[PATCH v16 16/26] 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>
---
 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 3f49dac03617..2b2991e5f344 100644
--- a/arch/x86/include/asm/page_64_types.h
+++ b/arch/x86/include/asm/page_64_types.h
@@ -97,6 +97,16 @@
 #define STACK_TOP		TASK_SIZE_LOW
 #define STACK_TOP_MAX		TASK_SIZE_MAX
 
+/*
+ * 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
+
 /*
  * Maximum kernel image size is limited to 1 GiB, due to the fixmap living
  * in the next 1 GiB (see level2_kernel_pgt in arch/x86/kernel/head_64.S).
diff --git a/include/linux/mm.h b/include/linux/mm.h
index b111f23a1be9..0bb6c265446d 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -2639,6 +2639,10 @@ 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);
@@ -2671,9 +2675,15 @@ 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;
+
+	if (gap != 0) {
+		vm_start -= gap;
 		if (vm_start > vma->vm_start)
 			vm_start = 0;
 	}
@@ -2683,9 +2693,15 @@ 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