Patch "writeback, cgroup: fix premature wb_put() in locked_inode_to_wb_and_lock_list()" has been added to the 4.5-stable tree

This is a note to let you know that I've just added the patch titled

writeback, cgroup: fix premature wb_put() in locked_inode_to_wb_and_lock_list()

to the 4.5-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
writeback-cgroup-fix-premature-wb_put-in-locked_inode_to_wb_and_lock_list.patch
and it can be found in the queue-4.5 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Tejun Heo <[email protected]>
Date: Fri, 18 Mar 2016 13:50:03 -0400
Subject: writeback, cgroup: fix premature wb_put() in locked_inode_to_wb_and_lock_list()

From: Tejun Heo <[email protected]>

commit 614a4e3773148a31f58dc174bbf578ceb63510c2 upstream.

locked_inode_to_wb_and_lock_list() wb_get()'s the wb associated with the target inode, unlocks inode, locks the wb's list_lock and verifies that the inode is still associated with the wb. To prevent the wb going away between dropping inode lock and acquiring list_lock, the wb is pinned while inode lock is held. The wb reference is put right after acquiring list_lock citing that the wb won't be dereferenced anymore.

This isn't true. If the inode is still associated with the wb, the inode has reference and it's safe to return the wb; however, if inode has been switched, the wb still needs to be unlocked which is a dereference and can lead to use-after-free if it it races with wb destruction.

Fix it by putting the reference after releasing list_lock.

---
 fs/fs-writeback.c |    8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

@@ -281,13 +281,15 @@ locked_inode_to_wb_and_lock_list(struct
 		wb_get(wb);
 		spin_unlock(&inode->i_lock);
 		spin_lock(&wb->list_lock);
-		wb_put(wb);		/* not gonna deref it anymore */
 
 		/* i_wb may have changed inbetween, can't use inode_to_wb() */
-		if (likely(wb == inode->i_wb))
-			return wb;	/* @inode already has ref */
+		if (likely(wb == inode->i_wb)) {
+			wb_put(wb);	/* @inode already has ref */
+			return wb;
+		}
 
 		spin_unlock(&wb->list_lock);
+		wb_put(wb);
 		cpu_relax();
 		spin_lock(&inode->i_lock);
 	}

Patches currently in stable-queue which might be from [email protected] are

queue-4.5/writeback-cgroup-fix-premature-wb_put-in-locked_inode_to_wb_and_lock_list.patch queue-4.5/writeback-cgroup-fix-use-of-the-wrong-bdi_writeback-which-mismatches-the-inode.patch queue-4.5/cgroup-ignore-css_sets-associated-with-dead-cgroups-during-migration.patch