On Fri, Jul 24, 2015 at 10:36:45PM -0700, Andy Lutomirski wrote:
> The modify_ldt syscall exposes a large attack surface and is
> unnecessary for modern userspace. Make it optional.
>
> Signed-off-by: Andy Lutomirski <l...@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 b3a1a5d77d92..ede52be845db 100644
> --- a/arch/x86/Kconfig
> +++ b/arch/x86/Kconfig
> @@ -1015,6 +1015,7 @@ 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
> @@ -2053,6 +2054,22 @@ 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
bool "Enable modify_ldt() for per-process Local Descriptor Table"
is how I'd call it.
> + default y
Is that "default y" going to turn into a "default n" after a grace
period?
> + ---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.
--
Regards/Gruss,
Boris.
ECO tip #101: Trim your mails when you reply.
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