mm: slub: really fix slab walking for init_on_free

Commit 1b7e816f ("mm: slub: Fix slab walking for init_on_free")
fixed one problem with the slab walking but missed a key detail: When
walking the list, the head and tail pointers need to be updated since we
end up reversing the list as a result.  Without doing this, bulk free is
broken.

One way this is exposed is a NULL pointer with slub_debug=F:

  =============================================================================
  BUG skbuff_head_cache (Tainted: G                T): Object already free
  -----------------------------------------------------------------------------

  INFO: Slab 0x000000000d2d2f8f objects=16 used=3 fp=0x0000000064309071 flags=0x3fff00000000201
  BUG: kernel NULL pointer dereference, address: 0000000000000000
  Oops: 0000 [#1] PREEMPT SMP PTI
  RIP: 0010:print_trailer+0x70/0x1d5
  Call Trace:
   <IRQ>
   free_debug_processing.cold.37+0xc9/0x149
   __slab_free+0x22a/0x3d0
   kmem_cache_free_bulk+0x415/0x420
   __kfree_skb_flush+0x30/0x40
   net_rx_action+0x2dd/0x480
   __do_softirq+0xf0/0x246
   irq_exit+0x93/0xb0
   do_IRQ+0xa0/0x110
   common_interrupt+0xf/0xf
   </IRQ>

Given we're now almost identical to the existing debugging code which
correctly walks the list, combine with that.

Link: https://lkml.kernel.org/r/20191104170303.GA50361@gandi.net
Link: http://lkml.kernel.org/r/20191106222208.26815-1-labbott@redhat.com
Fixes: 1b7e816f ("mm: slub: Fix slab walking for init_on_free")
Signed-off-by: Laura Abbott <labbott@redhat.com>
Reported-by: Thibaut Sautereau <thibaut.sautereau@clip-os.org>
Acked-by: David Rientjes <rientjes@google.com>
Tested-by: Alexander Potapenko <glider@google.com>
Acked-by: Alexander Potapenko <glider@google.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: <clipos@ssi.gouv.fr>
Cc: Christoph Lameter <cl@linux.com>
Cc: Pekka Enberg <penberg@kernel.org>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/slub.c

@@ -1433,12 +1433,15 @@ static inline bool slab_free_freelist_hook(struct kmem_cache *s,
 	void *old_tail = *tail ? *tail : *head;
 	int rsize;
 
-	if (slab_want_init_on_free(s)) {
-		void *p = NULL;
+	/* Head and tail of the reconstructed freelist */
+	*head = NULL;
+	*tail = NULL;
 
-		do {
-			object = next;
-			next = get_freepointer(s, object);
+	do {
+		object = next;
+		next = get_freepointer(s, object);
+
+		if (slab_want_init_on_free(s)) {
 			/*
 			 * Clear the object and the metadata, but don't touch
 			 * the redzone.
@@ -1448,29 +1451,8 @@ static inline bool slab_free_freelist_hook(struct kmem_cache *s,
 							   : 0;
 			memset((char *)object + s->inuse, 0,
 			       s->size - s->inuse - rsize);
-			set_freepointer(s, object, p);
-			p = object;
-		} while (object != old_tail);
-	}
-
-/*
- * Compiler cannot detect this function can be removed if slab_free_hook()
- * evaluates to nothing.  Thus, catch all relevant config debug options here.
- */
-#if defined(CONFIG_LOCKDEP)	||		\
-	defined(CONFIG_DEBUG_KMEMLEAK) ||	\
-	defined(CONFIG_DEBUG_OBJECTS_FREE) ||	\
-	defined(CONFIG_KASAN)
 
-	next = *head;
-
-	/* Head and tail of the reconstructed freelist */
-	*head = NULL;
-	*tail = NULL;
-
-	do {
-		object = next;
-		next = get_freepointer(s, object);
+		}
 		/* If object's reuse doesn't have to be delayed */
 		if (!slab_free_hook(s, object)) {
 			/* Move object to the new freelist */
@@ -1485,9 +1467,6 @@ static inline bool slab_free_freelist_hook(struct kmem_cache *s,
 		*tail = NULL;
 
 	return *head != NULL;
-#else
-	return true;
-#endif
 }
 
 static void *setup_object(struct kmem_cache *s, struct page *page,