kasan: preassign tags to objects with ctors or SLAB_TYPESAFE_BY_RCU

An object constructor can initialize pointers within this objects based on
the address of the object.  Since the object address might be tagged, we
need to assign a tag before calling constructor.

The implemented approach is to assign tags to objects with constructors
when a slab is allocated and call constructors once as usual.  The
downside is that such object would always have the same tag when it is
reallocated, so we won't catch use-after-frees on it.

Also pressign tags for objects from SLAB_TYPESAFE_BY_RCU caches, since
they can be validy accessed after having been freed.

Link: http://lkml.kernel.org/r/f158a8a74a031d66f0a9398a5b0ed453c37ba09a.1544099024.git.andreyknvl@google.comSigned-off-by: Andrey Konovalov <andreyknvl@google.com>
Reviewed-by: Andrey Ryabinin <aryabinin@virtuozzo.com>
Reviewed-by: Dmitry Vyukov <dvyukov@google.com>
Cc: Christoph Lameter <cl@linux.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Will Deacon <will.deacon@arm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/slab.c

@@ -2574,7 +2574,7 @@ static void cache_init_objs(struct kmem_cache *cachep,
 
 	for (i = 0; i < cachep->num; i++) {
 		objp = index_to_obj(cachep, page, i);
-		kasan_init_slab_obj(cachep, objp);
+		objp = kasan_init_slab_obj(cachep, objp);
 
 		/* constructor could break poison info */
 		if (DEBUG == 0 && cachep->ctor) {

mm/slub.c

@@ -1451,16 +1451,17 @@ static inline bool slab_free_freelist_hook(struct kmem_cache *s,
 #endif
 }
 
-static void setup_object(struct kmem_cache *s, struct page *page,
+static void *setup_object(struct kmem_cache *s, struct page *page,
 				void *object)
 {
 	setup_object_debug(s, page, object);
-	kasan_init_slab_obj(s, object);
+	object = kasan_init_slab_obj(s, object);
 	if (unlikely(s->ctor)) {
 		kasan_unpoison_object_data(s, object);
 		s->ctor(object);
 		kasan_poison_object_data(s, object);
 	}
+	return object;
 }
 
 /*
@@ -1568,16 +1569,16 @@ static bool shuffle_freelist(struct kmem_cache *s, struct page *page)
 	/* First entry is used as the base of the freelist */
 	cur = next_freelist_entry(s, page, &pos, start, page_limit,
 				freelist_count);
+	cur = setup_object(s, page, cur);
 	page->freelist = cur;
 
 	for (idx = 1; idx < page->objects; idx++) {
-		setup_object(s, page, cur);
 		next = next_freelist_entry(s, page, &pos, start, page_limit,
 			freelist_count);
+		next = setup_object(s, page, next);
 		set_freepointer(s, cur, next);
 		cur = next;
 	}
-	setup_object(s, page, cur);
 	set_freepointer(s, cur, NULL);
 
 	return true;
@@ -1599,7 +1600,7 @@ static struct page *allocate_slab(struct kmem_cache *s, gfp_t flags, int node)
 	struct page *page;
 	struct kmem_cache_order_objects oo = s->oo;
 	gfp_t alloc_gfp;
-	void *start, *p;
+	void *start, *p, *next;
 	int idx, order;
 	bool shuffle;
 
@@ -1651,13 +1652,16 @@ static struct page *allocate_slab(struct kmem_cache *s, gfp_t flags, int node)
 
 	if (!shuffle) {
 		for_each_object_idx(p, idx, s, start, page->objects) {
-			setup_object(s, page, p);
-			if (likely(idx < page->objects))
-				set_freepointer(s, p, p + s->size);
-			else
+			if (likely(idx < page->objects)) {
+				next = p + s->size;
+				next = setup_object(s, page, next);
+				set_freepointer(s, p, next);
+			} else
 				set_freepointer(s, p, NULL);
 		}
-		page->freelist = fixup_red_left(s, start);
+		start = fixup_red_left(s, start);
+		start = setup_object(s, page, start);
+		page->freelist = start;
 	}
 
 	page->inuse = page->objects;