From 76179b4dae212143faa023b71e44554c2854c2c0 Mon Sep 17 00:00:00 2001
From: Zheng Liang <zhengliang6@huawei.com>
Date: Fri, 24 Sep 2021 09:16:27 +0800
Subject: [PATCH] ovl: fix missing negative dentry check in ovl_rename()
Git-commit: a295aef603e109a47af355477326bd41151765b6
Patch-mainline: v5.15-rc5
References: CVE-2021-20321 bsc#1191647

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>
Acked-by: David Disseldorp <ddiss@suse.de>
---
 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 6509ec3cb373..3284363ebdfb 100644
--- a/fs/overlayfs/dir.c
+++ b/fs/overlayfs/dir.c
@@ -1160,9 +1160,13 @@ static int ovl_rename(struct inode *olddir, struct dentry *old,
 				goto out_dput;
 		}
 	} else {
-		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)
-- 
2.34.1