Re: [PATCH v2 1/2] mm: mmap: Add new /proc tunable for mmap_base ASLR.

From: Andrew Morton <akpm@linux-foundation.org>
To: ebiederm@xmission.com (Eric W. Biederman)
Cc: Daniel Cashman <dcashman@android.com>,
	linux-kernel@vger.kernel.org, linux@arm.linux.org.uk,
	keescook@chromium.org, mingo@kernel.org,
	linux-arm-kernel@lists.infradead.org, corbet@lwn.net,
	dzickus@redhat.com, xypron.glpk@gmx.de, jpoimboe@redhat.com,
	kirill.shutemov@linux.intel.com, n-horiguchi@ah.jp.nec.com,
	aarcange@redhat.com, mgorman@suse.de, tglx@linutronix.de,
	rientjes@google.com, linux-mm@kvack.org,
	linux-doc@vger.kernel.org, salyzyn@android.com, jeffv@google.com,
	nnk@google.com, dcashman <dcashman@google.com>
Subject: Re: [PATCH v2 1/2] mm: mmap: Add new /proc tunable for mmap_base ASLR.
Date: Tue, 3 Nov 2015 17:31:56 -0800	[thread overview]
Message-ID: <20151103173156.9ca17f52.akpm@linux-foundation.org> (raw)
In-Reply-To: <87k2pyppfk.fsf@x220.int.ebiederm.org>

On Tue, 03 Nov 2015 18:40:31 -0600 ebiederm@xmission.com (Eric W. Biederman) wrote:

> Andrew Morton <akpm@linux-foundation.org> writes:
> 
> > On Tue,  3 Nov 2015 10:10:03 -0800 Daniel Cashman <dcashman@android.com> wrote:
> >
> >> ASLR currently only uses 8 bits to generate the random offset for the
> >> mmap base address on 32 bit architectures. This value was chosen to
> >> prevent a poorly chosen value from dividing the address space in such
> >> a way as to prevent large allocations. This may not be an issue on all
> >> platforms. Allow the specification of a minimum number of bits so that
> >> platforms desiring greater ASLR protection may determine where to place
> >> the trade-off.
> >
> > Can we please include a very good description of the motivation for this
> > change?  What is inadequate about the current code, what value does the
> > enhancement have to our users, what real-world problems are being solved,
> > etc.
> >
> > Because all we have at present is "greater ASLR protection", which doesn't
> > really tell anyone anything.
> 
> The description seemed clear to me.
> 
> More random bits, more entropy, more work needed to brute force.
> 
> 8 bits only requires 256 tries (or a 1 in 256) chance to brute force
> something.

Of course, but that's not really very useful.

> We have seen in the last couple of months on Android how only having 8 bits
> doesn't help much.

Now THAT is important.  What happened here and how well does the
proposed fix improve things?  How much longer will a brute-force attack
take to succeed, with a particular set of kernel parameters?  Is the
new duration considered to be sufficiently long and if not, are there
alternative fixes we should be looking at?

Stuff like this.

> Each additional bit doubles the protection (and unfortunately also
> increases fragmentation of the userspace address space).

OK, so the benefit comes with a cost and people who are configuring
systems (and the people who are reviewing this patchset!) need to
understand the tradeoffs.  Please.