linux-kernel - Re: [PATCH v4 1/4] mm: mmap: Add new /proc tunable for mmap

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Date:	Mon, 30 Nov 2015 17:00:35 -0800
From:	Kees Cook <keescook@...omium.org>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Daniel Cashman <dcashman@...roid.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Russell King - ARM Linux <linux@....linux.org.uk>,
	Ingo Molnar <mingo@...nel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	Jonathan Corbet <corbet@....net>,
	Don Zickus <dzickus@...hat.com>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Heinrich Schuchardt <xypron.glpk@....de>, jpoimboe@...hat.com,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	n-horiguchi@...jp.nec.com, Andrea Arcangeli <aarcange@...hat.com>,
	Mel Gorman <mgorman@...e.de>,
	Thomas Gleixner <tglx@...utronix.de>,
	David Rientjes <rientjes@...gle.com>,
	Linux-MM <linux-mm@...ck.org>,
	"linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
	Mark Salyzyn <salyzyn@...roid.com>,
	Jeffrey Vander Stoep <jeffv@...gle.com>,
	Nick Kralevich <nnk@...gle.com>,
	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will.deacon@....com>,
	"H. Peter Anvin" <hpa@...or.com>,
	"x86@...nel.org" <x86@...nel.org>, Hector Marco <hecmargi@....es>,
	Borislav Petkov <bp@...e.de>,
	Daniel Cashman <dcashman@...gle.com>
Subject: Re: [PATCH v4 1/4] mm: mmap: Add new /proc tunable for mmap_base ASLR.

On Mon, Nov 30, 2015 at 4:18 PM, Andrew Morton
<akpm@...ux-foundation.org> wrote:
> On Mon, 30 Nov 2015 16:04:36 -0800 Kees Cook <keescook@...omium.org> wrote:
>
>> >> > +#ifdef CONFIG_HAVE_ARCH_MMAP_RND_BITS
>> >> > +   {
>> >> > +           .procname       = "mmap_rnd_bits",
>> >> > +           .data           = &mmap_rnd_bits,
>> >> > +           .maxlen         = sizeof(mmap_rnd_bits),
>> >> > +           .mode           = 0600,
>> >> > +           .proc_handler   = proc_dointvec_minmax,
>> >> > +           .extra1         = (void *) &mmap_rnd_bits_min,
>> >> > +           .extra2         = (void *) &mmap_rnd_bits_max,
>> >>
>> >> hm, why the typecasts?  They're unneeded and are omitted everywhere(?)
>> >> else in kernel/sysctl.c.
>> >
>> > Oh.  Casting away constness.
>> >
>> > What's the thinking here?  They can change at any time so they aren't
>> > const so we shouldn't declare them to be const?
>>
>> The _min and _max values shouldn't be changing: they're decided based
>> on the various CONFIG options that calculate the valid min/maxes. Only
>> mmap_rnd_bits itself should be changing.
>
> hmpf.
>
> From: Andrew Morton <akpm@...ux-foundation.org>
> Subject: include/linux/sysctl.h: make ctl_table.extra1/2 const
>
> Nothing should be altering these values.  Declare the pointed-to values to
> be const so we can actually use const values.
>
> Cc: Kees Cook <keescook@...omium.org>
> Cc: Daniel Cashman <dcashman@...roid.com>
> Signed-off-by: Andrew Morton <akpm@...ux-foundation.org>
> ---
>
>  include/linux/sysctl.h |    4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff -puN include/linux/sysctl.h~a include/linux/sysctl.h
> --- a/include/linux/sysctl.h~a
> +++ a/include/linux/sysctl.h
> @@ -111,8 +111,8 @@ struct ctl_table
>         struct ctl_table *child;        /* Deprecated */
>         proc_handler *proc_handler;     /* Callback for text formatting */
>         struct ctl_table_poll *poll;
> -       void *extra1;
> -       void *extra2;
> +       const void *extra1;
> +       const void *extra2;
>  };
>
>  struct ctl_node {
> diff -puN kernel/sysctl.c~a kernel/sysctl.c
> _
>

If allyesconfig builds with this, I'm all for it. :)

-Kees

-- 
Kees Cook
Chrome OS & Brillo Security
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