On Tue, Jul 21, 2015 at 4:34 PM, Andy Lutomirski <l...@amacapital.net> wrote: > On Tue, Jul 21, 2015 at 1:28 PM, Brian Gerst <brge...@gmail.com> wrote: >> On Tue, Jul 21, 2015 at 3:59 PM, Andy Lutomirski <l...@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 <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 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 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/