>one want to hide them.
> >>>
> >>>Signed-off-by: Serge Semin <fancer.lancer@...il.com>
> >>
> >>I personally like having that information because that helps debug and
> >>have a quick reference, but there appears to be a trend to remove this
> >>in the name of security:
> >>
> >>https://patchwork.kernel.org/patch/10124007/
> >>
> >>maybe hide this behind a configuration option?
> >
> >Yeah, arm code was the place I picked the function up.) But in my case
> >I've used %pK so the pointers would disappear from logging when
> >kptr_restrict sysctl is 1 or 2.
> >I agree, that we might need to make the printouts optional. If there is
> >any kernel config, which for instance increases the kernel security we
> >could also use it or anything else to discard the printouts at compile
> >time.
> 
> 
> Certainly, when KASLR is active it would be preferable to hide this
> information, so you could use CONFIG_RELOCATABLE. The existing KASLR stuff
> additionally hides this kind of information behind CONFIG_DEBUG_KERNEL, so
> that only people actively debugging the kernel see it:
> 
> http://elixir.free-electrons.com/linux/v4.15-rc8/source/arch/mips/kernel/setup.c#L604

Ok. I'll hide the printouts behind both of that config macros in the next patchset
version.

Regards,
-Sergey

> 
> Thanks,
> Matt
> 
> >
> >>-- 
> >>Florian