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SubjectRe: [PATCH v2 2/3] x86/ldt: Make modify_ldt optional
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On Tue, Jul 21, 2015 at 4:34 PM, 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.

You're right. I wasn't sure if Wine was ever changed to use that.

--
Brian Gerst


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