[PATCH v26 17/30] mm: Add guard pages around a shadow stack.

[Yu-cheng Yu]

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: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
Cc: Kees Cook <keescook@xxxxxxxxxxxx>
---
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 6ac9b3e9a865..3e9c84f21ef6 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -2660,6 +2660,7 @@ static inline struct vm_area_struct * find_vma_intersection(struct mm_struct * m
 	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;
@@ -2671,7 +2672,9 @@ 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;
@@ -2683,6 +2686,7 @@ 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