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

* 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


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