lkml.org 
[lkml]   [2015]   [Jul]   [21]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
From
Date
SubjectRe: [PATCH v2 2/3] x86/ldt: Make modify_ldt optional
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.

--Andy


\
 
 \ /
  Last update: 2015-07-21 22:41    [W:1.847 / U:0.452 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site