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

Yu-cheng Yu
2020-02-05T18:20:37Z

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@intel.com>
---
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