Re: [RFC PATCH v9 06/27] mm: Introduce VM_SHSTK for Shadow Stack memory

Kees Cook
2020-02-25T20:07:55Z

On Wed, Feb 05, 2020 at 10:19:14AM -0800, Yu-cheng Yu wrote:
> A Shadow Stack (SHSTK) PTE must be read-only and have _PAGE_DIRTY set.
> However, read-only and Dirty PTEs also exist for copy-on-write (COW) pages.
> These two cases are handled differently for page faults and a new VM flag
> is necessary for tracking SHSTK VMAs.
>
> v9:
> - Add VM_SHSTK case to arch_vma_name().
> - Revise the commit log to explain why a new VM flag is needed.
>
> Signed-off-by: Yu-cheng Yu <yu-cheng.yu@intel.com>

Reviewed-by: Kees Cook <keescook@chromium.org>

-Kees

> ---
> arch/x86/mm/mmap.c | 2 ++
> fs/proc/task_mmu.c | 3 +++
> include/linux/mm.h | 8 ++++++++
> 3 files changed, 13 insertions(+)
>
> diff --git a/arch/x86/mm/mmap.c b/arch/x86/mm/mmap.c
> index aae9a933dfd4..482813b4c659 100644
> --- a/arch/x86/mm/mmap.c
> +++ b/arch/x86/mm/mmap.c
> @@ -165,6 +165,8 @@ const char *arch_vma_name(struct vm_area_struct *vma)
> {
> if (vma->vm_flags & VM_MPX)
> return "[mpx]";
> + else if (vma->vm_flags & VM_SHSTK)
> + return "[shadow stack]";
> return NULL;
> }
>
> diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
> index 9442631fd4af..590b58ee008a 100644
> --- a/fs/proc/task_mmu.c
> +++ b/fs/proc/task_mmu.c
> @@ -687,6 +687,9 @@ static void show_smap_vma_flags(struct seq_file *m, struct vm_area_struct *vma)
> [ilog2(VM_PKEY_BIT4)] = "",
> #endif
> #endif /* CONFIG_ARCH_HAS_PKEYS */
> +#ifdef CONFIG_X86_INTEL_SHADOW_STACK_USER
> + [ilog2(VM_SHSTK)] = "ss",
> +#endif
> };
> size_t i;
>
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index cfaa8feecfe8..b5145fbe102e 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -298,11 +298,13 @@ extern unsigned int kobjsize(const void *objp);
> #define VM_HIGH_ARCH_BIT_2 34 /* bit only usable on 64-bit architectures */
> #define VM_HIGH_ARCH_BIT_3 35 /* bit only usable on 64-bit architectures */
> #define VM_HIGH_ARCH_BIT_4 36 /* bit only usable on 64-bit architectures */
> +#define VM_HIGH_ARCH_BIT_5 37 /* bit only usable on 64-bit architectures */
> #define VM_HIGH_ARCH_0 BIT(VM_HIGH_ARCH_BIT_0)
> #define VM_HIGH_ARCH_1 BIT(VM_HIGH_ARCH_BIT_1)
> #define VM_HIGH_ARCH_2 BIT(VM_HIGH_ARCH_BIT_2)
> #define VM_HIGH_ARCH_3 BIT(VM_HIGH_ARCH_BIT_3)
> #define VM_HIGH_ARCH_4 BIT(VM_HIGH_ARCH_BIT_4)
> +#define VM_HIGH_ARCH_5 BIT(VM_HIGH_ARCH_BIT_5)
> #endif /* CONFIG_ARCH_USES_HIGH_VMA_FLAGS */
>
> #ifdef CONFIG_ARCH_HAS_PKEYS
> @@ -340,6 +342,12 @@ extern unsigned int kobjsize(const void *objp);
> # define VM_MPX VM_NONE
> #endif
>
> +#ifdef CONFIG_X86_INTEL_SHADOW_STACK_USER
> +# define VM_SHSTK VM_HIGH_ARCH_5
> +#else
> +# define VM_SHSTK VM_NONE
> +#endif
> +
> #ifndef VM_GROWSUP
> # define VM_GROWSUP VM_NONE
> #endif
> --
> 2.21.0
>

--
Kees Cook