[RFC PATCH v9 06/27] mm: Introduce VM_SHSTK for Shadow Stack memory

[Yu-cheng Yu]

A Shadow Stack (SHSTK) PTE must be read-only and have _PAGE_DIRTY set.
However, read-only and Dirty PTEs also exist for copy-on-write (COW) pages.
These two cases are handled differently for page faults and a new VM flag
is necessary for tracking SHSTK VMAs.

v9:
- Add VM_SHSTK case to arch_vma_name().
- Revise the commit log to explain why a new VM flag is needed.

Signed-off-by: Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx>
---
 arch/x86/mm/mmap.c | 2 ++
 fs/proc/task_mmu.c | 3 +++
 include/linux/mm.h | 8 ++++++++
 3 files changed, 13 insertions(+)

diff --git a/arch/x86/mm/mmap.c b/arch/x86/mm/mmap.c
index aae9a933dfd4..482813b4c659 100644
--- a/arch/x86/mm/mmap.c
+++ b/arch/x86/mm/mmap.c
@@ -165,6 +165,8 @@ const char *arch_vma_name(struct vm_area_struct *vma)
 {
 	if (vma->vm_flags & VM_MPX)
 		return "[mpx]";
+	else if (vma->vm_flags & VM_SHSTK)
+		return "[shadow stack]";
 	return NULL;
 }
 
diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
index 9442631fd4af..590b58ee008a 100644
--- a/fs/proc/task_mmu.c
+++ b/fs/proc/task_mmu.c
@@ -687,6 +687,9 @@ static void show_smap_vma_flags(struct seq_file *m, struct vm_area_struct *vma)
 		[ilog2(VM_PKEY_BIT4)]	= "",
 #endif
 #endif /* CONFIG_ARCH_HAS_PKEYS */
+#ifdef CONFIG_X86_INTEL_SHADOW_STACK_USER
+		[ilog2(VM_SHSTK)]	= "ss",
+#endif
 	};
 	size_t i;
 
diff --git a/include/linux/mm.h b/include/linux/mm.h
index cfaa8feecfe8..b5145fbe102e 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -298,11 +298,13 @@ extern unsigned int kobjsize(const void *objp);
 #define VM_HIGH_ARCH_BIT_2	34	/* bit only usable on 64-bit architectures */
 #define VM_HIGH_ARCH_BIT_3	35	/* bit only usable on 64-bit architectures */
 #define VM_HIGH_ARCH_BIT_4	36	/* bit only usable on 64-bit architectures */
+#define VM_HIGH_ARCH_BIT_5	37	/* bit only usable on 64-bit architectures */
 #define VM_HIGH_ARCH_0	BIT(VM_HIGH_ARCH_BIT_0)
 #define VM_HIGH_ARCH_1	BIT(VM_HIGH_ARCH_BIT_1)
 #define VM_HIGH_ARCH_2	BIT(VM_HIGH_ARCH_BIT_2)
 #define VM_HIGH_ARCH_3	BIT(VM_HIGH_ARCH_BIT_3)
 #define VM_HIGH_ARCH_4	BIT(VM_HIGH_ARCH_BIT_4)
+#define VM_HIGH_ARCH_5	BIT(VM_HIGH_ARCH_BIT_5)
 #endif /* CONFIG_ARCH_USES_HIGH_VMA_FLAGS */
 
 #ifdef CONFIG_ARCH_HAS_PKEYS
@@ -340,6 +342,12 @@ extern unsigned int kobjsize(const void *objp);
 # define VM_MPX		VM_NONE
 #endif
 
+#ifdef CONFIG_X86_INTEL_SHADOW_STACK_USER
+# define VM_SHSTK	VM_HIGH_ARCH_5
+#else
+# define VM_SHSTK	VM_NONE
+#endif
+
 #ifndef VM_GROWSUP
 # define VM_GROWSUP	VM_NONE
 #endif
-- 
2.21.0