On Tue, Mar 16, 2021 at 08:10:43AM -0700, Yu-cheng Yu wrote:
> INCSSP(Q/D) increments shadow stack pointer and 'pops and discards' the
> first and the last elements in the range, effectively touches those memory
> areas.
>
> The maximum moving distance by INCSSPQ is 255 * 8 = 2040 bytes and
> 255 * 4 = 1020 bytes by INCSSPD. Both ranges are far from PAGE_SIZE.
> Thus, putting a gap page on both ends of a shadow stack prevents INCSSP,
> CALL, and RET from going beyond.
>
> Signed-off-by: Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx>
> Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx>
> ---
> arch/x86/include/asm/page_64_types.h | 10 ++++++++++
> include/linux/mm.h | 24 ++++++++++++++++++++----
> 2 files changed, 30 insertions(+), 4 deletions(-)
>
> diff --git a/arch/x86/include/asm/page_64_types.h b/arch/x86/include/asm/page_64_types.h
> index 64297eabad63..23e3d880ce6c 100644
> --- a/arch/x86/include/asm/page_64_types.h
> +++ b/arch/x86/include/asm/page_64_types.h
> @@ -115,4 +115,14 @@
> #define KERNEL_IMAGE_SIZE (512 * 1024 * 1024)
> #endif
>
> +/*
> + * Shadow stack pointer is moved by CALL, RET, and INCSSP(Q/D). INCSSPQ
> + * moves shadow stack pointer up to 255 * 8 = ~2 KB (~1KB for INCSSPD) and
> + * touches the first and the last element in the range, which triggers a
> + * page fault if the range is not in a shadow stack. Because of this,
> + * creating 4-KB guard pages around a shadow stack prevents these
> + * instructions from going beyond.
> + */
> +#define ARCH_SHADOW_STACK_GUARD_GAP PAGE_SIZE
> +
> #endif /* _ASM_X86_PAGE_64_DEFS_H */
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index af805ffde48e..9890e9f5a5e0 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -2619,6 +2619,10 @@ extern vm_fault_t filemap_page_mkwrite(struct vm_fault *vmf);
> int __must_check write_one_page(struct page *page);
> void task_dirty_inc(struct task_struct *tsk);
>
> +#ifndef ARCH_SHADOW_STACK_GUARD_GAP
> +#define ARCH_SHADOW_STACK_GUARD_GAP 0
> +#endif
> +
> extern unsigned long stack_guard_gap;
> /* Generic expand stack which grows the stack according to GROWS{UP,DOWN} */
> extern int expand_stack(struct vm_area_struct *vma, unsigned long address);
> @@ -2651,9 +2655,15 @@ static inline struct vm_area_struct * find_vma_intersection(struct mm_struct * m
> static inline unsigned long vm_start_gap(struct vm_area_struct *vma)
> {
> unsigned long vm_start = vma->vm_start;
> + unsigned long gap = 0;
>
> - if (vma->vm_flags & VM_GROWSDOWN) {
> - vm_start -= stack_guard_gap;
> + if (vma->vm_flags & VM_GROWSDOWN)
> + gap = stack_guard_gap;
> + else if (vma->vm_flags & VM_SHSTK)
> + gap = ARCH_SHADOW_STACK_GUARD_GAP;
Looks too x86-centric for generic code.
Maybe we can have a helper that would return gap for the given VMA?
The generic version of the helper would only return stack_guard_gap for
VM_GROWSDOWN. Arch code would override it to handle VM_SHSTK case too.
Similar can be done in vm_end_gap().
> +
> + if (gap != 0) {
> + vm_start -= gap;
> if (vm_start > vma->vm_start)
> vm_start = 0;
> }
> @@ -2663,9 +2673,15 @@ static inline unsigned long vm_start_gap(struct vm_area_struct *vma)
> static inline unsigned long vm_end_gap(struct vm_area_struct *vma)
> {
> unsigned long vm_end = vma->vm_end;
> + unsigned long gap = 0;
> +
> + if (vma->vm_flags & VM_GROWSUP)
> + gap = stack_guard_gap;
> + else if (vma->vm_flags & VM_SHSTK)
> + gap = ARCH_SHADOW_STACK_GUARD_GAP;
>
> - if (vma->vm_flags & VM_GROWSUP) {
> - vm_end += stack_guard_gap;
> + if (gap != 0) {
> + vm_end += gap;
> if (vm_end < vma->vm_end)
> vm_end = -PAGE_SIZE;
> }
> --
> 2.21.0
>
--
Kirill A. Shutemov
