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Date: Tue, 03 Nov 2015 18:40:31 -0600 From: ebiederm@...ssion.com (Eric W. Biederman) To: Andrew Morton <akpm@...ux-foundation.org> 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. 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. 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 -- 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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