Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1758249AbdCUWH4 (ORCPT ); Tue, 21 Mar 2017 18:07:56 -0400 Received: from mail-wr0-f194.google.com ([209.85.128.194]:34880 "EHLO mail-wr0-f194.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752625AbdCUWHz (ORCPT ); Tue, 21 Mar 2017 18:07:55 -0400 MIME-Version: 1.0 In-Reply-To: <20170321211648.xcgwigbv37ktxofx@angband.pl> References: <20170321174711.29880-1-dsafonov@virtuozzo.com> <20170321211648.xcgwigbv37ktxofx@angband.pl> From: Dmitry Safonov <0x7f454c46@gmail.com> Date: Wed, 22 Mar 2017 01:07:32 +0300 Message-ID: Subject: Re: [PATCHv3] x86/mm: set x32 syscall bit in SET_PERSONALITY() To: Adam Borowski Cc: Dmitry Safonov , open list , linux-mm@kvack.org, Andrei Vagin , Cyrill Gorcunov , Borislav Petkov , "Kirill A. Shutemov" , X86 ML , "H. Peter Anvin" , Andy Lutomirski , Ingo Molnar , Thomas Gleixner 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: 2512 Lines: 53 2017-03-22 0:16 GMT+03:00 Adam Borowski : > On Tue, Mar 21, 2017 at 08:47:11PM +0300, Dmitry Safonov wrote: >> After my changes to mmap(), its code now relies on the bitness of >> performing syscall. According to that, it chooses the base of allocation: >> mmap_base for 64-bit mmap() and mmap_compat_base for 32-bit syscall. >> It was done by: >> commit 1b028f784e8c ("x86/mm: Introduce mmap_compat_base() for >> 32-bit mmap()"). >> >> The code afterwards relies on in_compat_syscall() returning true for >> 32-bit syscalls. It's usually so while we're in context of application >> that does 32-bit syscalls. But during exec() it is not valid for x32 ELF. >> The reason is that the application hasn't yet done any syscall, so x32 >> bit has not being set. >> That results in -ENOMEM for x32 ELF files as there fired BAD_ADDR() >> in elf_map(), that is called from do_execve()->load_elf_binary(). >> For i386 ELFs it works as SET_PERSONALITY() sets TS_COMPAT flag. >> >> Set x32 bit before first return to userspace, during setting personality >> at exec(). This way we can rely on in_compat_syscall() during exec(). >> Do also the reverse: drop x32 syscall bit at SET_PERSONALITY for 64-bits. >> >> Fixes: commit 1b028f784e8c ("x86/mm: Introduce mmap_compat_base() for >> 32-bit mmap()") > > Tested: > with bash:x32, mksh:amd64, posh:i386, zsh:armhf (binfmt:qemu), fork+exec > works for every parent-child combination. > > Contrary to my naive initial reading of your fix, mixing syscalls from a > process of the wrong ABI also works as it did before. While using a glibc > wrapper will call the right version, x32 processes calling amd64 syscalls is > surprisingly common -- this brings seccomp joy. JFI: x32 mmap() syscall in 64 ELF should work even better - it has returned addresses over 4Gb in ia32 mmap()s, so I expect it did the same in x32 top-down allocation. (I guess you've mentioned the fixes-for patch). So the thing to check not also that mmap() returned address, but at least verify-dereference it with `mov' e.g. (or better - to parse /proc/self/maps) > I've attached a freestanding test case for write() and mmap(); it's > freestanding asm as most of you don't have an x32 toolchain at hand, sorry > for unfriendly error messages. > > So with these two patches: > x86/tls: Forcibly set the accessed bit in TLS segments > x86/mm: set x32 syscall bit in SET_PERSONALITY() > everything appears to be fine. Big thanks for the testing work, Adam! -- Dmitry