[PATCH v30 18/32] mm: Add guard pages around a shadow stack.
From: Yu-cheng Yu <hidden>
Date: 2021-08-30 18:17:44
Also in:
linux-api, linux-arch, linux-mm, lkml
Subsystem:
memory management - core, the rest, x86 architecture (32-bit and 64-bit), x86 mm · Maintainers:
Andrew Morton, David Hildenbrand, Linus Torvalds, Thomas Gleixner, Ingo Molnar, Borislav Petkov, Dave Hansen, Andy Lutomirski, Peter Zijlstra
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 <redacted> Reviewed-by: Kirill A. Shutemov <redacted> Cc: Kees Cook <redacted> --- v25: - Move SHADOW_STACK_GUARD_GAP to arch/x86/mm/mmap.c. v24: - Instead changing vm_*_gap(), create x86-specific versions. --- arch/x86/include/asm/page_types.h | 7 +++++ arch/x86/mm/mmap.c | 46 +++++++++++++++++++++++++++++++ include/linux/mm.h | 4 +++ 3 files changed, 57 insertions(+)
diff --git a/arch/x86/include/asm/page_types.h b/arch/x86/include/asm/page_types.h
index a506a411474d..e1533fdc08b4 100644
--- a/arch/x86/include/asm/page_types.h
+++ b/arch/x86/include/asm/page_types.h@@ -73,6 +73,13 @@ bool pfn_range_is_mapped(unsigned long start_pfn, unsigned long end_pfn); extern void initmem_init(void); +#define vm_start_gap vm_start_gap +struct vm_area_struct; +extern unsigned long vm_start_gap(struct vm_area_struct *vma); + +#define vm_end_gap vm_end_gap +extern unsigned long vm_end_gap(struct vm_area_struct *vma); + #endif /* !__ASSEMBLY__ */ #endif /* _ASM_X86_PAGE_DEFS_H */
diff --git a/arch/x86/mm/mmap.c b/arch/x86/mm/mmap.c
index f3f52c5e2fd6..81f9325084d3 100644
--- a/arch/x86/mm/mmap.c
+++ b/arch/x86/mm/mmap.c@@ -250,3 +250,49 @@ bool pfn_modify_allowed(unsigned long pfn, pgprot_t prot) return false; return true; } + +/* + * 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 SHADOW_STACK_GUARD_GAP PAGE_SIZE + +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) + gap = stack_guard_gap; + else if (vma->vm_flags & VM_SHADOW_STACK) + gap = SHADOW_STACK_GUARD_GAP; + + if (gap != 0) { + vm_start -= gap; + if (vm_start > vma->vm_start) + vm_start = 0; + } + return vm_start; +} + +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_SHADOW_STACK) + gap = SHADOW_STACK_GUARD_GAP; + + if (gap != 0) { + vm_end += gap; + if (vm_end < vma->vm_end) + vm_end = -PAGE_SIZE; + } + return vm_end; +}
diff --git a/include/linux/mm.h b/include/linux/mm.h
index 4548f75cef14..354f38d21eed 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h@@ -2729,6 +2729,7 @@ struct vm_area_struct *vma_lookup(struct mm_struct *mm, unsigned long addr) return vma; } +#ifndef vm_start_gap static inline unsigned long vm_start_gap(struct vm_area_struct *vma) { unsigned long vm_start = vma->vm_start;
@@ -2740,7 +2741,9 @@ static inline unsigned long vm_start_gap(struct vm_area_struct *vma) } return vm_start; } +#endif +#ifndef vm_end_gap static inline unsigned long vm_end_gap(struct vm_area_struct *vma) { unsigned long vm_end = vma->vm_end;
@@ -2752,6 +2755,7 @@ static inline unsigned long vm_end_gap(struct vm_area_struct *vma) } return vm_end; } +#endif static inline unsigned long vma_pages(struct vm_area_struct *vma) {
--
2.21.0