[PATCH] slub: Adds a way to handle freelist cycle in on_freelist()

[Lilith Gkini]

The on_freelist() doesn't have a way to handle the edgecase of having a
full freelist that doesn't end in NULL and instead has another valid
pointer in the slab as a result of a Use-After-Free or anything similar.

This case won't get caught by check_valid_pointer() and it will result in
nr incrementing to `slab->objects + 1`, corrupting the slab->inuse entry
later in the code by setting it to -1.

The Patch adds an if check to detect that case, notifies us and handles
the freelist and slab appropriately, as is the standard process in these
situations.

Furthermore the Patch changes the return type of the function from
int to bool as per codying style guidelines.

It also moves the `break;` line inside the `if (object) {` to make it more
obvious that the code breaks the while loop in that branch.

Signed-off-by: Lilith Persefoni Gkini <lilithgkini@xxxxxxxxx>
---
 mm/slub.c | 16 ++++++++++++----
 1 file changed, 12 insertions(+), 4 deletions(-)

diff --git a/mm/slub.c b/mm/slub.c
index 1f50129dcfb3..95e54ffd5330 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -1427,7 +1427,7 @@ static int check_slab(struct kmem_cache *s, struct slab *slab)
  * Determine if a certain object in a slab is on the freelist. Must hold the
  * slab lock to guarantee that the chains are in a consistent state.
  */
-static int on_freelist(struct kmem_cache *s, struct slab *slab, void *search)
+static bool on_freelist(struct kmem_cache *s, struct slab *slab, void *search)
 {
 	int nr = 0;
 	void *fp;
@@ -1437,26 +1437,34 @@ static int on_freelist(struct kmem_cache *s, struct slab *slab, void *search)
 	fp = slab->freelist;
 	while (fp && nr <= slab->objects) {
 		if (fp == search)
-			return 1;
+			return true;
 		if (!check_valid_pointer(s, slab, fp)) {
 			if (object) {
 				object_err(s, slab, object,
 					"Freechain corrupt");
 				set_freepointer(s, object, NULL);
+				break;
 			} else {
 				slab_err(s, slab, "Freepointer corrupt");
 				slab->freelist = NULL;
 				slab->inuse = slab->objects;
 				slab_fix(s, "Freelist cleared");
-				return 0;
+				return false;
 			}
-			break;
 		}
 		object = fp;
 		fp = get_freepointer(s, object);
 		nr++;
 	}
 
+	if (nr > slab->objects) {
+		slab_err(s, slab, "Freelist cycle detected");
+		slab->freelist = NULL;
+		slab->inuse = slab->objects;
+		slab_fix(s, "Freelist cleared");
+		return false;
+	}
+
 	max_objects = order_objects(slab_order(slab), s->size);
 	if (max_objects > MAX_OBJS_PER_PAGE)
 		max_objects = MAX_OBJS_PER_PAGE;
-- 
2.48.1