Re: [PATCH v5 3/4] arm64: mm: support ARCH_MMAP_RND_BITS.

[Will Deacon <will.deacon@arm.com>]

Hi Daniel,

On Tue, Dec 01, 2015 at 12:10:57PM -0800, Daniel Cashman wrote:
> From: dcashman <dcashman@google.com>
> 
> arm64: arch_mmap_rnd() uses STACK_RND_MASK 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 default values as new minimums.
> 
> Signed-off-by: Daniel Cashman <dcashman@android.com>
> ---
>  arch/arm64/Kconfig   | 31 +++++++++++++++++++++++++++++++
>  arch/arm64/mm/mmap.c |  8 ++++++--
>  2 files changed, 37 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> index 871f217..fb57649 100644
> --- a/arch/arm64/Kconfig
> +++ b/arch/arm64/Kconfig
> @@ -51,6 +51,8 @@ config ARM64
>  	select HAVE_ARCH_JUMP_LABEL
>  	select HAVE_ARCH_KASAN if SPARSEMEM_VMEMMAP && !(ARM64_16K_PAGES && ARM64_VA_BITS_48)
>  	select HAVE_ARCH_KGDB
> +	select HAVE_ARCH_MMAP_RND_BITS if MMU
> +	select HAVE_ARCH_MMAP_RND_COMPAT_BITS if MMU && COMPAT

You can drop the 'if MMU' bits, since we don't support !MMU on arm64.

>  	select HAVE_ARCH_SECCOMP_FILTER
>  	select HAVE_ARCH_TRACEHOOK
>  	select HAVE_BPF_JIT
> @@ -104,6 +106,35 @@ config ARCH_PHYS_ADDR_T_64BIT
>  config MMU
>  	def_bool y
>  
> +config ARCH_MMAP_RND_BITS_MIN
> +       default 15 if ARM64_64K_PAGES
> +       default 17 if ARM64_16K_PAGES
> +       default 19

Is this correct? We currently have a mask of 0x3ffff, so that's 18 bits.

> +config ARCH_MMAP_RND_BITS_MAX
> +       default 19 if ARM64_VA_BITS=36
> +       default 20 if ARM64_64K_PAGES && ARM64_VA_BITS=39
> +       default 22 if ARM64_16K_PAGES && ARM64_VA_BITS=39
> +       default 24 if ARM64_VA_BITS=39
> +       default 23 if ARM64_64K_PAGES && ARM64_VA_BITS=42
> +       default 25 if ARM64_16K_PAGES && ARM64_VA_BITS=42
> +       default 27 if ARM64_VA_BITS=42
> +       default 30 if ARM64_VA_BITS=47
> +       default 29 if ARM64_64K_PAGES && ARM64_VA_BITS=48
> +       default 31 if ARM64_16K_PAGES && ARM64_VA_BITS=48
> +       default 33 if ARM64_VA_BITS=48
> +       default 15 if ARM64_64K_PAGES
> +       default 17 if ARM64_16K_PAGES
> +       default 19

Could you add a comment above this with the formula
(VA_BITS - PAGE_SHIFT - 3), please, so that we can update this easily in
the future if we need to?

Will

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