Re: [PATCH] [RFC] Introduce mmap randomization

From: Jason Cooper <jason@lakedaemon.net>
To: Nick Kralevich <nnk@google.com>
Cc: "Roberts, William C" <william.c.roberts@intel.com>,
	"linux-mm@kvack.org" <linux-mm@kvack.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"kernel-hardening@lists.openwall.com" 
	<kernel-hardening@lists.openwall.com>,
	"akpm@linux-foundation.org" <akpm@linux-foundation.org>,
	"keescook@chromium.org" <keescook@chromium.org>,
	"gregkh@linuxfoundation.org" <gregkh@linuxfoundation.org>,
	"jeffv@google.com" <jeffv@google.com>,
	"salyzyn@android.com" <salyzyn@android.com>,
	"dcashman@android.com" <dcashman@android.com>
Subject: Re: [PATCH] [RFC] Introduce mmap randomization
Date: Thu, 28 Jul 2016 21:07:34 +0000	[thread overview]
Message-ID: <20160728210734.GU4541@io.lakedaemon.net> (raw)
In-Reply-To: <CAFJ0LnEZW7Y1zfN8v0_ckXQZn1n-UKEhf_tSmNOgHwrrnNnuMg@mail.gmail.com>

On Wed, Jul 27, 2016 at 09:59:35AM -0700, Nick Kralevich wrote:
> On Tue, Jul 26, 2016 at 1:59 PM, Jason Cooper <jason@lakedaemon.net> wrote:
> >> > One thing I didn't make clear in my commit message is why this is good. Right
> >> > now, if you know An address within in a process, you know all offsets done with
> >> > mmap(). For instance, an offset To libX can yield libY by adding/subtracting an
> >> > offset. This is meant to make rops a bit harder, or In general any mapping offset
> >> > mmore difficult to find/guess.
> >
> > Are you able to quantify how many bits of entropy you're imposing on the
> > attacker?  Is this a chair in the hallway or a significant increase in
> > the chances of crashing the program before finding the desired address?
> 
> Quantifying the effect of many security changes is extremely
> difficult, especially for a probabilistic defense like ASLR. I would
> urge us to not place too high of a proof bar on this change.
> Channeling Spender / grsecurity team, ASLR gets it's benefit not from
> it's high benefit, but from it's low cost of implementation
> (https://forums.grsecurity.net/viewtopic.php?f=7&t=3367). This patch
> certainly meets the low cost of implementation bar.

Ok, I buy that with the 64bit-only caveat.

> In the Project Zero Stagefright post
> (http://googleprojectzero.blogspot.com/2015/09/stagefrightened.html),
> we see that the linear allocation of memory combined with the low
> number of bits in the initial mmap offset resulted in a much more
> predictable layout which aided the attacker. The initial random mmap
> base range was increased by Daniel Cashman in
> d07e22597d1d355829b7b18ac19afa912cf758d1, but we've done nothing to
> address page relative attacks.
> 
> Inter-mmap randomization will decrease the predictability of later
> mmap() allocations, which should help make data structures harder to
> find in memory. In addition, this patch will also introduce unmapped
> gaps between pages, preventing linear overruns from one mapping to
> another another mapping. I am unable to quantify how much this will
> improve security, but it should be > 0.

One person calls "unmapped gaps between pages" a feature, others call it
a mess. ;-)

> I like Dave Hansen's suggestion that this functionality be limited to
> 64 bits, where concerns about running out of address space are
> essentially nil. I'd be supportive of this change if it was limited to
> 64 bits.

Agreed.

thx,

Jason.