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Date: Thu, 2 Feb 2017 06:25:22 -0800
From: Kees Cook <keescook@...omium.org>
To: Bhupesh Sharma <bhsharma@...hat.com>
Cc: "linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>,
"kernel-hardening@...ts.openwall.com" <kernel-hardening@...ts.openwall.com>, Daniel Cashman <dcashman@...gle.com>,
Michael Ellerman <mpe@...erman.id.au>, bhupesh.linux@...il.com,
Alexander Graf <agraf@...e.com>, Benjamin Herrenschmidt <benh@...nel.crashing.org>,
Paul Mackerras <paulus@...ba.org>, Anatolij Gustschin <agust@...x.de>,
Alistair Popple <alistair@...ple.id.au>, Matt Porter <mporter@...nel.crashing.org>,
Vitaly Bordug <vitb@...nel.crashing.org>, Scott Wood <oss@...error.net>,
Kumar Gala <galak@...nel.crashing.org>, Daniel Cashman <dcashman@...roid.com>
Subject: Re: [PATCH 1/2] powerpc: mm: support ARCH_MMAP_RND_BITS
On Wed, Feb 1, 2017 at 9:42 PM, Bhupesh Sharma <bhsharma@...hat.com> wrote:
> powerpc: arch_mmap_rnd() uses hard-coded values, (23-PAGE_SHIFT) for
> 32-bit and (30-PAGE_SHIFT) for 64-bit, 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.
>
> This patch makes sure that now powerpc mmap arch_mmap_rnd() approach
> is similar to other ARCHs like x86, arm64 and arm.
>
> Cc: Alexander Graf <agraf@...e.com>
> Cc: Benjamin Herrenschmidt <benh@...nel.crashing.org>
> Cc: Paul Mackerras <paulus@...ba.org>
> Cc: Michael Ellerman <mpe@...erman.id.au>
> Cc: Anatolij Gustschin <agust@...x.de>
> Cc: Alistair Popple <alistair@...ple.id.au>
> Cc: Matt Porter <mporter@...nel.crashing.org>
> Cc: Vitaly Bordug <vitb@...nel.crashing.org>
> Cc: Scott Wood <oss@...error.net>
> Cc: Kumar Gala <galak@...nel.crashing.org>
> Cc: Daniel Cashman <dcashman@...roid.com>
> Cc: Kees Cook <keescook@...omium.org>
> Signed-off-by: Bhupesh Sharma <bhsharma@...hat.com>
> ---
> arch/powerpc/Kconfig | 34 ++++++++++++++++++++++++++++++++++
> arch/powerpc/mm/mmap.c | 7 ++++---
> 2 files changed, 38 insertions(+), 3 deletions(-)
>
> diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
> index a8ee573fe610..b4a843f68705 100644
> --- a/arch/powerpc/Kconfig
> +++ b/arch/powerpc/Kconfig
> @@ -22,6 +22,38 @@ config MMU
> bool
> default y
>
> +config ARCH_MMAP_RND_BITS_MIN
> + default 5 if PPC_256K_PAGES && 32BIT
> + default 12 if PPC_256K_PAGES && 64BIT
> + default 7 if PPC_64K_PAGES && 32BIT
> + default 14 if PPC_64K_PAGES && 64BIT
> + default 9 if PPC_16K_PAGES && 32BIT
> + default 16 if PPC_16K_PAGES && 64BIT
> + default 11 if PPC_4K_PAGES && 32BIT
> + default 18 if PPC_4K_PAGES && 64BIT
> +
> +# max bits determined by the following formula:
> +# VA_BITS - PAGE_SHIFT - 4
> +# for e.g for 64K page and 64BIT = 48 - 16 - 4 = 28
> +config ARCH_MMAP_RND_BITS_MAX
> + default 10 if PPC_256K_PAGES && 32BIT
> + default 26 if PPC_256K_PAGES && 64BIT
> + default 12 if PPC_64K_PAGES && 32BIT
> + default 28 if PPC_64K_PAGES && 64BIT
> + default 14 if PPC_16K_PAGES && 32BIT
> + default 30 if PPC_16K_PAGES && 64BIT
> + default 16 if PPC_4K_PAGES && 32BIT
> + default 32 if PPC_4K_PAGES && 64BIT
> +
> +config ARCH_MMAP_RND_COMPAT_BITS_MIN
> + default 5 if PPC_256K_PAGES
> + default 7 if PPC_64K_PAGES
> + default 9 if PPC_16K_PAGES
> + default 11
> +
> +config ARCH_MMAP_RND_COMPAT_BITS_MAX
> + default 16
> +
> config HAVE_SETUP_PER_CPU_AREA
> def_bool PPC64
>
> @@ -100,6 +132,8 @@ config PPC
> select HAVE_EFFICIENT_UNALIGNED_ACCESS if !(CPU_LITTLE_ENDIAN && POWER7_CPU)
> select HAVE_KPROBES
> select HAVE_ARCH_KGDB
> + select HAVE_ARCH_MMAP_RND_BITS
> + select HAVE_ARCH_MMAP_RND_COMPAT_BITS if COMPAT
> select HAVE_KRETPROBES
> select HAVE_ARCH_TRACEHOOK
> select HAVE_MEMBLOCK
> diff --git a/arch/powerpc/mm/mmap.c b/arch/powerpc/mm/mmap.c
> index 2f1e44362198..babf59faab3b 100644
> --- a/arch/powerpc/mm/mmap.c
> +++ b/arch/powerpc/mm/mmap.c
> @@ -60,11 +60,12 @@ unsigned long arch_mmap_rnd(void)
> {
> unsigned long rnd;
>
> - /* 8MB for 32bit, 1GB for 64bit */
> +#ifdef CONFIG_COMPAT
> if (is_32bit_task())
> - rnd = get_random_long() % (1<<(23-PAGE_SHIFT));
> + rnd = get_random_long() & ((1UL << mmap_rnd_compat_bits) - 1);
> else
> - rnd = get_random_long() % (1UL<<(30-PAGE_SHIFT));
> +#endif
> + rnd = get_random_long() & ((1UL << mmap_rnd_bits) - 1);
>
> return rnd << PAGE_SHIFT;
> }
Awesome! This looks good to me based on my earlier analysis.
Reviewed-by: Kees Cook <keescook@...omium.org>
-Kees
--
Kees Cook
Pixel Security
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