[PATCH v4 3/7] mm/slab: Add kmem_buckets_destroy()

[Kees Cook <kees@kernel.org>]

kmem_buckets_create() intentionally had no "destroy" counterpart. Every
caller has lived in core kernel code and creates its set once at boot,
so nothing has needed to take one down. However, KUnit tests may be
built module, so we need it now to support the coming tests.

Some caches have size aliases, so the same pointer is stored at more
then one index, so we have to save it, clear all matching instances, and
then free the saved cache pointer. (This is what the bitmap was tracking
before in the "allocation failed" error path.)

When CONFIG_SLAB_BUCKETS=n the whole body compiles away, matching the
ZERO_SIZE_PTR that kmem_buckets_create() hands back in that configuration.

Link: https://lore.kernel.org/all/20240809073309.2134488-1-kees@kernel.org/
Assisted-by: LLM
Signed-off-by: Kees Cook <kees@kernel.org>
---
Cc: Vlastimil Babka <vbabka@kernel.org>
Cc: Harry Yoo <harry@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Hao Li <hao.li@linux.dev>
Cc: Christoph Lameter <cl@gentwo.org>
Cc: David Rientjes <rientjes@google.com>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Cc: <linux-mm@kvack.org>
Cc: Pedro Falcato <pfalcato@suse.de>
Cc: Kuniyuki Iwashima <kuniyu@google.com>
Cc: <linux-hardening@vger.kernel.org>
---
 include/linux/slab.h |  1 +
 mm/slab_common.c     | 49 +++++++++++++++++++++++++++++++++++++-------
 2 files changed, 43 insertions(+), 7 deletions(-)

diff --git a/include/linux/slab.h b/include/linux/slab.h
index cda126def67a..18a2351f9084 100644
--- a/include/linux/slab.h
+++ b/include/linux/slab.h
@@ -893,6 +893,7 @@ void kmem_cache_free(struct kmem_cache *s, void *objp);
 kmem_buckets *kmem_buckets_create(const char *name, slab_flags_t flags,
 				  unsigned int useroffset, unsigned int usersize,
 				  void (*ctor)(void *));
+void kmem_buckets_destroy(kmem_buckets *bucket);
 
 /*
  * Bulk allocation and freeing operations. These are accelerated in an
diff --git a/mm/slab_common.c b/mm/slab_common.c
index cc58f192f349..eb29cfb2f0a9 100644
--- a/mm/slab_common.c
+++ b/mm/slab_common.c
@@ -434,12 +434,9 @@ kmem_buckets *kmem_buckets_create(const char *name, slab_flags_t flags,
 				  unsigned int usersize,
 				  void (*ctor)(void *))
 {
-	unsigned long mask = 0;
 	unsigned int idx;
 	kmem_buckets *b;
 
-	BUILD_BUG_ON(ARRAY_SIZE(kmalloc_caches[KMALLOC_NORMAL]) > BITS_PER_LONG);
-
 	/*
 	 * When the separate buckets API is not built in, just return
 	 * a non-NULL value for the kmem_buckets pointer, which will be
@@ -493,7 +490,6 @@ kmem_buckets *kmem_buckets_create(const char *name, slab_flags_t flags,
 			kfree(cache_name);
 			if (WARN_ON(!(*b)[aligned_idx]))
 				goto fail;
-			set_bit(aligned_idx, &mask);
 		}
 		if (idx != aligned_idx)
 			(*b)[idx] = (*b)[aligned_idx];
@@ -502,14 +498,53 @@ kmem_buckets *kmem_buckets_create(const char *name, slab_flags_t flags,
 	return b;
 
 fail:
-	for_each_set_bit(idx, &mask, ARRAY_SIZE(kmalloc_caches[KMALLOC_NORMAL]))
-		kmem_cache_destroy((*b)[idx]);
-	kmem_cache_free(kmem_buckets_cache, b);
+	kmem_buckets_destroy(b);
 
 	return NULL;
 }
 EXPORT_SYMBOL(kmem_buckets_create);
 
+/**
+ * kmem_buckets_destroy - Destroy a set of caches made by kmem_buckets_create()
+ * @bucket: The set to destroy, which may be NULL.
+ *
+ * Destroys each cache in @bucket and then frees @bucket itself. As for
+ * kmem_cache_destroy(), every object allocated from @bucket must have been
+ * freed beforehand, and @bucket must not be used afterwards.
+ *
+ * Context: Process context. May sleep, as kmem_cache_destroy() takes the
+ *	    slab mutex and can wait on RCU callbacks for each cache.
+ */
+void kmem_buckets_destroy(kmem_buckets *bucket)
+{
+	unsigned int idx, i;
+
+	if (!IS_ENABLED(CONFIG_SLAB_BUCKETS) || ZERO_OR_NULL_PTR(bucket))
+		return;
+
+	for (idx = 0; idx < ARRAY_SIZE(kmalloc_caches[KMALLOC_NORMAL]); idx++) {
+		struct kmem_cache *cache = (*bucket)[idx];
+
+		if (!cache)
+			continue;
+
+		/*
+		 * Sizes below arch_slab_minalign() share one cache, which
+		 * kmem_buckets_create() then stores at each of their indices.
+		 * Drop every reference to it before destroying it, so that no
+		 * later pass reads a pointer to a cache that is already gone.
+		 */
+		for (i = idx; i < ARRAY_SIZE(kmalloc_caches[KMALLOC_NORMAL]); i++)
+			if ((*bucket)[i] == cache)
+				(*bucket)[i] = NULL;
+
+		kmem_cache_destroy(cache);
+	}
+
+	kmem_cache_free(kmem_buckets_cache, bucket);
+}
+EXPORT_SYMBOL(kmem_buckets_destroy);
+
 /*
  * For a given kmem_cache, kmem_cache_destroy() should only be called
  * once or there will be a use-after-free problem. The actual deletion
-- 
2.34.1