From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755985AbbKCTTt (ORCPT ); Tue, 3 Nov 2015 14:19:49 -0500 Received: from mail-io0-f181.google.com ([209.85.223.181]:35770 "EHLO mail-io0-f181.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755874AbbKCTTp (ORCPT ); Tue, 3 Nov 2015 14:19:45 -0500 MIME-Version: 1.0 In-Reply-To: <1446574204-15567-2-git-send-email-dcashman@android.com> References: <1446574204-15567-1-git-send-email-dcashman@android.com> <1446574204-15567-2-git-send-email-dcashman@android.com> Date: Tue, 3 Nov 2015 11:19:44 -0800 X-Google-Sender-Auth: zzDh0XVH_ZSH1630Z8vB4wTRlDU Message-ID: Subject: Re: [PATCH v2 2/2] arm: mm: support ARCH_MMAP_RND_BITS. From: Kees Cook To: Daniel Cashman Cc: LKML , Russell King - ARM Linux , Andrew Morton , Ingo Molnar , "linux-arm-kernel@lists.infradead.org" , Jonathan Corbet , Don Zickus , "Eric W. Biederman" , Heinrich Schuchardt , jpoimboe@redhat.com, "Kirill A. Shutemov" , n-horiguchi@ah.jp.nec.com, Andrea Arcangeli , Mel Gorman , Thomas Gleixner , David Rientjes , Linux-MM , "linux-doc@vger.kernel.org" , Mark Salyzyn , Jeffrey Vander Stoep , Nick Kralevich , dcashman Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Nov 3, 2015 at 10:10 AM, Daniel Cashman wrote: > From: dcashman > > 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 Acked-by: Kees Cook 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. > --- > Changes in v2: > - Changed arch/arm/Kconfig and arch/arm/mm/mmap.c to reflect changes > in [PATCH v2 1/2], specifically the movement of variables to global > rather than arch-specific files. > > arch/arm/Kconfig | 10 ++++++++++ > arch/arm/mm/mmap.c | 3 +-- > 2 files changed, 11 insertions(+), 2 deletions(-) > > diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig > index 639411f..47d7561 100644 > --- a/arch/arm/Kconfig > +++ b/arch/arm/Kconfig > @@ -35,6 +35,7 @@ config ARM > select HAVE_ARCH_BITREVERSE if (CPU_32v7M || CPU_32v7) && !CPU_32v6 > select HAVE_ARCH_JUMP_LABEL if !XIP_KERNEL && !CPU_ENDIAN_BE32 > select HAVE_ARCH_KGDB if !CPU_ENDIAN_BE32 > + select HAVE_ARCH_MMAP_RND_BITS > select HAVE_ARCH_SECCOMP_FILTER if (AEABI && !OABI_COMPAT) > select HAVE_ARCH_TRACEHOOK > select HAVE_BPF_JIT > @@ -306,6 +307,15 @@ config MMU > Select if you want MMU-based virtualised addressing space > support by paged memory management. If unsure, say 'Y'. > > +config ARCH_MMAP_RND_BITS_MIN > + default 8 > + > +config ARCH_MMAP_RND_BITS_MAX > + default 14 if MMU && PAGE_OFFSET=0x40000000 > + default 15 if MMU && PAGE_OFFSET=0x80000000 > + default 16 if MMU > + default 8 > + > # > # The "ARM system type" choice list is ordered alphabetically by option > # text. Please add new entries in the option alphabetic order. > diff --git a/arch/arm/mm/mmap.c b/arch/arm/mm/mmap.c > index 407dc78..c938693 100644 > --- a/arch/arm/mm/mmap.c > +++ b/arch/arm/mm/mmap.c > @@ -173,8 +173,7 @@ unsigned long arch_mmap_rnd(void) > { > unsigned long rnd; > > - /* 8 bits of randomness in 20 address space bits */ > - rnd = (unsigned long)get_random_int() % (1 << 8); > + rnd = (unsigned long)get_random_int() % (1 << mmap_rnd_bits); > > return rnd << PAGE_SHIFT; > } I like this getting pulled closer and closer to having arch_mmap_rnd() be identical across all architectures, and then we can just pull it out and leave the true variable: the entropy size. Do you have patches for x86 and arm64? -Kees > -- > 2.6.0.rc2.230.g3dd15c0 > -- Kees Cook Chrome OS Security