On 11/3/15 5:31 PM, Andrew Morton wrote: > 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. The direct motivation here was in response to the libstagefright vulnerabilities that affected Android, specifically to information provided by Google's project zero at: http://googleprojectzero.blogspot.com/2015/09/stagefrightened.html The attack there specifically used the limited randomness used in generating the mmap base address as part of a brute-force-based exploit. In this particular case, the attack was against the mediaserver process on Android, which was limited to respawning every 5 seconds, giving the attacker an average expected success rate of defeating the mmap ASLR after over 10 minutes (128 tries at 5 seconds each). With change to the maximum proposed value of 16 bits, this would change to over 45 hours (32768 tries), which would make the user of such a system much more likely to notice such an attack. I understand the desire for this clarification, and will happily try to improve the explanation for this change, especially so that those considering use of this option understand the tradeoffs, but I also view this as one particular hardening change which is a component of making attacks such as these harder, rather than the only solution. As for the clarification itself, where would you like it? I could include a cover letter for this patch-set, elaborate more in the commit message itself, add more to the Kconfig help description, or some combination of the above. Thank You, Dan -- 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>