Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754970AbbGUUfA (ORCPT ); Tue, 21 Jul 2015 16:35:00 -0400 Received: from mail-lb0-f180.google.com ([209.85.217.180]:33528 "EHLO mail-lb0-f180.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752299AbbGUUe7 (ORCPT ); Tue, 21 Jul 2015 16:34:59 -0400 MIME-Version: 1.0 In-Reply-To: References: <06f4a63325ac221868770a8d63207d74b160df89.1437508486.git.luto@kernel.org> From: Andy Lutomirski Date: Tue, 21 Jul 2015 13:34:38 -0700 Message-ID: Subject: Re: [PATCH v2 2/3] x86/ldt: Make modify_ldt optional To: Brian Gerst Cc: Andy Lutomirski , 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: 2741 Lines: 63 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. --Andy -- 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/