Messages in this thread | | | From | (Eric W. Biederman) | Date | Tue, 03 Nov 2015 18:40:31 -0600 | Subject | Re: [PATCH v2 1/2] mm: mmap: Add new /proc tunable for mmap_base ASLR. |
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Andrew Morton <akpm@linux-foundation.org> writes:
> On Tue, 3 Nov 2015 10:10:03 -0800 Daniel Cashman <dcashman@android.com> 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.
We have seen in the last couple of months on Android how only having 8 bits doesn't help much.
Each additional bit doubles the protection (and unfortunately also increases fragmentation of the userspace address space).
Eric
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