FAILED: patch "[PATCH] ovl: check whiteout in ovl_create_over_whiteout()" failed to apply to 4.4-stable tree

The patch below does not apply to the 4.4-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From: Miklos Szeredi <[email protected]>
Date: Wed, 31 Oct 2018 12:15:23 +0100
Subject: [PATCH] ovl: check whiteout in ovl_create_over_whiteout()

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 <[email protected]>

index ce1857fb5434..c6289147c787 100644
@@ -462,6 +462,10 @@ static int ovl_create_over_whiteout(struct dentry *dentry, struct inode *inode,
 	if (IS_ERR(upper))
 		goto out_unlock;
 
+	err = -ESTALE;
+	if (d_is_negative(upper) || !IS_WHITEOUT(d_inode(upper)))
+		goto out_dput;
+
 	newdentry = ovl_create_temp(workdir, cattr);
 	err = PTR_ERR(newdentry);
 	if (IS_ERR(newdentry))