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Re: [PATCH v2 2/2] arm: mm: support ARCH_MMAP_RND_BITS.

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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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[PATCH v2 1/2] mm: mmap: Add new /proc tunable for mmap_base ASLR. Daniel Cashman <dcashman@android.com> - 2015-11-03 19:20 +0100
  [PATCH v2 2/2] arm: mm: support ARCH_MMAP_RND_BITS. Daniel Cashman <dcashman@android.com> - 2015-11-03 19:20 +0100
    Re: [PATCH v2 2/2] arm: mm: support ARCH_MMAP_RND_BITS. Kees Cook <keescook@chromium.org> - 2015-11-03 20:30 +0100
      Re: [PATCH v2 2/2] arm: mm: support ARCH_MMAP_RND_BITS. Russell King - ARM Linux <linux@arm.linux.org.uk> - 2015-11-03 23:40 +0100
        Re: [PATCH v2 2/2] arm: mm: support ARCH_MMAP_RND_BITS. Kees Cook <keescook@chromium.org> - 2015-11-04 00:20 +0100
          Re: [PATCH v2 2/2] arm: mm: support ARCH_MMAP_RND_BITS. Daniel Cashman <dcashman@android.com> - 2015-11-04 19:30 +0100
      Re: [PATCH v2 2/2] arm: mm: support ARCH_MMAP_RND_BITS. Daniel Cashman <dcashman@android.com> - 2015-11-04 00:20 +0100
        Re: [PATCH v2 2/2] arm: mm: support ARCH_MMAP_RND_BITS. Kees Cook <keescook@chromium.org> - 2015-11-04 00:30 +0100
          Re: [PATCH v2 2/2] arm: mm: support ARCH_MMAP_RND_BITS. Daniel Cashman <dcashman@android.com> - 2015-11-04 19:40 +0100
            Re: [PATCH v2 2/2] arm: mm: support ARCH_MMAP_RND_BITS. Daniel Cashman <dcashman@android.com> - 2015-11-05 19:50 +0100
              Re: [PATCH v2 2/2] arm: mm: support ARCH_MMAP_RND_BITS. Kees Cook <keescook@chromium.org> - 2015-11-06 22:00 +0100
                Re: [PATCH v2 2/2] arm: mm: support ARCH_MMAP_RND_BITS. Michael Ellerman <mpe@ellerman.id.au> - 2015-11-09 04:50 +0100
                Re: [PATCH v2 2/2] arm: mm: support ARCH_MMAP_RND_BITS. Daniel Cashman <dcashman@android.com> - 2015-11-09 20:00 +0100
                Re: [PATCH v2 2/2] arm: mm: support ARCH_MMAP_RND_BITS. Kees Cook <keescook@chromium.org> - 2015-11-09 22:30 +0100
  Re: [PATCH v2 1/2] mm: mmap: Add new /proc tunable for mmap_base ASLR. Kees Cook <keescook@chromium.org> - 2015-11-03 20:20 +0100
  Re: [PATCH v2 1/2] mm: mmap: Add new /proc tunable for mmap_base  ASLR. Andrew Morton <akpm@linux-foundation.org> - 2015-11-04 01:10 +0100
    Re: [PATCH v2 1/2] mm: mmap: Add new /proc tunable for mmap_base ASLR. ebiederm@xmission.com (Eric W. Biederman) - 2015-11-04 01:50 +0100
      Re: [PATCH v2 1/2] mm: mmap: Add new /proc tunable for mmap_base  ASLR. Andrew Morton <akpm@linux-foundation.org> - 2015-11-04 02:30 +0100
        Re: [PATCH v2 1/2] mm: mmap: Add new /proc tunable for mmap_base  ASLR. Daniel Cashman <dcashman@android.com> - 2015-11-04 20:40 +0100
          Re: [PATCH v2 1/2] mm: mmap: Add new /proc tunable for mmap_base  ASLR. Andrew Morton <akpm@linux-foundation.org> - 2015-11-04 23:10 +0100
          Re: [PATCH v2 1/2] mm: mmap: Add new /proc tunable for mmap_base ASLR. ebiederm@xmission.com (Eric W. Biederman) - 2015-11-04 23:30 +0100
            Re: [PATCH v2 1/2] mm: mmap: Add new /proc tunable for mmap_base ASLR. Kees Cook <keescook@chromium.org> - 2015-11-04 23:40 +0100
  Re: [PATCH v2 1/2] mm: mmap: Add new /proc tunable for mmap_base  ASLR. Michal Hocko <mhocko@kernel.org> - 2015-11-04 10:50 +0100
    Re: [PATCH v2 1/2] mm: mmap: Add new /proc tunable for mmap_base ASLR. ebiederm@xmission.com (Eric W. Biederman) - 2015-11-04 20:40 +0100
      Re: [PATCH v2 1/2] mm: mmap: Add new /proc tunable for mmap_base  ASLR. Daniel Cashman <dcashman@android.com> - 2015-11-04 20:40 +0100

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