[PATCH 4.14 121/124] ovl: check whiteout in ovl_create_over_whiteout()

Greg Kroah-Hartman
2018-11-19T16:52:24Z

From: Miklos Szeredi <mszeredi@redhat.com>

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")
Cc: <stable@vger.kernel.org> # v3.18
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
fs/overlayfs/dir.c | 4 ++++
1 file changed, 4 insertions(+)

--- a/fs/overlayfs/dir.c
+++ b/fs/overlayfs/dir.c
@@ -392,6 +392,10 @@ static int ovl_create_over_whiteout(stru
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;