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Date: Tue, 3 Nov 2015 17:31:56 -0800 From: Andrew Morton <akpm@...ux-foundation.org> To: ebiederm@...ssion.com (Eric W. Biederman) Cc: Daniel Cashman <dcashman@...roid.com>, linux-kernel@...r.kernel.org, linux@....linux.org.uk, keescook@...omium.org, mingo@...nel.org, linux-arm-kernel@...ts.infradead.org, corbet@....net, dzickus@...hat.com, xypron.glpk@....de, jpoimboe@...hat.com, kirill.shutemov@...ux.intel.com, n-horiguchi@...jp.nec.com, aarcange@...hat.com, mgorman@...e.de, tglx@...utronix.de, rientjes@...gle.com, linux-mm@...ck.org, linux-doc@...r.kernel.org, salyzyn@...roid.com, jeffv@...gle.com, nnk@...gle.com, dcashman <dcashman@...gle.com> Subject: Re: [PATCH v2 1/2] mm: mmap: Add new /proc tunable for mmap_base ASLR. On Tue, 03 Nov 2015 18:40:31 -0600 ebiederm@...ssion.com (Eric W. Biederman) wrote: > Andrew Morton <akpm@...ux-foundation.org> writes: > > > On Tue, 3 Nov 2015 10:10:03 -0800 Daniel Cashman <dcashman@...roid.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. 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 from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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