linux-kernel - Re: [PATCH v2 2/3] x86/ldt: Make modify

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Date:	Tue, 21 Jul 2015 23:23:02 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	Brian Gerst <brgerst@...il.com>, Andy Lutomirski <luto@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	"security@...nel.org" <security@...nel.org>,
	X86 ML <x86@...nel.org>, Borislav Petkov <bp@...en8.de>,
	Sasha Levin <sasha.levin@...cle.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Boris Ostrovsky <boris.ostrovsky@...cle.com>
Subject: Re: [PATCH v2 2/3] x86/ldt: Make modify_ldt optional

On Tue, Jul 21, 2015 at 11:06 PM, Ingo Molnar <mingo@...nel.org> wrote:
>
> * Andy Lutomirski <luto@...capital.net> wrote:
>
>> On Tue, Jul 21, 2015 at 1:28 PM, Brian Gerst <brgerst@...il.com> wrote:
>> > On Tue, Jul 21, 2015 at 3:59 PM, Andy Lutomirski <luto@...nel.org> wrote:
>> >>
>> >> The modify_ldt syscall exposes a large attack surface and is unnecessary for
>> >> modern userspace.  Make it optional.
>> >>
>> >> Signed-off-by: Andy Lutomirski <luto@...nel.org>
>> >> ---
>> >>  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
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