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SubjectRe: [PATCH v2 2/3] x86/ldt: Make modify_ldt optional
On Tue, Jul 21, 2015 at 11:06 PM, Ingo Molnar <mingo@kernel.org> wrote:
>
> * 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
>
> ;-)

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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