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>
Cc: Kees Cook <redacted>
Cc: Kirill A. Shutemov <redacted>
---
v24:
- Instead changing vm_*_gap(), create x86-specific versions.

 arch/x86/include/asm/page_types.h | 17 +++++++++++++++
 arch/x86/mm/mmap.c                | 36 +++++++++++++++++++++++++++++++
 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..3a5529bcfd76 100644
--- a/arch/x86/include/asm/page_types.h
+++ b/arch/x86/include/asm/page_types.h

@@ -73,6 +73,23 @@ bool pfn_range_is_mapped(unsigned long start_pfn, unsigned long end_pfn);
 
 extern void initmem_init(void);
 
+/*
+ * 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