[PATCH v2 2/2] mm/kmemleak: Fix UAF bug in kmemleak_scan()

[Waiman Long]

Commit 6edda04ccc7c ("mm/kmemleak: prevent soft lockup in first
object iteration loop of kmemleak_scan()") fixes soft lockup problem
in kmemleak_scan() by periodically doing a cond_resched(). It does
take a reference of the current object before doing it. Unfortunately,
if the object has been deleted from the object_list, the next object
pointed to by its next pointer may no longer be valid after coming
back from cond_resched(). This can result in use-after-free and other
nasty problem.

Fix this problem by adding a del_state flag into kmemleak_object
structure to synchronize the object deletion process between
kmemleak_cond_resched() and __remove_object() to make sure that the
object remained in the object_list in the duration of the cond_resched()
call.

Fixes: 6edda04ccc7c ("mm/kmemleak: prevent soft lockup in first object iteration loop of kmemleak_scan()")
Signed-off-by: Waiman Long <longman@xxxxxxxxxx>
---
 mm/kmemleak.c | 35 +++++++++++++++++++++++++++++------
 1 file changed, 29 insertions(+), 6 deletions(-)

diff --git a/mm/kmemleak.c b/mm/kmemleak.c
index e7cb521236bf..0ece170fc9ef 100644
--- a/mm/kmemleak.c
+++ b/mm/kmemleak.c
@@ -13,11 +13,12 @@
  *
  * The following locks and mutexes are used by kmemleak:
  *
- * - kmemleak_lock (raw_spinlock_t): protects the object_list modifications and
- *   accesses to the object_tree_root (or object_phys_tree_root). The
- *   object_list is the main list holding the metadata (struct kmemleak_object)
- *   for the allocated memory blocks. The object_tree_root and object_phys_tree_root
- *   are red black trees used to look-up metadata based on a pointer to the
+ * - kmemleak_lock (raw_spinlock_t): protects the object_list as well as
+ *   del_state modifications and accesses to the object_tree_root (or
+ *   object_phys_tree_root). The object_list is the main list holding the
+ *   metadata (struct kmemleak_object) for the allocated memory blocks.
+ *   The object_tree_root and object_phys_tree_root are red
+ *   black trees used to look-up metadata based on a pointer to the
  *   corresponding memory block. The object_phys_tree_root is for objects
  *   allocated with physical address. The kmemleak_object structures are
  *   added to the object_list and object_tree_root (or object_phys_tree_root)
@@ -147,6 +148,7 @@ struct kmemleak_object {
 	struct rcu_head rcu;		/* object_list lockless traversal */
 	/* object usage count; object freed when use_count == 0 */
 	atomic_t use_count;
+	unsigned int del_state;		/* deletion state */
 	unsigned long pointer;
 	size_t size;
 	/* pass surplus references to this pointer */
@@ -177,6 +179,11 @@ struct kmemleak_object {
 /* flag set for object allocated with physical address */
 #define OBJECT_PHYS		(1 << 4)
 
+/* set when __remove_object() called */
+#define DELSTATE_REMOVED	(1 << 0)
+/* set to temporarily prevent deletion from object_list */
+#define DELSTATE_NO_DELETE	(1 << 1)
+
 #define HEX_PREFIX		"    "
 /* number of bytes to print per line; must be 16 or 32 */
 #define HEX_ROW_SIZE		16
@@ -567,7 +574,9 @@ static void __remove_object(struct kmemleak_object *object)
 	rb_erase(&object->rb_node, object->flags & OBJECT_PHYS ?
 				   &object_phys_tree_root :
 				   &object_tree_root);
-	list_del_rcu(&object->object_list);
+	if (!(object->del_state & DELSTATE_NO_DELETE))
+		list_del_rcu(&object->object_list);
+	object->del_state |= DELSTATE_REMOVED;
 }
 
 /*
@@ -633,6 +642,7 @@ static void __create_object(unsigned long ptr, size_t size,
 	object->count = 0;			/* white color initially */
 	object->jiffies = jiffies;
 	object->checksum = 0;
+	object->del_state = 0;
 
 	/* task information */
 	if (in_hardirq()) {
@@ -1470,9 +1480,22 @@ static void kmemleak_cond_resched(struct kmemleak_object *object)
 	if (!get_object(object))
 		return;	/* Try next object */
 
+	raw_spin_lock_irq(&kmemleak_lock);
+	if (object->del_state & DELSTATE_REMOVED)
+		goto unlock_put;	/* Object removed */
+	object->del_state |= DELSTATE_NO_DELETE;
+	raw_spin_unlock_irq(&kmemleak_lock);
+
 	rcu_read_unlock();
 	cond_resched();
 	rcu_read_lock();
+
+	raw_spin_lock_irq(&kmemleak_lock);
+	if (object->del_state & DELSTATE_REMOVED)
+		list_del_rcu(&object->object_list);
+	object->del_state &= ~DELSTATE_NO_DELETE;
+unlock_put:
+	raw_spin_unlock_irq(&kmemleak_lock);
 	put_object(object);
 }
 
-- 
2.31.1