Re: WARNING in lock_release

[Al Viro <viro@ZenIV.linux.org.uk>]

On Thu, Nov 16, 2017 at 02:56:00AM -0800, syzbot wrote:
> Hello,
> 
> syzkaller hit the following crash on
> 5515cf16e270538121e4fa9283fed86c6cfd8c9c
> git://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git/master
> compiler: gcc (GCC) 7.1.1 20170620
> .config is attached
> Raw console output is attached.
> C reproducer is attached
> syzkaller reproducer is attached. See https://goo.gl/kgGztJ
> for information about syzkaller reproducers

Hmm...   That's alloc_super() buggering off on allocation failure and
hitting up_write(s->s_umount) in destroy_unused_super(), since it has
not done
        init_rwsem(&s->s_umount);
        lockdep_set_class(&s->s_umount, &type->s_umount_key);
        down_write_nested(&s->s_umount, SINGLE_DEPTH_NESTING);
part yet.  The sucker is just all-zeroes here.  The easiest way to fix
that would probably be to move that bit of initialization in the very
beginning...

diff --git a/fs/super.c b/fs/super.c
index 8ca15415351a..2808aeaf5337 100644
--- a/fs/super.c
+++ b/fs/super.c
@@ -190,6 +190,24 @@ static struct super_block *alloc_super(struct file_system_type *type, int flags,
 
 	INIT_LIST_HEAD(&s->s_mounts);
 	s->s_user_ns = get_user_ns(user_ns);
+	init_rwsem(&s->s_umount);
+	lockdep_set_class(&s->s_umount, &type->s_umount_key);
+	/*
+	 * sget() can have s_umount recursion.
+	 *
+	 * When it cannot find a suitable sb, it allocates a new
+	 * one (this one), and tries again to find a suitable old
+	 * one.
+	 *
+	 * In case that succeeds, it will acquire the s_umount
+	 * lock of the old one. Since these are clearly distrinct
+	 * locks, and this object isn't exposed yet, there's no
+	 * risk of deadlocks.
+	 *
+	 * Annotate this by putting this lock in a different
+	 * subclass.
+	 */
+	down_write_nested(&s->s_umount, SINGLE_DEPTH_NESTING);
 
 	if (security_sb_alloc(s))
 		goto fail;
@@ -217,25 +235,6 @@ static struct super_block *alloc_super(struct file_system_type *type, int flags,
 		goto fail;
 	if (list_lru_init_memcg(&s->s_inode_lru))
 		goto fail;
-
-	init_rwsem(&s->s_umount);
-	lockdep_set_class(&s->s_umount, &type->s_umount_key);
-	/*
-	 * sget() can have s_umount recursion.
-	 *
-	 * When it cannot find a suitable sb, it allocates a new
-	 * one (this one), and tries again to find a suitable old
-	 * one.
-	 *
-	 * In case that succeeds, it will acquire the s_umount
-	 * lock of the old one. Since these are clearly distrinct
-	 * locks, and this object isn't exposed yet, there's no
-	 * risk of deadlocks.
-	 *
-	 * Annotate this by putting this lock in a different
-	 * subclass.
-	 */
-	down_write_nested(&s->s_umount, SINGLE_DEPTH_NESTING);
 	s->s_count = 1;
 	atomic_set(&s->s_active, 1);
 	mutex_init(&s->s_vfs_rename_mutex);