[PATCH v2 0/2] mm/kmemleak: Simplify kmemleak_cond_resched() & fix UAF

[PATCH v2 0/2] mm/kmemleak: Simplify kmemleak_cond_resched() & fix UAF

[Waiman Long]

It was found that a KASAN use-after-free error was reported in the
kmemleak_scan() function. After further examination, it is believe
that even though a reference is taken from the current object, it does
not prevent the object pointed to by the next pointer from going away
after a cond_resched().

To fix that, additional flags are added to make sure that the current
object won't be removed from the object_list during the duration of
the cond_resched() to ensure the validity of the next pointer.

While making the change, I also simplify the current usage of
kmemleak_cond_resched() to make it easier to understand.

Waiman Long (2):
  mm/kmemleak: Simplify kmemleak_cond_resched() usage
  mm/kmemleak: Fix UAF bug in kmemleak_scan()

 [v2: Update patch 2 to prevent object_list removal of current object]

 mm/kmemleak.c | 83 +++++++++++++++++++++++++--------------------------
 1 file changed, 41 insertions(+), 42 deletions(-)

-- 
2.31.1

[PATCH v2 1/2] mm/kmemleak: Simplify kmemleak_cond_resched() usage

[Waiman Long]

The presence of a pinned argument and the 64k loop count make
kmemleak_cond_resched() a bit more complex to read. The pinned argument
is used only by first kmemleak_scan() loop.

Simplify the usage of kmemleak_cond_resched() by removing the pinned
argument and always do a get_object()/put_object() sequence. In
addition, the 64k loop is removed by using need_resched() to decide if
kmemleak_cond_resched() should be called.

Signed-off-by: Waiman Long <longman@redhat.com>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
---
 mm/kmemleak.c | 48 ++++++++++++------------------------------------
 1 file changed, 12 insertions(+), 36 deletions(-)

diff --git a/mm/kmemleak.c b/mm/kmemleak.c
index 267332904354..e7cb521236bf 100644
--- a/mm/kmemleak.c
+++ b/mm/kmemleak.c
@@ -1463,22 +1463,17 @@ static void scan_gray_list(void)
 /*
  * Conditionally call resched() in an object iteration loop while making sure
  * that the given object won't go away without RCU read lock by performing a
- * get_object() if !pinned.
- *
- * Return: false if can't do a cond_resched() due to get_object() failure
- *	   true otherwise
+ * get_object() if necessaary.
  */
-static bool kmemleak_cond_resched(struct kmemleak_object *object, bool pinned)
+static void kmemleak_cond_resched(struct kmemleak_object *object)
 {
-	if (!pinned && !get_object(object))
-		return false;
+	if (!get_object(object))
+		return;	/* Try next object */
 
 	rcu_read_unlock();
 	cond_resched();
 	rcu_read_lock();
-	if (!pinned)
-		put_object(object);
-	return true;
+	put_object(object);
 }
 
 /*
@@ -1492,15 +1487,12 @@ static void kmemleak_scan(void)
 	struct zone *zone;
 	int __maybe_unused i;
 	int new_leaks = 0;
-	int loop_cnt = 0;
 
 	jiffies_last_scan = jiffies;
 
 	/* prepare the kmemleak_object's */
 	rcu_read_lock();
 	list_for_each_entry_rcu(object, &object_list, object_list) {
-		bool obj_pinned = false;
-
 		raw_spin_lock_irq(&object->lock);
 #ifdef DEBUG
 		/*
@@ -1526,19 +1518,13 @@ static void kmemleak_scan(void)
 
 		/* reset the reference count (whiten the object) */
 		object->count = 0;
-		if (color_gray(object) && get_object(object)) {
+		if (color_gray(object) && get_object(object))
 			list_add_tail(&object->gray_list, &gray_list);
-			obj_pinned = true;
-		}
 
 		raw_spin_unlock_irq(&object->lock);
 
-		/*
-		 * Do a cond_resched() every 64k objects to avoid soft lockup.
-		 */
-		if (!(++loop_cnt & 0xffff) &&
-		    !kmemleak_cond_resched(object, obj_pinned))
-			loop_cnt--; /* Try again on next object */
+		if (need_resched())
+			kmemleak_cond_resched(object);
 	}
 	rcu_read_unlock();
 
@@ -1605,14 +1591,9 @@ static void kmemleak_scan(void)
 	 * scan and color them gray until the next scan.
 	 */
 	rcu_read_lock();
-	loop_cnt = 0;
 	list_for_each_entry_rcu(object, &object_list, object_list) {
-		/*
-		 * Do a cond_resched() every 64k objects to avoid soft lockup.
-		 */
-		if (!(++loop_cnt & 0xffff) &&
-		    !kmemleak_cond_resched(object, false))
-			loop_cnt--;	/* Try again on next object */
+		if (need_resched())
+			kmemleak_cond_resched(object);
 
 		/*
 		 * This is racy but we can save the overhead of lock/unlock
@@ -1647,14 +1628,9 @@ static void kmemleak_scan(void)
 	 * Scanning result reporting.
 	 */
 	rcu_read_lock();
-	loop_cnt = 0;
 	list_for_each_entry_rcu(object, &object_list, object_list) {
-		/*
-		 * Do a cond_resched() every 64k objects to avoid soft lockup.
-		 */
-		if (!(++loop_cnt & 0xffff) &&
-		    !kmemleak_cond_resched(object, false))
-			loop_cnt--;	/* Try again on next object */
+		if (need_resched())
+			kmemleak_cond_resched(object);
 
 		/*
 		 * This is racy but we can save the overhead of lock/unlock
-- 
2.31.1

[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@redhat.com>
---
 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