[PATCH] Add Prefer_MAP_32BIT_EXEC for Silvermont

[Mike Frysinger]

On 11 Dec 2015 12:59, Andi Kleen wrote:
> Zack Weinberg <zackw@panix.com> writes:
> > Just to back up this assertion: 16 bits of base address randomization
> > was brute-forceable in less than five minutes (on average) in 2004,
> > per http://www.cs.columbia.edu/~locasto/projects/candidacy/papers/shacham2004ccs.pdf
> > .  Digging into the kernel a little, it appears that MAP_32BIT (in
> > 4.3) selects a page-aligned address at random from within a 1GB (not
> > 2GB) space; that's *thirteen* bits of randomness, so we don't even
> > have to have the argument about how many more than 16 bits it would
> > take to be good enough in 2016; clearly *fewer* than 16 bits is
> > unacceptable.
> 
> The patch brings ASLR into a similar ballpark as with 32bit (plus minus
> 1 or 2 bits)
> 
> You're basically arguing that running 32bit is unacceptable, and that the
> main motivation for users to use 64bit is security and not performance.

yes, this is exactly why Chrome OS (and many Google systems) use 64bit,
to the point where we've sacrificed performance.  it's also the only
thing that's held us back from moving forward with x32.

> I don't think users would agree with you on that. 32bit is widely used,
> and clearly users find the risk from that acceptable. Also there
> are no security advisories around that ask users to stop using 32bit.

users don't understand or care.  appealing to the masses here doesn't
make sense.
-mike
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