From: Miklos Szeredi <mszeredi@redhat.com>
Date: Wed, 31 Oct 2018 12:15:23 +0100
Subject: [4.14.y PATCH] ovl: check whiteout in ovl_create_over_whiteout()

commit 5e1275808630ea3b2c97c776f40e475017535f72 upstream.

Kaixuxia repors that it's possible to crash overlayfs by removing the
whiteout on the upper layer before creating a directory over it.  This is a
reproducer:

 mkdir lower upper work merge
 touch lower/file
 mount -t overlay overlay -olowerdir=lower,upperdir=upper,workdir=work merge
 rm merge/file
 ls -al merge/file
 rm upper/file
 ls -al merge/
 mkdir merge/file

Before commencing with a vfs_rename(..., RENAME_EXCHANGE) verify that the
lookup of "upper" is positive and is a whiteout, and return ESTALE
otherwise.

Reported by: kaixuxia <xiakaixu1987@gmail.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
Fixes: e9be9d5e76e3 ("overlay filesystem")
---
 fs/overlayfs/dir.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/fs/overlayfs/dir.c b/fs/overlayfs/dir.c
index cc961a3bd3bd..dff146041424 100644
--- a/fs/overlayfs/dir.c
+++ b/fs/overlayfs/dir.c
@@ -392,6 +392,10 @@ static int ovl_create_over_whiteout(struct dentry *dentry, struct inode *inode,
 	if (IS_ERR(upper))
 		goto out_dput;
 
+	err = -ESTALE;
+	if (d_is_negative(upper) || !IS_WHITEOUT(d_inode(upper)))
+		goto out_dput2;
+
 	err = ovl_create_real(wdir, newdentry, cattr, hardlink, true);
 	if (err)
 		goto out_dput2;
-- 
2.14.5

