[PATCH v1] mm: abstract shadow stack vma behind arch_is_shadow_stack_vma

[Deepak Gupta <debug@xxxxxxxxxxxx>]

x86 has used VM_SHADOW_STACK (alias to VM_HIGH_ARCH_5) to encode shadow
stack VMA. VM_SHADOW_STACK is thus not possible on 32bit. Some arches may
need a way to encode shadow stack on 32bit and 64bit both and they may
encode this information differently in VMAs.

This patch changes checks of VM_SHADOW_STACK flag in generic code to call
to a function `arch_is_shadow_stack_vma` which will return true if arch
supports shadow stack and vma is shadow stack else stub returns false.

Signed-off-by: Deepak Gupta <debug@xxxxxxxxxxxx>
---
 include/linux/mm.h | 15 ++++++++++++++-
 mm/gup.c           |  2 +-
 mm/internal.h      |  2 +-
 3 files changed, 16 insertions(+), 3 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 418d26608ece..9586e7bbd2aa 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -352,8 +352,21 @@ extern unsigned int kobjsize(const void *objp);
  * for more details on the guard size.
  */
 # define VM_SHADOW_STACK	VM_HIGH_ARCH_5
+
+static inline bool arch_is_shadow_stack_vma(vm_flags_t vm_flags)
+{
+	return (vm_flags & VM_SHADOW_STACK) ? true : false;
+}
+
 #else
+
 # define VM_SHADOW_STACK	VM_NONE
+
+static inline bool arch_is_shadow_stack_vma(vm_flags_t vm_flags)
+{
+	return false;
+}
+
 #endif
 
 #if defined(CONFIG_X86)
@@ -3452,7 +3465,7 @@ static inline unsigned long stack_guard_start_gap(struct vm_area_struct *vma)
 		return stack_guard_gap;
 
 	/* See reasoning around the VM_SHADOW_STACK definition */
-	if (vma->vm_flags & VM_SHADOW_STACK)
+	if (arch_is_shadow_stack_vma(vma->vm_flags))
 		return PAGE_SIZE;
 
 	return 0;
diff --git a/mm/gup.c b/mm/gup.c
index 231711efa390..dcc2aa079163 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -1051,7 +1051,7 @@ static int check_vma_flags(struct vm_area_struct *vma, unsigned long gup_flags)
 		    !writable_file_mapping_allowed(vma, gup_flags))
 			return -EFAULT;
 
-		if (!(vm_flags & VM_WRITE) || (vm_flags & VM_SHADOW_STACK)) {
+		if (!(vm_flags & VM_WRITE) || arch_is_shadow_stack_vma(vm_flags)) {
 			if (!(gup_flags & FOLL_FORCE))
 				return -EFAULT;
 			/* hugetlb does not support FOLL_FORCE|FOLL_WRITE. */
diff --git a/mm/internal.h b/mm/internal.h
index b61034bd50f5..05a6b47c3ca1 100644
--- a/mm/internal.h
+++ b/mm/internal.h
@@ -572,7 +572,7 @@ static inline bool is_exec_mapping(vm_flags_t flags)
  */
 static inline bool is_stack_mapping(vm_flags_t flags)
 {
-	return ((flags & VM_STACK) == VM_STACK) || (flags & VM_SHADOW_STACK);
+	return ((flags & VM_STACK) == VM_STACK) || arch_is_shadow_stack_vma(flags);
 }
 
 /*
-- 
2.43.0