Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S965171AbbKCXSu (ORCPT ); Tue, 3 Nov 2015 18:18:50 -0500 Received: from mail-ig0-f171.google.com ([209.85.213.171]:37793 "EHLO mail-ig0-f171.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S965073AbbKCXSs (ORCPT ); Tue, 3 Nov 2015 18:18:48 -0500 MIME-Version: 1.0 In-Reply-To: <20151103223904.GG8644@n2100.arm.linux.org.uk> References: <1446574204-15567-1-git-send-email-dcashman@android.com> <1446574204-15567-2-git-send-email-dcashman@android.com> <20151103223904.GG8644@n2100.arm.linux.org.uk> Date: Tue, 3 Nov 2015 15:18:47 -0800 X-Google-Sender-Auth: i9Qh-Djv2YWjTTVPYQuckIFkhO4 Message-ID: Subject: Re: [PATCH v2 2/2] arm: mm: support ARCH_MMAP_RND_BITS. From: Kees Cook To: Russell King - ARM Linux Cc: Daniel Cashman , LKML , 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 Content-Length: 2121 Lines: 46 On Tue, Nov 3, 2015 at 2:39 PM, Russell King - ARM Linux wrote: > On Tue, Nov 03, 2015 at 11:19:44AM -0800, Kees Cook wrote: >> 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. > > Yes, it looks sane, though I do wonder whether there should also be > a Kconfig option to allow archtectures to specify the default, instead > of the default always being the minimum randomisation. I can see scope > to safely pushing our mmap randomness default to 12, especially on 3GB > setups, as we already have 11 bits of randomness on the sigpage and if > enabled, 13 bits on the heap. My thinking is that the there shouldn't be a reason to ever have a minimum that was below the default. I have no objection with it, but it seems needless. Frankly minimum is "0", really, so I don't think it makes much sense to have default != arch minimum. I actually view "arch minimum" as "known good", so if we are happy with raising the "known good" value, that should be the new minimum. -Kees -- Kees Cook Chrome OS Security -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/