Re: [PATCH v3 3/4] arm64: mm: support ARCH_MMAP_RND_BITS.

Will Deacon
2015-11-23T15:05:08Z

On Wed, Nov 18, 2015 at 03:20:07PM -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@google.com>
> ---
> arch/arm64/Kconfig | 23 +++++++++++++++++++++++
> arch/arm64/mm/mmap.c | 6 ++++--
> 2 files changed, 27 insertions(+), 2 deletions(-)
>
> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> index 9ac16a4..be38e4c 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
> select HAVE_ARCH_KGDB
> + select HAVE_ARCH_MMAP_RND_BITS
> + select HAVE_ARCH_MMAP_RND_COMPAT_BITS if COMPAT
> select HAVE_ARCH_SECCOMP_FILTER
> select HAVE_ARCH_TRACEHOOK
> select HAVE_BPF_JIT
> @@ -104,6 +106,27 @@ config ARCH_PHYS_ADDR_T_64BIT
> config MMU
> def_bool y
>
> +config ARCH_MMAP_RND_BITS_MIN
> + default 15 if ARM64_64K_PAGES
> + default 19
> +
> +config ARCH_MMAP_RND_BITS_MAX
> + default 20 if ARM64_64K_PAGES && ARCH_VA_BITS=39
> + default 24 if ARCH_VA_BITS=39
> + default 23 if ARM64_64K_PAGES && ARCH_VA_BITS=42
> + default 27 if ARCH_VA_BITS=42
> + default 29 if ARM64_64K_PAGES && ARCH_VA_BITS=48
> + default 33 if ARCH_VA_BITS=48
> + default 15 if ARM64_64K_PAGES
> + default 19
> +
> +config ARCH_MMAP_RND_COMPAT_BITS_MIN
> + default 7 if ARM64_64K_PAGES
> + default 11

FYI: we now support 16k pages too, so this might need updating. It would
be much nicer if this was somehow computed rather than have the results
all open-coded like this.

Will