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

[Kees Cook]

On Mon, Nov 9, 2015 at 10:56 AM, Daniel Cashman <dcashman@xxxxxxxxxxx> wrote:
> On 11/08/2015 07:47 PM, Michael Ellerman wrote:
>> On Fri, 2015-11-06 at 12:52 -0800, Kees Cook wrote:
>>> 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:
>>>
>>> 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.
>
> I may be misunderstanding the suggestion, or perhaps simply too
> conservative in my desire to prevent bad values, but I still think we
> would have need for two min-max ranges.  If using a single
> mmap_rnd_bits_compat value, there are two approaches: to either use
> mmap_rnd_bits for 32-bit applications and then add the compat value for
> 64-bit or the opposite, to have mmap_rnd_bits be the default and
> subtract the compat value for the 32-bit applications.  In either case,
> the compat value would need to be sensibly bounded, and that bounding
> depends on acceptable values for both 32 and 64 bit applications.

Yeah, I think I wasn't thinking about this well. I think two sysctls
should be fine: mmap_rnd_bits and mmap_compat_rnd_bits. And
internally, we'd have a second set of CONFIGs (and ranges) to deal
with CONFIG_COMPAT.

-Kees

-- 
Kees Cook
Chrome OS Security
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