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Date: Sun, 23 Apr 2017 21:53:21 +1000 (AEST)
From: Michael Ellerman <>
To: Michael Ellerman <>,
Subject: Re: [v2] powerpc/mm: Add support for runtime configuration of ASLR limits

On Thu, 2017-04-20 at 14:36:20 UTC, Michael Ellerman wrote:
> Add powerpc support for mmap_rnd_bits and mmap_rnd_compat_bits, which are two
> sysctls that allow a user to configure the number of bits of randomness used for
> Because of the way the Kconfig for ARCH_MMAP_RND_BITS is defined, we have to
> construct at least the MIN value in Kconfig, vs in a header which would be more
> natural. Given that we just go ahead and do it all in Kconfig.
> At least according to the code (the documentation makes no mention of it), the
> value is defined as the number of bits of randomisation *of the page*, not the
> address. This makes some sense, with larger page sizes more of the low bits are
> forced to zero, which would reduce the randomisation if we didn't take the
> PAGE_SIZE into account. However it does mean the min/max values have to change
> depending on the PAGE_SIZE in order to actually limit the amount of address
> space consumed by the randomisation.
> The result of that is that we have to define the default values based on both
> 32-bit vs 64-bit, but also the configured PAGE_SIZE. Furthermore now that we
> have 128TB address space support on Book3S, we also have to take that into
> account.
> Finally we can wire up the value in arch_mmap_rnd().
> Signed-off-by: Michael Ellerman <>
> Signed-off-by: Bhupesh Sharma <>
> Reviewed-by: Kees Cook <>
> Reviewed-by: Aneesh Kumar K.V <>

Applied to powerpc next.


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