Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755896AbbGUU3A (ORCPT ); Tue, 21 Jul 2015 16:29:00 -0400 Received: from mail-oi0-f46.google.com ([209.85.218.46]:34993 "EHLO mail-oi0-f46.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752510AbbGUU27 (ORCPT ); Tue, 21 Jul 2015 16:28:59 -0400 MIME-Version: 1.0 In-Reply-To: <06f4a63325ac221868770a8d63207d74b160df89.1437508486.git.luto@kernel.org> References: <06f4a63325ac221868770a8d63207d74b160df89.1437508486.git.luto@kernel.org> Date: Tue, 21 Jul 2015 16:28:58 -0400 Message-ID: Subject: Re: [PATCH v2 2/3] x86/ldt: Make modify_ldt optional From: Brian Gerst To: Andy Lutomirski Cc: Peter Zijlstra , Steven Rostedt , "security@kernel.org" , X86 ML , Borislav Petkov , Sasha Levin , Linux Kernel Mailing List , Konrad Rzeszutek Wilk , Boris Ostrovsky 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: 2535 Lines: 59 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. -- Brian Gerst -- 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/