[PATCH v6 18/41] mm: Introduce VM_SHADOW_STACK for shadow stack memory

[Rick Edgecombe]

From: Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx>

The x86 Control-flow Enforcement Technology (CET) feature includes a new
type of memory called shadow stack. This shadow stack memory has some
unusual properties, which requires some core mm changes to function
properly.

A shadow stack 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. Introduce
VM_SHADOW_STACK to track shadow stack VMAs.

Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx>
Tested-by: Pengfei Xu <pengfei.xu@xxxxxxxxx>
Tested-by: John Allen <john.allen@xxxxxxx>
Signed-off-by: Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx>
Reviewed-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
Signed-off-by: Rick Edgecombe <rick.p.edgecombe@xxxxxxxxx>
Cc: Kees Cook <keescook@xxxxxxxxxxxx>

---
v6:
 - Add comment about VM_SHADOW_STACK not being allowed with VM_SHARED
   (David Hildenbrand)

v3:
 - Drop arch specific change in arch_vma_name(). The memory can show as
   anonymous (Kirill)
 - Change CONFIG_ARCH_HAS_SHADOW_STACK to CONFIG_X86_USER_SHADOW_STACK
   in show_smap_vma_flags() (Boris)
---
 Documentation/filesystems/proc.rst | 1 +
 fs/proc/task_mmu.c                 | 3 +++
 include/linux/mm.h                 | 8 ++++++++
 3 files changed, 12 insertions(+)

diff --git a/Documentation/filesystems/proc.rst b/Documentation/filesystems/proc.rst
index e224b6d5b642..115843e8cce3 100644
--- a/Documentation/filesystems/proc.rst
+++ b/Documentation/filesystems/proc.rst
@@ -564,6 +564,7 @@ encoded manner. The codes are the following:
     mt    arm64 MTE allocation tags are enabled
     um    userfaultfd missing tracking
     uw    userfaultfd wr-protect tracking
+    ss    shadow stack page
     ==    =======================================
 
 Note that there is no guarantee that every flag and associated mnemonic will
diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
index af1c49ae11b1..9e2cefe47749 100644
--- a/fs/proc/task_mmu.c
+++ b/fs/proc/task_mmu.c
@@ -711,6 +711,9 @@ static void show_smap_vma_flags(struct seq_file *m, struct vm_area_struct *vma)
 #ifdef CONFIG_HAVE_ARCH_USERFAULTFD_MINOR
 		[ilog2(VM_UFFD_MINOR)]	= "ui",
 #endif /* CONFIG_HAVE_ARCH_USERFAULTFD_MINOR */
+#ifdef CONFIG_X86_USER_SHADOW_STACK
+		[ilog2(VM_SHADOW_STACK)] = "ss",
+#endif
 	};
 	size_t i;
 
diff --git a/include/linux/mm.h b/include/linux/mm.h
index e6f1789c8e69..76e0a09aeffe 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -315,11 +315,13 @@ 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
@@ -335,6 +337,12 @@ extern unsigned int kobjsize(const void *objp);
 #endif
 #endif /* CONFIG_ARCH_HAS_PKEYS */
 
+#ifdef CONFIG_X86_USER_SHADOW_STACK
+# define VM_SHADOW_STACK	VM_HIGH_ARCH_5 /* Should not be set with VM_SHARED */
+#else
+# define VM_SHADOW_STACK	VM_NONE
+#endif
+
 #if defined(CONFIG_X86)
 # define VM_PAT		VM_ARCH_1	/* PAT reserves whole VMA at once (x86) */
 #elif defined(CONFIG_PPC)
-- 
2.17.1