Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755931AbbGVGXY (ORCPT ); Wed, 22 Jul 2015 02:23:24 -0400 Received: from mail-lb0-f181.google.com ([209.85.217.181]:33308 "EHLO mail-lb0-f181.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753138AbbGVGXX (ORCPT ); Wed, 22 Jul 2015 02:23:23 -0400 MIME-Version: 1.0 In-Reply-To: <20150722060643.GA25466@gmail.com> References: <06f4a63325ac221868770a8d63207d74b160df89.1437508486.git.luto@kernel.org> <20150722060643.GA25466@gmail.com> From: Andy Lutomirski Date: Tue, 21 Jul 2015 23:23:02 -0700 Message-ID: Subject: Re: [PATCH v2 2/3] x86/ldt: Make modify_ldt optional To: Ingo Molnar 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 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: 3358 Lines: 80 On Tue, Jul 21, 2015 at 11:06 PM, Ingo Molnar wrote: > > * 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 > > ;-) You don't say? It appears that Wine uses set_thread_area with a fallback to modify_ldt for 32-bit binaries and arch_prctl for 64-bit. --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/