From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755397AbbLAAEk (ORCPT ); Mon, 30 Nov 2015 19:04:40 -0500 Received: from mail-ig0-f170.google.com ([209.85.213.170]:34808 "EHLO mail-ig0-f170.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754418AbbLAAEi (ORCPT ); Mon, 30 Nov 2015 19:04:38 -0500 MIME-Version: 1.0 In-Reply-To: <20151130160118.e43a2e53a59e347a95a94d5c@linux-foundation.org> References: <1448578785-17656-1-git-send-email-dcashman@android.com> <1448578785-17656-2-git-send-email-dcashman@android.com> <20151130155412.b1a087f4f6f4d4180ab4472d@linux-foundation.org> <20151130160118.e43a2e53a59e347a95a94d5c@linux-foundation.org> Date: Mon, 30 Nov 2015 16:04:36 -0800 X-Google-Sender-Auth: 52P-mW69RRbc6Uzd_x7m6qJD5YI Message-ID: Subject: Re: [PATCH v4 1/4] mm: mmap: Add new /proc tunable for mmap_base ASLR. From: Kees Cook To: Andrew Morton Cc: Daniel Cashman , LKML , Russell King - ARM Linux , 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 , Catalin Marinas , Will Deacon , "H. Peter Anvin" , "x86@kernel.org" , Hector Marco , Borislav Petkov , Daniel Cashman 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 Mon, Nov 30, 2015 at 4:01 PM, Andrew Morton wrote: > On Mon, 30 Nov 2015 15:54:12 -0800 Andrew Morton wrote: > >> On Thu, 26 Nov 2015 14:59:42 -0800 Daniel Cashman wrote: >> >> > ASLR only uses as few as 8 bits to generate the random offset for the >> > mmap base address on 32 bit architectures. This value was chosen to >> > prevent a poorly chosen value from dividing the address space in such >> > a way as to prevent large allocations. This may not be an issue on all >> > platforms. Allow the specification of a minimum number of bits so that >> > platforms desiring greater ASLR protection may determine where to place >> > the trade-off. >> > >> > --- a/kernel/sysctl.c >> > +++ b/kernel/sysctl.c >> > @@ -1568,6 +1568,28 @@ static struct ctl_table vm_table[] = { >> > .mode = 0644, >> > .proc_handler = proc_doulongvec_minmax, >> > }, >> > +#ifdef CONFIG_HAVE_ARCH_MMAP_RND_BITS >> > + { >> > + .procname = "mmap_rnd_bits", >> > + .data = &mmap_rnd_bits, >> > + .maxlen = sizeof(mmap_rnd_bits), >> > + .mode = 0600, >> > + .proc_handler = proc_dointvec_minmax, >> > + .extra1 = (void *) &mmap_rnd_bits_min, >> > + .extra2 = (void *) &mmap_rnd_bits_max, >> >> hm, why the typecasts? They're unneeded and are omitted everywhere(?) >> else in kernel/sysctl.c. > > Oh. Casting away constness. > > What's the thinking here? They can change at any time so they aren't > const so we shouldn't declare them to be const? The _min and _max values shouldn't be changing: they're decided based on the various CONFIG options that calculate the valid min/maxes. Only mmap_rnd_bits itself should be changing. -Kees -- Kees Cook Chrome OS & Brillo Security