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

[ebiederm@xmission.com (Eric W. Biederman)]

Daniel Cashman <dcashman@android.com> writes:

> On 10/28/2015 08:41 PM, Eric W. Biederman wrote:
>> Dan Cashman <dcashman@android.com> writes:
>> 
>>>>> This all would be much cleaner if the arm architecture code were just to
>>>>> register the sysctl itself.
>>>>>
>>>>> As it sits this looks like a patchset that does not meaninfully bisect,
>>>>> and would result in code that is hard to trace and understand.
>>>>
>>>> I believe the intent is to follow up with more architecture specific
>>>> patches to allow each architecture to define the number of bits to use
>>>
>>> Yes.  I included these patches together because they provide mutual
>>> context, but each has a different outcome and they could be taken
>>> separately.
>> 
>> They can not.  The first patch is incomplete by itself.
>
> Could you be more specific in what makes the first patch incomplete?  Is
> it because it is essentially a no-op without additional architecture
> changes (e.g. the second patch) or is it specifically because it
> introduces and uses the three "mmap_rnd_bits*" variables without
> defining them?  If the former, I'd like to avoid combining the general
> procfs change with any architecture-specific one(s).  If the latter, I
> hope the proposal below addresses that.

A bit of both.  The fact that the code can not compile in the first
patch because of missing variables is distressing.  Having the arch
specific code as a separate patch is fine, but they need to remain in
the same patchset.

>>> The arm architecture-specific portion allows the changing
>>> of the number of bits used for mmap ASLR, useful even without the
>>> sysctl.  The sysctl patch (patch 1) provides another way of setting
>>> this value, and the hope is that this will be adopted across multiple
>>> architectures, with the arm changes (patch 2) providing an example.  I
>>> hope to follow this with changes to arm64 and x86, for example.
>> 
>> If you want to make the code generic.  Please maximize the sharing.
>> That is please define the variables in a generic location, as well
>> as the Kconfig variables (if possible).
>> 
>> As it is you have an architecture specific piece of code that can not be
>> reused without duplicating code, and that is just begging for problems.
>
> I think it would make sense to move the variable definitions into
> mm/mmap.c, included conditionally based on the presence of
> CONFIG_ARCH_MMAP_RND_BITS.
>
> As for the Kconfigs, I am open to suggestions.  I considered declaring
> and documenting ARCH_MMAP_RND_BITS in arch/Kconfig, but I would like it
> to be bounded in range by the _MIN and _MAX values, which necessarily
> must be defined in the arch-specific Kconfigs.  Thus, we'd have
> ARCH_MMAP_RND_BITS declared in arch/Kconfig as it currently is in
> arch/arm/Kconfig defaulting to _MIN, and would declare both the _MIN and
> _MAX in arch/Kconfig, while specifying default values in
> arch/${ARCH}/Kconfig.
>
> Would these changes be more acceptable?

Yes.  I don't think you can do much about the Kconfigs so I would not
worry about that too much.

Eric