Re: [PATCH v2 1/2] mm: mmap: Add new /proc tunable for mmap_base ASLR.

[Andrew Morton]

On Tue, 03 Nov 2015 18:40:31 -0600 ebiederm@xxxxxxxxxxxx (Eric W. Biederman) wrote:

> Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> writes:
> 
> > On Tue,  3 Nov 2015 10:10:03 -0800 Daniel Cashman <dcashman@xxxxxxxxxxx> wrote:
> >
> >> ASLR currently only uses 8 bits to generate the random offset for the
> >> mmap base address on 32 bit architectures. This value was chosen to
> >> prevent a poorly chosen value from dividing the address space in such
> >> a way as to prevent large allocations. This may not be an issue on all
> >> platforms. Allow the specification of a minimum number of bits so that
> >> platforms desiring greater ASLR protection may determine where to place
> >> the trade-off.
> >
> > Can we please include a very good description of the motivation for this
> > change?  What is inadequate about the current code, what value does the
> > enhancement have to our users, what real-world problems are being solved,
> > etc.
> >
> > Because all we have at present is "greater ASLR protection", which doesn't
> > really tell anyone anything.
> 
> The description seemed clear to me.
> 
> More random bits, more entropy, more work needed to brute force.
> 
> 8 bits only requires 256 tries (or a 1 in 256) chance to brute force
> something.

Of course, but that's not really very useful.

> We have seen in the last couple of months on Android how only having 8 bits
> doesn't help much.

Now THAT is important.  What happened here and how well does the
proposed fix improve things?  How much longer will a brute-force attack
take to succeed, with a particular set of kernel parameters?  Is the
new duration considered to be sufficiently long and if not, are there
alternative fixes we should be looking at?

Stuff like this.

> Each additional bit doubles the protection (and unfortunately also
> increases fragmentation of the userspace address space).

OK, so the benefit comes with a cost and people who are configuring
systems (and the people who are reviewing this patchset!) need to
understand the tradeoffs.  Please.
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