[PATCH] Add Prefer_MAP_32BIT_EXEC for Silvermont

[H.J. Lu]

On Fri, Dec 11, 2015 at 6:01 PM, Jeff Law <law@redhat.com> wrote:
> On 12/11/2015 05:31 PM, Zack Weinberg wrote:
>>
>> On Fri, Dec 11, 2015 at 7:14 PM, Andi Kleen <andi@firstfloor.org> wrote:
>>>>
>>>> And I'd argue that this is killing ASLR at a level that it should be
>>>> an opt-out rather than opt-in.  Crippling ASLR is, IMHO,
>>>> unacceptable.
>>>
>>>
>>> You're arguing then that running 32bit code is unacceptable.
>>
>>
>> I don't see that that follows.
>>
>> Right now, 32-bit code has security margin X and 64-bit code has
>> security margin Y > X.  The proposed patch *reduces* the security
>> margin of 64-bit code from Y to X (give or take).  That may be, and
>> IMHO is, an unacceptable change *even if* X is agreed to be adequate,
>> or anyway the best that can be done for 32-bit.
>
> Exactly.  For a 64 bit application, this change will essentially cripple
> ASLR if I understand the patch correctly.  That is unacceptable to me and
> likely to Red Hat as a whole.
>
>>
>> Fundamentally, my issue here is that there are people right now
>> depending on this security margin to be Y, so a glibc upgrade should
>> not silently remove that.  It is a compatibility break of the worst
>> kind: completely invisible in normal operation, but the system no
>> longer has a property you were counting on to protect you under
>> abnormal (adversarial) conditions.
>
> Right.  And in fact, ASLR is the margin by which some currently known
> vulnerabilities have not been turned into proof of concept exploits.
>
> ASLR, while not perfect, while bypassable via various information leaks, is
> still a vital component in the overall security profile for Linux,
> particularly for 64 bit OSs & applications.
>

Here is the updated patch to make it opt-in.  OK for master?

-- 
H.J.
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