Messages in this thread | | | Date | Tue, 3 Nov 2015 15:18:47 -0800 | Subject | Re: [PATCH v2 2/2] arm: mm: support ARCH_MMAP_RND_BITS. | From | Kees Cook <> |
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On Tue, Nov 3, 2015 at 2:39 PM, Russell King - ARM Linux <linux@arm.linux.org.uk> wrote: > On Tue, Nov 03, 2015 at 11:19:44AM -0800, Kees Cook wrote: >> On Tue, Nov 3, 2015 at 10:10 AM, Daniel Cashman <dcashman@android.com> wrote: >> > From: dcashman <dcashman@google.com> >> > >> > arm: arch_mmap_rnd() uses a hard-code value of 8 to generate the >> > random offset for the mmap base address. This value represents a >> > compromise between increased ASLR effectiveness and avoiding >> > address-space fragmentation. Replace it with a Kconfig option, which >> > is sensibly bounded, so that platform developers may choose where to >> > place this compromise. Keep 8 as the minimum acceptable value. >> > >> > Signed-off-by: Daniel Cashman <dcashman@google.com> >> >> Acked-by: Kees Cook <keescook@chromium.org> >> >> Russell, if you don't see any problems here, it might make sense not >> to put this through the ARM patch tracker since it depends on the 1/2, >> and I think x86 and arm64 (and possibly other arch) changes are coming >> too. > > Yes, it looks sane, though I do wonder whether there should also be > a Kconfig option to allow archtectures to specify the default, instead > of the default always being the minimum randomisation. I can see scope > to safely pushing our mmap randomness default to 12, especially on 3GB > setups, as we already have 11 bits of randomness on the sigpage and if > enabled, 13 bits on the heap.
My thinking is that the there shouldn't be a reason to ever have a minimum that was below the default. I have no objection with it, but it seems needless. Frankly minimum is "0", really, so I don't think it makes much sense to have default != arch minimum. I actually view "arch minimum" as "known good", so if we are happy with raising the "known good" value, that should be the new minimum.
-Kees
-- Kees Cook Chrome OS Security
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