Re: [PATCH v2 2/3] x86/ldt: Make modify_ldt optional

[Ingo Molnar <mingo@kernel.org>]

* Andy Lutomirski <luto@amacapital.net> wrote:

> On Tue, Jul 21, 2015 at 1:28 PM, Brian Gerst <brgerst@gmail.com> wrote:
> > On Tue, Jul 21, 2015 at 3:59 PM, Andy Lutomirski <luto@kernel.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@kernel.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

;-)

Thanks,

	Ingo