Re: [PATCH v2 2/2] arm: mm: support ARCH_MMAP_RND_BITS.

[Michael Ellerman <mpe@ellerman.id.au>]

On Fri, 2015-11-06 at 12:52 -0800, Kees Cook wrote:
> On Thu, Nov 5, 2015 at 10:44 AM, Daniel Cashman <dcashman@android.com> wrote:
> > On 11/04/2015 10:30 AM, Daniel Cashman wrote:
> > > On 11/3/15 3:21 PM, Kees Cook wrote:
> > > > On Tue, Nov 3, 2015 at 3:14 PM, Daniel Cashman <dcashman@android.com> wrote:
> > > > > On 11/03/2015 11:19 AM, Kees Cook wrote:
> > > > > > Do you have patches for x86 and arm64?
> > > > > 
> > > > > I was holding off on those until I could gauge upstream reception.  If
> > > > > desired, I could put those together and add them as [PATCH 3/4] and
> > > > > [PATCH 4/4].
> > > > 
> > > > If they're as trivial as I'm hoping, yeah, let's toss them in now. If
> > > > not, skip 'em. PowerPC, MIPS, and s390 should be relatively simple
> > > > too, but one or two of those have somewhat stranger calculations when
> > > > I looked, so their Kconfigs may not be as clean.
> > > 
> > > Creating the patches should be simple, it's the choice of minimum and
> > > maximum values for each architecture that I'd be most concerned about.
> > > I'll put them together, though, and the ranges can be changed following
> > > discussion with those more knowledgeable, if needed.  I also don't have
> > > devices on which to test the PowerPC, MIPS and s390 changes, so I'll
> > > need someone's help for that.
> > 
> > Actually, in preparing the x86 and arm64 patches, it became apparent
> > that the current patch-set does not address 32-bit executables running
> > on 64-bit systems (compatibility mode), since only one procfs
> > mmap_rnd_bits variable is created and exported. Some possible solutions:
> 
> How about a single new CONFIG+sysctl that is the compat delta. For
> example, on x86, it's 20 bits. Then we don't get splashed with a whole
> new set of min/maxes, but we can reasonably control compat?

Do you mean in addition to mmap_rnd_bits?

So we'd end up with mmap_rnd_bits and also mmap_rnd_bits_compat_delta?
(naming TBD)

If so yeah I think that would work.

It would have the nice property of allowing you to add some more randomness to
all processes by bumping mmap_rnd_bits. But at the same time if you want to add
a lot more randomness to 64-bit processes, but just a bit (or none) to 32-bit
processes you can also do that.

cheers

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