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..0fbee6dcd3ca 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, JMP, 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 09f07d07a8ff..80cda65bb1ae 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
