[PATCH v2 1/2] kasan: catch invalid free before SLUB reinitializes the object

[Jann Horn]

Currently, when KASAN is combined with init-on-free behavior, the
initialization happens before KASAN's "invalid free" checks.

More importantly, a subsequent commit will want to use the object metadata
region to store an rcu_head, and we should let KASAN check that the object
pointer is valid before that. (Otherwise that change will make the existing
testcase kmem_cache_invalid_free fail.)

So add a new KASAN hook that allows KASAN to pre-validate a
kmem_cache_free() operation before SLUB actually starts modifying the
object or its metadata.

Signed-off-by: Jann Horn <jannh@xxxxxxxxxx>
---
 include/linux/kasan.h | 10 ++++++++++
 mm/kasan/common.c     | 51 +++++++++++++++++++++++++++++++++++++++------------
 mm/slub.c             |  7 +++++++
 3 files changed, 56 insertions(+), 12 deletions(-)

diff --git a/include/linux/kasan.h b/include/linux/kasan.h
index 70d6a8f6e25d..eee8ca1dcb40 100644
--- a/include/linux/kasan.h
+++ b/include/linux/kasan.h
@@ -175,6 +175,16 @@ static __always_inline void * __must_check kasan_init_slab_obj(
 	return (void *)object;
 }
 
+bool __kasan_slab_pre_free(struct kmem_cache *s, void *object,
+			unsigned long ip);
+static __always_inline bool kasan_slab_pre_free(struct kmem_cache *s,
+						void *object)
+{
+	if (kasan_enabled())
+		return __kasan_slab_pre_free(s, object, _RET_IP_);
+	return false;
+}
+
 bool __kasan_slab_free(struct kmem_cache *s, void *object,
 			unsigned long ip, bool init);
 static __always_inline bool kasan_slab_free(struct kmem_cache *s,
diff --git a/mm/kasan/common.c b/mm/kasan/common.c
index 85e7c6b4575c..7c7fc6ce7eb7 100644
--- a/mm/kasan/common.c
+++ b/mm/kasan/common.c
@@ -208,31 +208,52 @@ void * __must_check __kasan_init_slab_obj(struct kmem_cache *cache,
 	return (void *)object;
 }
 
-static inline bool poison_slab_object(struct kmem_cache *cache, void *object,
-				      unsigned long ip, bool init)
+enum free_validation_result {
+	KASAN_FREE_IS_IGNORED,
+	KASAN_FREE_IS_VALID,
+	KASAN_FREE_IS_INVALID
+};
+
+static enum free_validation_result check_slab_free(struct kmem_cache *cache,
+						void *object, unsigned long ip)
 {
-	void *tagged_object;
+	void *tagged_object = object;
 
-	if (!kasan_arch_is_ready())
-		return false;
+	if (is_kfence_address(object) || !kasan_arch_is_ready())
+		return KASAN_FREE_IS_IGNORED;
 
-	tagged_object = object;
 	object = kasan_reset_tag(object);
 
 	if (unlikely(nearest_obj(cache, virt_to_slab(object), object) != object)) {
 		kasan_report_invalid_free(tagged_object, ip, KASAN_REPORT_INVALID_FREE);
-		return true;
+		return KASAN_FREE_IS_INVALID;
 	}
 
-	/* RCU slabs could be legally used after free within the RCU period. */
-	if (unlikely(cache->flags & SLAB_TYPESAFE_BY_RCU))
-		return false;
-
 	if (!kasan_byte_accessible(tagged_object)) {
 		kasan_report_invalid_free(tagged_object, ip, KASAN_REPORT_DOUBLE_FREE);
-		return true;
+		return KASAN_FREE_IS_INVALID;
 	}
 
+	return KASAN_FREE_IS_VALID;
+}
+
+static inline bool poison_slab_object(struct kmem_cache *cache, void *object,
+				      unsigned long ip, bool init)
+{
+	void *tagged_object = object;
+	enum free_validation_result valid = check_slab_free(cache, object, ip);
+
+	if (valid == KASAN_FREE_IS_IGNORED)
+		return false;
+	if (valid == KASAN_FREE_IS_INVALID)
+		return true;
+
+	object = kasan_reset_tag(object);
+
+	/* RCU slabs could be legally used after free within the RCU period. */
+	if (unlikely(cache->flags & SLAB_TYPESAFE_BY_RCU))
+		return false;
+
 	kasan_poison(object, round_up(cache->object_size, KASAN_GRANULE_SIZE),
 			KASAN_SLAB_FREE, init);
 
@@ -242,6 +263,12 @@ static inline bool poison_slab_object(struct kmem_cache *cache, void *object,
 	return false;
 }
 
+bool __kasan_slab_pre_free(struct kmem_cache *cache, void *object,
+				unsigned long ip)
+{
+	return check_slab_free(cache, object, ip) == KASAN_FREE_IS_INVALID;
+}
+
 bool __kasan_slab_free(struct kmem_cache *cache, void *object,
 				unsigned long ip, bool init)
 {
diff --git a/mm/slub.c b/mm/slub.c
index 4927edec6a8c..34724704c52d 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -2170,6 +2170,13 @@ bool slab_free_hook(struct kmem_cache *s, void *x, bool init)
 	if (kfence_free(x))
 		return false;
 
+	/*
+	 * Give KASAN a chance to notice an invalid free operation before we
+	 * modify the object.
+	 */
+	if (kasan_slab_pre_free(s, x))
+		return false;
+
 	/*
 	 * As memory initialization might be integrated into KASAN,
 	 * kasan_slab_free and initialization memset's must be

-- 
2.45.2.1089.g2a221341d9-goog