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. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>