[PATCH v4 5/7] mm/slab: Provide kmalloc type fallback for bucket allocations

[Kees Cook <kees@kernel.org>]

kmem_buckets_create() clones kmalloc_caches[KMALLOC_NORMAL].
kmalloc_slab() figures out the kmalloc type the caller asks for, but
then ignored it whenever a bucket set was in use, returning a normal
cache regardless. This would be a problem if a caller asked for GFP_DMA,
__GFP_ACCOUNT, etc. None of the current users do this, so there is
problem, but it makes adding new users fragile. For example, skb data[1]
needs to handle GFP_DMA (rarely) and __GFP_ACCOUNT (often).

Send those allocations to the general caches instead so nothing breaks and
regular allocations remain isolated with the bucket. The kmem_bucket_type
enum contains only a single item here, but will be expanded in the next
patch.

Built and tests pass with ARCH=x86_64 defconfig with GCC 16.2.0, with
CONFIG_SLAB_BUCKETS as y and n.

Assisted-by: LLM
Link: https://lore.kernel.org/all/04debe19-bbe8-4b5f-9668-753d1f97832d@redhat.com/ [1]
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   | 13 +++++++++++
 mm/slab.h              | 23 ++++++++++++++++--
 lib/tests/slub_kunit.c | 53 ++++++++++++++++++++++++++++++++++++++++++
 3 files changed, 87 insertions(+), 2 deletions(-)

diff --git a/include/linux/slab.h b/include/linux/slab.h
index 18a2351f9084..ab9ab3d34847 100644
--- a/include/linux/slab.h
+++ b/include/linux/slab.h
@@ -742,6 +742,19 @@ typedef struct kmem_cache * kmem_buckets[KMALLOC_SHIFT_HIGH + 1];
 
 extern kmem_buckets kmalloc_caches[NR_KMALLOC_TYPES];
 
+/*
+ * The kmalloc types a bucket set can hold a copy of. This is deliberately not
+ * enum kmalloc_cache_type: the KMALLOC_PARTITION copies are all "normal" to a
+ * bucket set, which already separates what they were there to separate, so
+ * indexing by those would mean up to KMALLOC_PARTITION_CACHES_NR unusable
+ * rows per set. Allocations of any type not listed here are served by the
+ * general caches.
+ */
+enum kmem_bucket_type {
+	KMEM_BUCKET_NORMAL = 0,
+	NR_KMEM_BUCKET_TYPES
+};
+
 /*
  * Define gfp bits that should not be set for KMALLOC_NORMAL.
  */
diff --git a/mm/slab.h b/mm/slab.h
index 8fd6835e4235..7f1bfee83b92 100644
--- a/mm/slab.h
+++ b/mm/slab.h
@@ -421,6 +421,26 @@ static inline unsigned int size_index_elem(unsigned int bytes)
 	return (bytes - 1) / 8;
 }
 
+/*
+ * Which set of buckets to use for the given kmalloc_cache_type. If not
+ * handled by the kmem_buckets, fall back to general caches.
+ */
+static inline kmem_buckets *
+kmalloc_choose_bucket(kmem_buckets *bucket, enum kmalloc_cache_type type)
+{
+	enum kmem_bucket_type btype;
+
+	if (!bucket)
+		return &kmalloc_caches[type];
+
+	if (type <= KMALLOC_PARTITION_END)
+		btype = KMEM_BUCKET_NORMAL;
+	else
+		return &kmalloc_caches[type];	/* No set holds a row for it. */
+
+	return &bucket[btype];
+}
+
 /*
  * Find the kmem_cache structure that serves a given size of
  * allocation
@@ -438,8 +458,7 @@ kmalloc_slab(size_t size, kmem_buckets *b, gfp_t flags, kmalloc_token_t token,
 	if (alloc_flags & SLAB_ALLOC_NO_OBJ_EXT)
 		type = KMALLOC_NO_OBJ_EXT;
 
-	if (!b)
-		b = &kmalloc_caches[type];
+	b = kmalloc_choose_bucket(b, type);
 	if (size <= 192)
 		index = kmalloc_size_index[size_index_elem(size)];
 	else
diff --git a/lib/tests/slub_kunit.c b/lib/tests/slub_kunit.c
index d6467dd5cf9b..823607e06248 100644
--- a/lib/tests/slub_kunit.c
+++ b/lib/tests/slub_kunit.c
@@ -683,6 +683,58 @@ static void test_kmem_buckets_destroy(struct kunit *test)
 	KUNIT_EXPECT_EQ(test, 2, slab_errors);
 }
 
+/*
+ * A bucket set holds only the kmalloc types it was created with, so an
+ * allocation that asks for a different one has to come from the general
+ * caches. Check that it does, rather than being served a normal cache that
+ * does not satisfy what the flags asked for.
+ */
+static void test_kmem_buckets_type_fallback(struct kunit *test)
+{
+	struct kmem_cache *c;
+	kmem_buckets *b;
+	void *p;
+
+	if (!IS_ENABLED(CONFIG_SLAB_BUCKETS))
+		kunit_skip(test, "needs CONFIG_SLAB_BUCKETS");
+
+	b = kmem_buckets_create("test_buckets", 0, 0, INT_MAX, NULL);
+	KUNIT_ASSERT_BUCKETS_CREATED(test, b);
+
+	/* A plain allocation stays isolated in the bucket set. */
+	p = kmem_buckets_alloc(b, 128, GFP_KERNEL);
+	KUNIT_ASSERT_NOT_NULL(test, p);
+	c = cache_of(p);
+	kfree(p);
+	KUNIT_ASSERT_NOT_NULL(test, c);
+
+	KUNIT_EXPECT_TRUE_MSG(test, strstarts(c->name, "test_buckets-"),
+			      "expected a bucket cache, got %s", c->name);
+
+	/* One that needs ZONE_DMA cannot, so it falls back. */
+	if (IS_ENABLED(CONFIG_ZONE_DMA)) {
+		p = kmem_buckets_alloc(b, 128, GFP_KERNEL | GFP_DMA);
+		KUNIT_ASSERT_NOT_NULL(test, p);
+		c = cache_of(p);
+		kfree(p);
+		KUNIT_ASSERT_NOT_NULL(test, c);
+
+		KUNIT_EXPECT_TRUE_MSG(test, strstarts(c->name, "dma-kmalloc-"),
+				      "expected a DMA cache, got %s", c->name);
+	}
+
+	/* Nor can one that has to be accounted. */
+	if (IS_ENABLED(CONFIG_MEMCG) && !mem_cgroup_kmem_disabled()) {
+		p = kmem_buckets_alloc(b, 128, GFP_KERNEL | __GFP_ACCOUNT);
+		KUNIT_ASSERT_NOT_NULL(test, p);
+		c = virt_to_slab(p)->slab_cache;
+		kfree(p);
+
+		KUNIT_EXPECT_TRUE_MSG(test, strstarts(c->name, "kmalloc-cg-"),
+				      "expected an accounted cache, got %s", c->name);
+	}
+}
+
 static struct kunit_case test_cases[] = {
 	KUNIT_CASE(test_clobber_zone),
 
@@ -709,6 +761,7 @@ static struct kunit_case test_cases[] = {
 	KUNIT_CASE(test_kmem_buckets_alignment),
 	KUNIT_CASE(test_kmem_buckets_disabled),
 	KUNIT_CASE(test_kmem_buckets_destroy),
+	KUNIT_CASE(test_kmem_buckets_type_fallback),
 	{}
 };
 
-- 
2.34.1