ovl: fix oops in ovl_indexdir_cleanup() with nfs_export=on

Mounting with nfs_export=on, xfstests overlay/031 triggers a kernel panic
since v5.8-rc1 overlayfs updates.

 overlayfs: orphan index entry (index/00fb1..., ftype=4000, nlink=2)
 BUG: kernel NULL pointer dereference, address: 0000000000000030
 RIP: 0010:ovl_cleanup_and_whiteout+0x28/0x220 [overlay]

Bisect point at commit c21c839b ("ovl: whiteout inode sharing")

Minimal reproducer:
--------------------------------------------------
rm -rf l u w m
mkdir -p l u w m
mkdir -p l/testdir
touch l/testdir/testfile
mount -t overlay -o lowerdir=l,upperdir=u,workdir=w,nfs_export=on overlay m
echo 1 > m/testdir/testfile
umount m
rm -rf u/testdir
mount -t overlay -o lowerdir=l,upperdir=u,workdir=w,nfs_export=on overlay m
umount m
--------------------------------------------------

When mount with nfs_export=on, and fail to verify an orphan index, we're
cleaning this index from indexdir by calling ovl_cleanup_and_whiteout().
This dereferences ofs->workdir, that was earlier set to NULL.

The design was that ovl->workdir will point at ovl->indexdir, but we are
assigning ofs->indexdir to ofs->workdir only after ovl_indexdir_cleanup().
There is no reason not to do it sooner, because once we get success from
ofs->indexdir = ovl_workdir_create(... there is no turning back.
Reported-and-tested-by: Murphy Zhou <jencce.kernel@gmail.com>
Fixes: c21c839b ("ovl: whiteout inode sharing")
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

fs/overlayfs/super.c

@@ -1354,6 +1354,12 @@ static int ovl_get_indexdir(struct super_block *sb, struct ovl_fs *ofs,
 
 	ofs->indexdir = ovl_workdir_create(ofs, OVL_INDEXDIR_NAME, true);
 	if (ofs->indexdir) {
+		/* index dir will act also as workdir */
+		iput(ofs->workdir_trap);
+		ofs->workdir_trap = NULL;
+		dput(ofs->workdir);
+		ofs->workdir = dget(ofs->indexdir);
+
 		err = ovl_setup_trap(sb, ofs->indexdir, &ofs->indexdir_trap,
 				     "indexdir");
 		if (err)
@@ -1852,20 +1858,12 @@ static int ovl_fill_super(struct super_block *sb, void *data, int silent)
 		sb->s_flags |= SB_RDONLY;
 
 	if (!(ovl_force_readonly(ofs)) && ofs->config.index) {
-		/* index dir will act also as workdir */
-		dput(ofs->workdir);
-		ofs->workdir = NULL;
-		iput(ofs->workdir_trap);
-		ofs->workdir_trap = NULL;
-
 		err = ovl_get_indexdir(sb, ofs, oe, &upperpath);
 		if (err)
 			goto out_free_oe;
 
 		/* Force r/o mount with no index dir */
-		if (ofs->indexdir)
-			ofs->workdir = dget(ofs->indexdir);
-		else
+		if (!ofs->indexdir)
 			sb->s_flags |= SB_RDONLY;
 	}