mm: vmalloc: add flag preventing guard hole allocation

For instrumenting global variables KASan will shadow memory backing memory
for modules.  So on module loading we will need to allocate memory for
shadow and map it at address in shadow that corresponds to the address
allocated in module_alloc().

__vmalloc_node_range() could be used for this purpose, except it puts a
guard hole after allocated area.  Guard hole in shadow memory should be a
problem because at some future point we might need to have a shadow memory
at address occupied by guard hole.  So we could fail to allocate shadow
for module_alloc().

Add a new vm_struct flag 'VM_NO_GUARD' indicating that vm area doesn't
have a guard hole.

Signed-off-by: Andrey Ryabinin <[email protected]>
Cc: Dmitry Vyukov <[email protected]>
Cc: Konstantin Serebryany <[email protected]>
Cc: Dmitry Chernenkov <[email protected]>
Signed-off-by: Andrey Konovalov <[email protected]>
Cc: Yuri Gribov <[email protected]>
Cc: Konstantin Khlebnikov <[email protected]>
Cc: Sasha Levin <[email protected]>
Cc: Christoph Lameter <[email protected]>
Cc: Joonsoo Kim <[email protected]>
Cc: Dave Hansen <[email protected]>
Cc: Andi Kleen <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: "H. Peter Anvin" <[email protected]>
Cc: Christoph Lameter <[email protected]>
Cc: Pekka Enberg <[email protected]>
Cc: David Rientjes <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

include/linux/vmalloc.h

@@ -16,6 +16,7 @@ struct vm_area_struct;		/* vma defining user mapping in mm_types.h */
 #define VM_USERMAP		0x00000008	/* suitable for remap_vmalloc_range */
 #define VM_VPAGES		0x00000010	/* buffer for pages was vmalloc'ed */
 #define VM_UNINITIALIZED	0x00000020	/* vm_struct is not fully initialized */
+#define VM_NO_GUARD		0x00000040      /* don't add guard page */
 /* bits [20..32] reserved for arch specific ioremap internals */
 
 /*
@@ -96,8 +97,12 @@ void vmalloc_sync_all(void);
 
 static inline size_t get_vm_area_size(const struct vm_struct *area)
 {
-	/* return actual size without guard page */
-	return area->size - PAGE_SIZE;
+	if (!(area->flags & VM_NO_GUARD))
+		/* return actual size without guard page */
+		return area->size - PAGE_SIZE;
+	else
+		return area->size;
+
 }
 
 extern struct vm_struct *get_vm_area(unsigned long size, unsigned long flags);

mm/vmalloc.c

@@ -1324,10 +1324,8 @@ static struct vm_struct *__get_vm_area_node(unsigned long size,
 	if (unlikely(!area))
 		return NULL;
 
-	/*
-	 * We always allocate a guard page.
-	 */
-	size += PAGE_SIZE;
+	if (!(flags & VM_NO_GUARD))
+		size += PAGE_SIZE;
 
 	va = alloc_vmap_area(size, align, start, end, node, gfp_mask);
 	if (IS_ERR(va)) {