From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-lf0-f70.google.com (mail-lf0-f70.google.com [209.85.215.70]) by kanga.kvack.org (Postfix) with ESMTP id E057F6B000A for ; Mon, 26 Mar 2018 15:45:36 -0400 (EDT) Received: by mail-lf0-f70.google.com with SMTP id p202-v6so6403462lfe.3 for ; Mon, 26 Mar 2018 12:45:36 -0700 (PDT) Received: from mail-sor-f65.google.com (mail-sor-f65.google.com. [209.85.220.65]) by mx.google.com with SMTPS id q22sor2320736ljc.0.2018.03.26.12.45.35 for (Google Transport Security); Mon, 26 Mar 2018 12:45:35 -0700 (PDT) Content-Type: text/plain; charset=utf-8 Mime-Version: 1.0 (Mac OS X Mail 11.2 \(3445.5.20\)) Subject: Re: [RFC PATCH v2 0/2] Randomization of address chosen by mmap. From: Ilya Smith In-Reply-To: <20180326084650.GC5652@dhcp22.suse.cz> Date: Mon, 26 Mar 2018 22:45:31 +0300 Content-Transfer-Encoding: quoted-printable Message-Id: <01A133F4-27DF-4AE2-80D6-B0368BF758CD@gmail.com> References: <1521736598-12812-1-git-send-email-blackzert@gmail.com> <20180323124806.GA5624@bombadil.infradead.org> <651E0DB6-4507-4DA1-AD46-9C26ED9792A8@gmail.com> <20180326084650.GC5652@dhcp22.suse.cz> Sender: owner-linux-mm@kvack.org List-ID: To: Michal Hocko Cc: Matthew Wilcox , rth@twiddle.net, ink@jurassic.park.msu.ru, mattst88@gmail.com, vgupta@synopsys.com, linux@armlinux.org.uk, tony.luck@intel.com, fenghua.yu@intel.com, ralf@linux-mips.org, jejb@parisc-linux.org, Helge Deller , benh@kernel.crashing.org, paulus@samba.org, mpe@ellerman.id.au, schwidefsky@de.ibm.com, heiko.carstens@de.ibm.com, ysato@users.sourceforge.jp, dalias@libc.org, davem@davemloft.net, tglx@linutronix.de, mingo@redhat.com, hpa@zytor.com, x86@kernel.org, nyc@holomorphy.com, viro@zeniv.linux.org.uk, arnd@arndb.de, gregkh@linuxfoundation.org, deepa.kernel@gmail.com, Hugh Dickins , kstewart@linuxfoundation.org, pombredanne@nexb.com, Andrew Morton , steve.capper@arm.com, punit.agrawal@arm.com, aneesh.kumar@linux.vnet.ibm.com, npiggin@gmail.com, Kees Cook , bhsharma@redhat.com, riel@redhat.com, nitin.m.gupta@oracle.com, "Kirill A. Shutemov" , Dan Williams , Jan Kara , ross.zwisler@linux.intel.com, Jerome Glisse , Andrea Arcangeli , Oleg Nesterov , linux-alpha@vger.kernel.org, LKML , linux-snps-arc@lists.infradead.org, linux-ia64@vger.kernel.org, linux-metag@vger.kernel.org, linux-mips@linux-mips.org, linux-parisc@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, linux-s390@vger.kernel.org, linux-sh@vger.kernel.org, sparclinux@vger.kernel.org, Linux-MM > On 26 Mar 2018, at 11:46, Michal Hocko wrote: >=20 > On Fri 23-03-18 20:55:49, Ilya Smith wrote: >>=20 >>> On 23 Mar 2018, at 15:48, Matthew Wilcox = wrote: >>>=20 >>> On Thu, Mar 22, 2018 at 07:36:36PM +0300, Ilya Smith wrote: >>>> Current implementation doesn't randomize address returned by mmap. >>>> All the entropy ends with choosing mmap_base_addr at the process >>>> creation. After that mmap build very predictable layout of address >>>> space. It allows to bypass ASLR in many cases. This patch make >>>> randomization of address on any mmap call. >>>=20 >>> Why should this be done in the kernel rather than libc? libc is = perfectly >>> capable of specifying random numbers in the first argument of mmap. >> Well, there is following reasons: >> 1. It should be done in any libc implementation, what is not possible = IMO; >=20 > Is this really so helpful? Yes, ASLR is one of very important mitigation techniques which are = really used=20 to protect applications. If there is no ASLR, it is very easy to exploit=20= vulnerable application and compromise the system. We can=E2=80=99t just = fix all the=20 vulnerabilities right now, thats why we have mitigations - techniques = which are=20 makes exploitation more hard or impossible in some cases. Thats why it is helpful. >=20 >> 2. User mode is not that layer which should be responsible for = choosing >> random address or handling entropy; >=20 > Why? Because of the following reasons: 1. To get random address you should have entropy. These entropy = shouldn=E2=80=99t be=20 exposed to attacker anyhow, the best case is to get it from kernel. So = this is a syscall. 2. You should have memory map of your process to prevent remapping or = big fragmentation. Kernel already has this map. You will got another one in = libc. And any non-libc user of mmap (via syscall, etc) will make hole in your = map. This one also decrease performance cause you any way call syscall_mmap=20= which will try to find some address for you in worst case, but after you = already did some computing on it. 3. The more memory you use in userland for these proposal, the easier = for attacker to leak it or use in exploitation techniques. 4. It is so easy to fix Kernel function and so hard to support memory management from userspace. >=20 >> 3. Memory fragmentation is unpredictable in this case >>=20 >> Off course user mode could use random =E2=80=98hint=E2=80=99 address, = but kernel may >> discard this address if it is occupied for example and allocate just = before >> closest vma. So this solution doesn=E2=80=99t give that much security = like=20 >> randomization address inside kernel. >=20 > The userspace can use the new MAP_FIXED_NOREPLACE to probe for the > address range atomically and chose a different range on failure. >=20 This algorithm should track current memory. If he doesn=E2=80=99t he may = cause infinite loop while trying to choose memory. And each iteration increase = time needed on allocation new memory, what is not preferred by any libc = library developer. Thats why I did this patch. Thanks, Ilya