Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754906AbbGVGGt (ORCPT ); Wed, 22 Jul 2015 02:06:49 -0400 Received: from mail-wi0-f179.google.com ([209.85.212.179]:36501 "EHLO mail-wi0-f179.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751589AbbGVGGs (ORCPT ); Wed, 22 Jul 2015 02:06:48 -0400 Date: Wed, 22 Jul 2015 08:06:43 +0200 From: Ingo Molnar To: Andy Lutomirski Cc: Brian Gerst , Andy Lutomirski , Peter Zijlstra , Steven Rostedt , "security@kernel.org" , X86 ML , Borislav Petkov , Sasha Levin , Linux Kernel Mailing List , Konrad Rzeszutek Wilk , Boris Ostrovsky Subject: Re: [PATCH v2 2/3] x86/ldt: Make modify_ldt optional Message-ID: <20150722060643.GA25466@gmail.com> References: <06f4a63325ac221868770a8d63207d74b160df89.1437508486.git.luto@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 3089 Lines: 76 * Andy Lutomirski wrote: > On Tue, Jul 21, 2015 at 1:28 PM, Brian Gerst wrote: > > On Tue, Jul 21, 2015 at 3:59 PM, Andy Lutomirski wrote: > >> > >> The modify_ldt syscall exposes a large attack surface and is unnecessary for > >> modern userspace. Make it optional. > >> > >> Signed-off-by: Andy Lutomirski > >> --- > >> arch/x86/Kconfig | 17 +++++++++++++++++ > >> arch/x86/include/asm/mmu.h | 2 ++ > >> arch/x86/include/asm/mmu_context.h | 31 +++++++++++++++++++++++-------- > >> arch/x86/kernel/Makefile | 3 ++- > >> arch/x86/kernel/cpu/perf_event.c | 4 ++++ > >> arch/x86/kernel/process_64.c | 2 ++ > >> arch/x86/kernel/step.c | 2 ++ > >> kernel/sys_ni.c | 1 + > >> 8 files changed, 53 insertions(+), 9 deletions(-) > >> > >> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig > >> index 55bced17dc95..a7ff3980bd65 100644 > >> --- a/arch/x86/Kconfig > >> +++ b/arch/x86/Kconfig > >> @@ -1009,6 +1009,7 @@ config VM86 > >> config X86_16BIT > >> bool "Enable support for 16-bit segments" if EXPERT > >> default y > >> + depends on MODIFY_LDT_SYSCALL > >> ---help--- > >> This option is required by programs like Wine to run 16-bit > >> protected mode legacy code on x86 processors. Disabling > >> @@ -2047,6 +2048,22 @@ config CMDLINE_OVERRIDE > >> This is used to work around broken boot loaders. This should > >> be set to 'N' under normal conditions. > >> > >> +config MODIFY_LDT_SYSCALL > >> + bool "Enable the LDT (local descriptor table)" if EXPERT > >> + default y > >> + ---help--- > >> + Linux can allow user programs to install a per-process x86 > >> + Local Descriptor Table (LDT) using the modify_ldt(2) system > >> + call. This is required to run 16-bit or segmented code such as > >> + DOSEMU or some Wine programs. It is also used by some very old > >> + threading libraries. > >> + > >> + Enabling this feature adds a small amount of overhead to > >> + context switches and increases the low-level kernel attack > >> + surface. Disabling it removes the modify_ldt(2) system call. > >> + > >> + Saying 'N' here may make sense for embedded or server kernels. > >> + > > > > I believe Wine still uses the LDT for thread-local data, even for 32 > > and 64-bit programs. This is separate from the Linux runtime TLS. > > > > Really? I thought the whole reason we had three set_thread_area slots > was for Wine. Too bad we have to guess, if only we had the Wine source code under a nicely accessible Git archive or so to check? git clone git://source.winehq.org/git/wine.git ;-) Thanks, Ingo -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/