mm/slab: add naive detection of double free

Similar to commit ce6fa91 ("mm/slub.c: add a naive detection of
double free or corruption"), add a very cheap double-free check for SLAB
under CONFIG_SLAB_FREELIST_HARDENED.  With this added, the
"SLAB_FREE_DOUBLE" LKDTM test passes under SLAB:

  lkdtm: Performing direct entry SLAB_FREE_DOUBLE
  lkdtm: Attempting double slab free ...
  ------------[ cut here ]------------
  WARNING: CPU: 2 PID: 2193 at mm/slab.c:757 ___cache _free+0x325/0x390

[[email protected]: fix misplaced __free_one()]
  Link: http://lkml.kernel.org/r/202006261306.0D82A2B@keescook
  Link: https://lore.kernel.org/lkml/[email protected]

Signed-off-by: Kees Cook <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Acked-by: Vlastimil Babka <[email protected]>
Acked-by: Randy Dunlap <[email protected]>	[build tested]
Cc: Roman Gushchin <[email protected]>
Cc: Christoph Lameter <[email protected]>
Cc: Alexander Popov <[email protected]>
Cc: Pekka Enberg <[email protected]>
Cc: David Rientjes <[email protected]>
Cc: Joonsoo Kim <[email protected]>
Cc: Vinayak Menon <[email protected]>
Cc: Matthew Garrett <[email protected]>
Cc: Jann Horn <[email protected]>
Cc: Vijayanand Jitta <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Linus Torvalds <[email protected]>

mm/slab.c

@@ -588,6 +588,16 @@ static int transfer_objects(struct array_cache *to,
 	return nr;
 }
 
+/* &alien->lock must be held by alien callers. */
+static __always_inline void __free_one(struct array_cache *ac, void *objp)
+{
+	/* Avoid trivial double-free. */
+	if (IS_ENABLED(CONFIG_SLAB_FREELIST_HARDENED) &&
+	    WARN_ON_ONCE(ac->avail > 0 && ac->entry[ac->avail - 1] == objp))
+		return;
+	ac->entry[ac->avail++] = objp;
+}
+
 #ifndef CONFIG_NUMA
 
 #define drain_alien_cache(cachep, alien) do { } while (0)
@@ -767,7 +777,7 @@ static int __cache_free_alien(struct kmem_cache *cachep, void *objp,
 			STATS_INC_ACOVERFLOW(cachep);
 			__drain_alien_cache(cachep, ac, page_node, &list);
 		}
-		ac->entry[ac->avail++] = objp;
+		__free_one(ac, objp);
 		spin_unlock(&alien->lock);
 		slabs_destroy(cachep, &list);
 	} else {
@@ -3466,7 +3476,7 @@ void ___cache_free(struct kmem_cache *cachep, void *objp,
 		}
 	}
 
-	ac->entry[ac->avail++] = objp;
+	__free_one(ac, objp);
 }
 
 /**