mm/kmemleak: simplify kmemleak_cond_resched() usage

Patch series "mm/kmemleak: Simplify kmemleak_cond_resched() & fix UAF", v2.

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.

This patch (of 2):

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.

Link: https://lkml.kernel.org/r/20230119040111.350923-1-longman@redhat.com
Link: https://lkml.kernel.org/r/20230119040111.350923-2-longman@redhat.com
Signed-off-by: Waiman Long <longman@redhat.com>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Cc: Muchun Song <songmuchun@bytedance.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/mm/kmemleak.c b/mm/kmemleak.c
index 55dc8b8..69327b7 100644 (file)
--- a/mm/kmemleak.c
+++ b/mm/kmemleak.c
@@ -1472,22 +1472,17 @@ 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);
 }
 
 /*
@@ -1501,15 +1496,12 @@ 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
                /*
@@ -1535,19 +1527,13 @@ 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();
 
@@ -1614,14 +1600,9 @@ 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
@@ -1656,14 +1637,9 @@ 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