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

From: Kees Cook
Date: Fri Nov 06 2015 - 15:52:43 EST

On Thu, Nov 5, 2015 at 10:44 AM, Daniel Cashman <dcashman@xxxxxxxxxxx> 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@xxxxxxxxxxx> 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:
> 1) Create a second set for compatibility, e.g. mmap_rnd_compat_bits,
> mmap_rnd_compat_bits_min, mmap_rnd_compat_bits_max and export it as with
> mmap_rnd_bits. This provides the most control and is truest to the
> spirit of this patch, but pollutes the Kconfigs and procfs a bit more,
> especially if we ever need a mmap_rnd_64compat_bits...
> 2) Get rid of the arch-independent nature of this patch and instead let
> each arch define its own Kconfig values and procfs entries. Essentially
> the same outcome as the above, but with less disruption in the common
> kernel code, although also with a potentially variable ABI.
> 3) Default to the lowest-supported, e.g. arm64 running with
> CONFIG_COMPAT would be limited to the same range as arm. This solution
> I think is highly undesirable, as it actually makes things worse for
> existing 64-bit platforms.
> 4) Support setting the COMPAT values by Kconfig, but don't expose them
> via procfs. This keeps the procfs change simple and gets most of its
> benefits.
> 5) Leave the COMPAT values specified in code, and only adjust introduce
> config and tunable options for the 64-bit processes. Basically keep
> this patch-set as-is and not give any benefit to compatible applications.
> My preference would be for either solutions 1 or 4, but would love
> feedback and/or other solutions. Thoughts?

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?


Kees Cook
Chrome OS Security
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