[PATCH 2/2] configfs: unhash the dentry before dropping the item in rmdir

configfs_get_config_item() treats a hashed dentry as proof that
sd->s_element is a live config_item.  configfs_rmdir() breaks that:
simple_rmdir() leaves the dentry hashed, the last reference to the item is
dropped right after, and the dentry is only unhashed by d_delete() once
->rmdir() has returned.  configfs_symlink() resolves its target holding no
lock on it, so get_target() can land in that window:

  BUG: KASAN: slab-use-after-free in config_item_get+0x26/0x90
   get_target fs/configfs/symlink.c:128 [inline]
   configfs_symlink+0x4ab/0x1030 fs/configfs/symlink.c:185

Unhash in configfs_remove_dir(), while the item is still guaranteed to be
there.  A reference obtained just before that stays harmless, as
create_link() rechecks CONFIGFS_USET_DROPPING, already set by
configfs_detach_prep().  Both configfs_unregister_subsystem() paths
d_drop() after detaching, so this only makes rmdir match them.

Reported-by: [email protected]
Closes: https://syzkaller.appspot.com/bug?extid=6b16e3d085833cbf3e25
Fixes: 7063fbf22611 ("[PATCH] configfs: User-driven configuration filesystem")
Cc: [email protected]
Signed-off-by: Vasileios Almpanis <[email protected]>
---
 fs/configfs/dir.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/fs/configfs/dir.c b/fs/configfs/dir.c
index 3c88f13f1ca2..eda80c2a2d38 100644
--- a/fs/configfs/dir.c
+++ b/fs/configfs/dir.c
@@ -416,6 +416,15 @@ static void configfs_remove_dir(struct dentry *d)
 	if (d_really_is_positive(d)) {
 		if (unlikely(simple_rmdir(d_inode(parent), d)))
 			pr_warn("remove_dir (%pd): attributes remain", d);
+		else
+			/*
+			 * configfs_get_config_item() takes a hashed dentry as
+			 * proof that ->s_element is still alive.  Our caller
+			 * is about to drop the last reference to the item and
+			 * the VFS will not unhash until after we return, so
+			 * unhash it here.
+			 */
+			d_drop(d);
 	}
 
 	pr_debug(" o %pd removing done (%d)\n", d, d_count(d));
-- 
2.47.3