linux-kernel - Re: [PATCH 1/2] security: Change name of CONFIG_DEBUG

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Message-Id: <20170119113305.GB5110@osiris>
Date:   Thu, 19 Jan 2017 12:33:05 +0100
From:   Heiko Carstens <heiko.carstens@...ibm.com>
To:     Mark Rutland <mark.rutland@....com>
Cc:     Laura Abbott <labbott@...hat.com>,
        Kees Cook <keescook@...omium.org>,
        Jason Wessel <jason.wessel@...driver.com>,
        Jonathan Corbet <corbet@....net>,
        Russell King <linux@...linux.org.uk>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        "James E.J. Bottomley" <jejb@...isc-linux.org>,
        Helge Deller <deller@....de>,
        Martin Schwidefsky <schwidefsky@...ibm.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
        Rob Herring <robh@...nel.org>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Len Brown <len.brown@...el.com>, Pavel Machek <pavel@....cz>,
        Jessica Yu <jeyu@...hat.com>, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-parisc@...r.kernel.org, linux-s390@...r.kernel.org,
        linux-pm@...r.kernel.org, kernel-hardening@...ts.openwall.com
Subject: Re: [PATCH 1/2] security: Change name of CONFIG_DEBUG_RODATA

On Thu, Jan 19, 2017 at 10:56:46AM +0000, Mark Rutland wrote:
> > +config HARDENED_PAGE_MAPPINGS
> > +	bool "Mark kernel mappings with stricter permissions (RO/W^X)"
> > +	default y
> > +	depends on ARCH_HAS_HARDENED_MAPPINGS
> > +	help
> > +          If this is set, kernel text and rodata memory will be made read-only,
> > +	  and non-text memory will be made non-executable. This provides
> > +	  protection against certain security attacks (e.g. executing the heap
> > +	  or modifying text).
> > +
> > +	  Unless your system has known restrictions or performance issues, it
> > +	  is recommended to say Y here.
> 
> It's somewhat unfortunate that this means the feature is no longer
> mandatory on arm64 (and s390+x86). We have a boot-time switch to turn
> the protections off, so I was hoping we could make this mandatory on all
> architectures with support.
> 
> It would be good to see if we could make this mandatory for arm and
> parisc, or if it really needs to be optional for either of those.

Looks like the config option is a no-op on parisc just like it is on
s390. Irrelavant of the config option at least on s390 the page tables for
kernel text and rodata will be read-only anyway.

This works since ages and I don't see a reason why this should be
changed. Also trying to disable this with the "rodata=" command line option
does not work at least on s390, and I guess this is true for parisc as
well.

The only thing implemented with CONFIG_DEBUG_RODATA on both architectures
is to emit a message that states memory has been protected
(mark_rodata_ro).
This just avoids a wrong "Kernel memory protection disabled." message.

So yes, I'd really like to keep this option mandatory.