linux-kernel - Re: [PATCH v3 1/4] mm: mmap: Add new /proc tunable for mmap

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Date:	Wed, 25 Nov 2015 11:16:46 -0800
From:	Daniel Cashman <dcashman@...roid.com>
To:	Kees Cook <keescook@...omium.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Cc:	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 v3 1/4] mm: mmap: Add new /proc tunable for mmap_base
 ASLR.

On 11/24/2015 04:47 PM, Kees Cook wrote:
> On Tue, Nov 24, 2015 at 4:40 PM, Andrew Morton
> <akpm@...ux-foundation.org> wrote:
>> On Wed, 18 Nov 2015 15:20:05 -0800 Daniel Cashman <dcashman@...roid.com> wrote:
>>
>>> --- a/kernel/sysctl.c
>>> +++ b/kernel/sysctl.c
>>> @@ -1568,6 +1568,28 @@ static struct ctl_table vm_table[] = {
>>>               .mode           = 0644,
>>>               .proc_handler   = proc_doulongvec_minmax,
>>>       },
>>> +#ifdef CONFIG_HAVE_ARCH_MMAP_RND_BITS
>>> +     {
>>> +             .procname       = "mmap_rnd_bits",
>>> +             .data           = &mmap_rnd_bits,
>>> +             .maxlen         = sizeof(mmap_rnd_bits),
>>> +             .mode           = 0644,
>>
>> Is there any harm in permitting the attacker to read these values?
>>
>> And is there any benefit in permitting non-attackers to read them?
> 
> I'm on the fence. Things like kernel/randomize_va_space is 644. But
> since I don't see a benefit in exposing them, let's make them all 600
> instead -- it's a new interface, better to keep it narrower now.

Is there any harm in allowing the attacker to read these values? Nothing
immediately comes to mind.  It is a form of information leakage, and I
guess a local attacker could use this information to calibrate an attack
or decide whether or not brute-forcing is a worthy approach, but this
easily could be leaked in other ways as well.

Is there a benefit to allowing non-attackers to read them?  Possibly
could be used in tests seeking to verify the system environment, but
again, this could be discovered in other ways.

I like Kees' suggestion of starting narrow and granting if need arises.

>>>
>>> +#ifdef CONFIG_HAVE_ARCH_MMAP_RND_BITS
>>> +int mmap_rnd_bits_min = CONFIG_ARCH_MMAP_RND_BITS_MIN;
>>> +int mmap_rnd_bits_max = CONFIG_ARCH_MMAP_RND_BITS_MAX;
>>> +int mmap_rnd_bits = CONFIG_ARCH_MMAP_RND_BITS;
>>> +#endif
>>> +#ifdef CONFIG_HAVE_ARCH_MMAP_RND_COMPAT_BITS
>>> +int mmap_rnd_compat_bits_min = CONFIG_ARCH_MMAP_RND_COMPAT_BITS_MIN;
>>> +int mmap_rnd_compat_bits_max = CONFIG_ARCH_MMAP_RND_COMPAT_BITS_MAX;
>>> +int mmap_rnd_compat_bits = CONFIG_ARCH_MMAP_RND_COMPAT_BITS;
>>
>> These could be __read_mostly.
>>
>> If one believes in such things.  One effect of __read_mostly is to
>> clump the write-often stuff into the same cachelines and I've never
>> been convinced that one outweighs the other...
> 
> The _min and _max values should be const, actually, since they're
> build-time selected. The _bits could easily be __read_mostly, yeah.

Yes, one would generally expect these to never be touched, and even if
they were, the threshold of __read_mostly would certainly be crossed.

-Dan

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