[PATCH] Add Prefer_MAP_32BIT_EXEC for Silvermont

[Zack Weinberg]

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.

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.

zw