[PATCH] ovl: fix missing negative dentry check in ovl_rename()

["Masami Ichikawa(CIP)" <masami256@gmail.com>]

From: Zheng Liang <zhengliang6@huawei.com>

From: Zheng Liang <zhengliang6@huawei.com>

commit a295aef603e109a47af355477326bd41151765b6 upstream.

The following reproducer

  mkdir lower upper work merge
  touch lower/old
  touch lower/new
  mount -t overlay overlay -olowerdir=lower,upperdir=upper,workdir=work merge
  rm merge/new
  mv merge/old merge/new & unlink upper/new

may result in this race:

PROCESS A:
  rename("merge/old", "merge/new");
  overwrite=true,ovl_lower_positive(old)=true,
  ovl_dentry_is_whiteout(new)=true -> flags |= RENAME_EXCHANGE

PROCESS B:
  unlink("upper/new");

PROCESS A:
  lookup newdentry in new_upperdir
  call vfs_rename() with negative newdentry and RENAME_EXCHANGE

Fix by adding the missing check for negative newdentry.

Signed-off-by: Zheng Liang <zhengliang6@huawei.com>
Fixes: e9be9d5e76e3 ("overlay filesystem")
Cc: <stable@vger.kernel.org> # v3.18
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
Reference: CVE-2021-20321
Signed-off-by: Masami Ichikawa(CIP) <masami.ichikawa@cybertrust.co.jp>
---
 fs/overlayfs/dir.c | 10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/fs/overlayfs/dir.c b/fs/overlayfs/dir.c
index eedacae889b9..80bf0ab52e81 100644
--- a/fs/overlayfs/dir.c
+++ b/fs/overlayfs/dir.c
@@ -824,9 +824,13 @@ static int ovl_rename2(struct inode *olddir, struct dentry *old,
 		}
 	} else {
 		new_create = true;
-		if (!d_is_negative(newdentry) &&
-		    (!new_opaque || !ovl_is_whiteout(newdentry)))
-			goto out_dput;
+		if (!d_is_negative(newdentry)) {
+			if (!new_opaque || !ovl_is_whiteout(newdentry))
+				goto out_dput;
+		} else {
+			if (flags & RENAME_EXCHANGE)
+				goto out_dput;
+		}
 	}
 
 	if (olddentry == trap)
-- 
2.33.0