On Mon, Jul 27, 2015 at 10:29 PM, 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
Reviewed-by: Kees Cook
> ---
> 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
> + default y
> + ---help---
> + Linux can allow user programs to install a per-process x86
nit: looks like a spaces vs tabs issue in the line above?
> +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.
> +
> source "kernel/livepatch/Kconfig"
>
> endmenu
> diff --git a/arch/x86/include/asm/mmu.h b/arch/x86/include/asm/mmu.h
> index 364d27481a52..55234d5e7160 100644
> --- a/arch/x86/include/asm/mmu.h
> +++ b/arch/x86/include/asm/mmu.h
> @@ -9,7 +9,9 @@
> * we put the segment information here.
> */
> typedef struct {
> +#ifdef CONFIG_MODIFY_LDT_SYSCALL
> struct ldt_struct *ldt;
> +#endif
>
> #ifdef CONFIG_X86_64
> /* True if mm supports a task running in 32 bit compatibility mode. */
> diff --git a/arch/x86/include/asm/mmu_context.h
> b/arch/x86/include/asm/mmu_context.h
> index 3fcff70c398e..08094eded318 100644
> --- a/arch/x86/include/asm/mmu_context.h
> +++ b/arch/x86/include/asm/mmu_context.h
> @@ -33,6 +33,7 @@ static inline void load_mm_cr4(struct mm_struct *mm)
> static inline void load_mm_cr4(struct mm_struct *mm) {}
> #endif
>
> +#ifdef CONFIG_MODIFY_LDT_SYSCALL
> /*
> * ldt_structs can be allocated, used, and freed, but they are never
> * modified while live.
> @@ -48,10 +49,24 @@ struct ldt_struct {
> int size;
> };
>
> +/*
> + * Used for LDT copy/destruction.
> + */
> +int init_new_context(struct task_struct *tsk, struct mm_struct *mm);
> +void destroy_context(struct mm_struct *mm);
> +#else /* CONFIG_MODIFY_LDT_SYSCALL */
> +static inline int init_new_context(struct task_struct *tsk,
> + struct mm_struct *mm)
> +{
> + return 0;
> +}
> +static inline void destroy_context(struct mm_struct *mm) {}
> +#endif
> +
> static inline void load_mm_ldt(struct mm_struct *mm)
> {
> +#ifdef CONFIG_MODIFY_LDT_SYSCALL
> struct ldt_struct *ldt;
> - DEBUG_LOCKS_WARN_ON(!irqs_disabled());
>
> /* lockless_dereference synchronizes with smp_store_release */
> ldt = lockless_dereference(mm->context.ldt);
> @@ -74,14 +89,12 @@ static inline void load_mm_ldt(struct mm_struct *mm)
> set_ldt(ldt->entries, ldt->size);
> else
> clear_LDT();
> -}
> -
> -/*
> - * Used for LDT copy/destruction.
> - */
> -int init_new_context(struct task_struct *tsk, struct mm_struct *mm);
> -void destroy_context(struct mm_struct *mm);
> +#else
> + clear_LDT();
> +#endif
>
> + DEBUG_LOCKS_WARN_ON(!irqs_disabled());
> +}
>
> static inline void enter_lazy_tlb(struct mm_struct *mm, struct task_struct
> *tsk)
> {
> @@ -113,6 +126,7 @@ static inline void switch_mm(struct mm_struct *prev,
> struct mm_struct *next,
> /* Load per-mm CR4 state */
> load_mm_cr4(next);
>
> +#ifdef CONFIG_MODIFY_LDT_SYSCALL
> /*
> * Load the LDT, if the LDT is different.
> *
> @@ -127,6 +141,7 @@ static inline void switch_mm(struct mm_struct *prev,
> struct mm_struct *next,
>
