linux-kernel - [PATCH v7 10/10] lib/dlock-list: Fix use-after-unlock problem in dlist_for_each_entry

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Date:   Fri, 27 Oct 2017 16:10:53 -0400
From:   Waiman Long <longman@...hat.com>
To:     Alexander Viro <viro@...iv.linux.org.uk>, Jan Kara <jack@...e.com>,
        Jeff Layton <jlayton@...chiereds.net>,
        "J. Bruce Fields" <bfields@...ldses.org>,
        Tejun Heo <tj@...nel.org>,
        Christoph Lameter <cl@...ux-foundation.org>
Cc:     linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Andi Kleen <andi@...stfloor.org>,
        Dave Chinner <dchinner@...hat.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Davidlohr Bueso <dave@...olabs.net>,
        Waiman Long <longman@...hat.com>
Subject: [PATCH v7 10/10] lib/dlock-list: Fix use-after-unlock problem in dlist_for_each_entry_safe()

The dlist_for_each_entry_safe() macro in include/linux/dlock-list has
a use-after-unlock problem where racing condition can happen because
of a lack of spinlock protection.  Fortunately, this macro is not
currently being used in the kernel.

This patch changes the dlist_for_each_entry_safe() macro so that the
call to __dlock_list_next_list() is deferred until the next entry is
being used. That should eliminate the use-after-unlock problem.

Reported-by: Boqun Feng <boqun.feng@...il.com>
Signed-off-by: Waiman Long <longman@...hat.com>
---
 include/linux/dlock-list.h | 28 +++++++++++++++++-----------
 1 file changed, 17 insertions(+), 11 deletions(-)

diff --git a/include/linux/dlock-list.h b/include/linux/dlock-list.h
index 02c5f4d..f4b7657 100644
--- a/include/linux/dlock-list.h
+++ b/include/linux/dlock-list.h
@@ -191,17 +191,17 @@ extern void dlock_list_add(struct dlock_list_node *node,
 }
 
 /**
- * dlock_list_first_entry - get the first element from a list
+ * dlock_list_next_list_entry - get first element from next list in iterator
  * @iter  : The dlock list iterator.
- * @type  : The type of the struct this is embedded in.
+ * @pos   : A variable of the struct that is embedded in.
  * @member: The name of the dlock_list_node within the struct.
- * Return : Pointer to the next entry or NULL if all the entries are iterated.
+ * Return : Pointer to first entry or NULL if all the lists are iterated.
  */
-#define dlock_list_first_entry(iter, type, member)			\
+#define dlock_list_next_list_entry(iter, pos, member)			\
 	({								\
 		struct dlock_list_node *_n;				\
 		_n = __dlock_list_next_entry(NULL, iter);		\
-		_n ? list_entry(_n, type, member) : NULL;		\
+		_n ? list_entry(_n, typeof(*pos), member) : NULL;	\
 	})
 
 /**
@@ -231,7 +231,7 @@ extern void dlock_list_add(struct dlock_list_node *node,
  * This iteration function is designed to be used in a while loop.
  */
 #define dlist_for_each_entry(pos, iter, member)				\
-	for (pos = dlock_list_first_entry(iter, typeof(*(pos)), member);\
+	for (pos = dlock_list_next_list_entry(iter, pos, member);	\
 	     pos != NULL;						\
 	     pos = dlock_list_next_entry(pos, iter, member))
 
@@ -245,14 +245,20 @@ extern void dlock_list_add(struct dlock_list_node *node,
  * This iteration macro is safe with respect to list entry removal.
  * However, it cannot correctly iterate newly added entries right after the
  * current one.
+ *
+ * The call to __dlock_list_next_list() is deferred until the next entry
+ * is being iterated to avoid use-after-unlock problem.
  */
 #define dlist_for_each_entry_safe(pos, n, iter, member)			\
-	for (pos = dlock_list_first_entry(iter, typeof(*(pos)), member);\
+	for (pos = NULL;						\
 	    ({								\
-		bool _b = (pos != NULL);				\
-		if (_b)							\
-			n = dlock_list_next_entry(pos, iter, member);	\
-		_b;							\
+		if (!pos ||						\
+		   (&(pos)->member.list == &(iter)->entry->list))	\
+			pos = dlock_list_next_list_entry(iter, pos,	\
+							 member);	\
+		if (pos)						\
+			n = list_next_entry(pos, member.list);		\
+		pos;							\
 	    });								\
 	    pos = n)
 
-- 
1.8.3.1