From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756345AbcHVSh2 (ORCPT ); Mon, 22 Aug 2016 14:37:28 -0400 Received: from mx1.redhat.com ([209.132.183.28]:40606 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755561AbcHVShW (ORCPT ); Mon, 22 Aug 2016 14:37:22 -0400 Subject: Re: [PATCH v4] powerpc: Do not make the entire heap executable To: "Aneesh Kumar K.V" , linuxppc-dev@lists.ozlabs.org References: <20160810130030.5268-1-dvlasenk@redhat.com> <874m6ejf81.fsf@linux.vnet.ibm.com> Cc: Jason Gunthorpe , Benjamin Herrenschmidt , Paul Mackerras , Kees Cook , Oleg Nesterov , Michael Ellerman , Florian Weimer , linux-mm@kvack.org, linux-kernel@vger.kernel.org From: Denys Vlasenko Message-ID: <47a2e87e-5299-a009-8a65-5171b33967a1@redhat.com> Date: Mon, 22 Aug 2016 20:37:05 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.2.0 MIME-Version: 1.0 In-Reply-To: <874m6ejf81.fsf@linux.vnet.ibm.com> Content-Type: text/plain; charset=windows-1252; format=flowed Content-Transfer-Encoding: 7bit X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.32]); Mon, 22 Aug 2016 18:37:09 +0000 (UTC) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 08/21/2016 05:47 PM, Aneesh Kumar K.V wrote: > Denys Vlasenko writes: > >> On 32-bit powerpc the ELF PLT sections of binaries (built with --bss-plt, >> or with a toolchain which defaults to it) look like this: >> >> [17] .sbss NOBITS 0002aff8 01aff8 000014 00 WA 0 0 4 >> [18] .plt NOBITS 0002b00c 01aff8 000084 00 WAX 0 0 4 >> [19] .bss NOBITS 0002b090 01aff8 0000a4 00 WA 0 0 4 >> >> Which results in an ELF load header: >> >> Type Offset VirtAddr PhysAddr FileSiz MemSiz Flg Align >> LOAD 0x019c70 0x00029c70 0x00029c70 0x01388 0x014c4 RWE 0x10000 >> >> This is all correct, the load region containing the PLT is marked as >> executable. Note that the PLT starts at 0002b00c but the file mapping ends at >> 0002aff8, so the PLT falls in the 0 fill section described by the load header, >> and after a page boundary. >> >> Unfortunately the generic ELF loader ignores the X bit in the load headers >> when it creates the 0 filled non-file backed mappings. It assumes all of these >> mappings are RW BSS sections, which is not the case for PPC. >> >> gcc/ld has an option (--secure-plt) to not do this, this is said to incur >> a small performance penalty. >> >> Currently, to support 32-bit binaries with PLT in BSS kernel maps *entire >> brk area* with executable rights for all binaries, even --secure-plt ones. > > > Is this going to break any application ? I am asking because you > mentioned the patch is lightly tested. I booted powerpc64 machine with RHEL7 installation, it did not catch fire. > x86 do have a > > #define VM_DATA_DEFAULT_FLAGS \ > (((current->personality & READ_IMPLIES_EXEC) ? VM_EXEC : 0 ) | \ > VM_READ | VM_WRITE | VM_MAYREAD | VM_MAYWRITE | VM_MAYEXEC) > > ie, it can force a read implies exec mode. Do we need that ? powerpc64 never had that. 32-bit mode may need it, since before this patch all 32-bit tasks were unconditionally getting VM_DATA_DEFAULT_FLAGS with VM_EXEC bit. I'll send an updated patch.