From mboxrd@z Thu Jan 1 00:00:00 1970 From: Kees Cook Subject: Re: ppc elf_map breakage with MAP_FIXED_NOREPLACE Date: Fri, 2 Feb 2018 07:55:14 +1100 Message-ID: References: <20180126140415.GD5027@dhcp22.suse.cz> <15da8c87-e6db-13aa-01c8-a913656bfdb6@linux.vnet.ibm.com> <6db9b33d-fd46-c529-b357-3397926f0733@linux.vnet.ibm.com> <20180129132235.GE21609@dhcp22.suse.cz> <87k1w081e7.fsf@concordia.ellerman.id.au> <20180130094205.GS21609@dhcp22.suse.cz> <5eccdc1b-6a10-b48a-c63f-295f69473d97@linux.vnet.ibm.com> <20180131131937.GA6740@dhcp22.suse.cz> <20180201131007.GJ21609@dhcp22.suse.cz> <20180201134026.GK21609@dhcp22.suse.cz> Mime-Version: 1.0 Content-Type: text/plain; charset="UTF-8" Return-path: Received: from mail-vk0-f65.google.com ([209.85.213.65]:40382 "EHLO mail-vk0-f65.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751677AbeBAUzP (ORCPT ); Thu, 1 Feb 2018 15:55:15 -0500 Received: by mail-vk0-f65.google.com with SMTP id h69so12170222vke.7 for ; Thu, 01 Feb 2018 12:55:15 -0800 (PST) In-Reply-To: <20180201134026.GK21609@dhcp22.suse.cz> Sender: linux-next-owner@vger.kernel.org List-ID: To: Michal Hocko Cc: Anshuman Khandual , Michael Ellerman , "akpm@linux-foundation.org" , mm-commits@vger.kernel.org, LKML , Linux-MM , "linux-fsdevel@vger.kernel.org" , Linux-Next , Stephen Rothwell , Mark Brown , Linus Torvalds On Fri, Feb 2, 2018 at 12:40 AM, Michal Hocko wrote: > On Thu 01-02-18 14:10:07, Michal Hocko wrote: > Thanks a lot to Michael Matz for his background. He has pointed me to > the following two segments from your binary[1] > LOAD 0x0000000000000000 0x0000000010000000 0x0000000010000000 > 0x0000000000013a8c 0x0000000000013a8c R E 10000 > LOAD 0x000000000001fd40 0x000000001002fd40 0x000000001002fd40 > 0x00000000000002c0 0x00000000000005e8 RW 10000 > LOAD 0x0000000000020328 0x0000000010030328 0x0000000010030328 > 0x0000000000000384 0x00000000000094a0 RW 10000 > > That binary has two RW LOAD segments, the first crosses a page border > into the second > 0x1002fd40 (LOAD2-vaddr) + 0x5e8 (LOAD2-memlen) == 0x10030328 (LOAD3-vaddr) > > He says > : This is actually an artifact of RELRO machinism. The first RW mapping > : will be remapped as RO after relocations are applied (to increase > : security). > : Well, to be honest, normal relro binaries also don't have more than > : two LOAD segments, so whatever RHEL did to their compilation options, > : it's something in addition to just relro (which can be detected by > : having a GNU_RELRO program header) > : But it definitely has something to do with relro, it's just not the > : whole story yet. > > I am still trying to wrap my head around all this, but it smells rather > dubious to map different segments over the same page. Is this something > that might happen widely and therefore MAP_FIXED_NOREPLACE is a no-go > when loading ELF segments? Or is this a special case we can detect? Eww. FWIW, I would expect that to be rare and detectable. -Kees -- Kees Cook Pixel Security