ovl: fix sgid on directory

When creating directory in workdir, the group/sgid inheritance from the
parent dir was omitted completely.  Fix this by calling inode_init_owner()
on overlay inode and using the resulting uid/gid/mode to create the file.

Unfortunately the sgid bit can be stripped off due to umask, so need to
reset the mode in this case in workdir before moving the directory in
place.
Reported-by: Eryu Guan <eguan@redhat.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

fs/overlayfs/dir.c

@@ -357,6 +357,21 @@ static int ovl_create_over_whiteout(struct dentry *dentry, struct inode *inode,
 	if (err)
 		goto out_dput2;
 
+	/*
+	 * mode could have been mutilated due to umask (e.g. sgid directory)
+	 */
+	if (!S_ISLNK(stat->mode) && newdentry->d_inode->i_mode != stat->mode) {
+		struct iattr attr = {
+			.ia_valid = ATTR_MODE,
+			.ia_mode = stat->mode,
+		};
+		inode_lock(newdentry->d_inode);
+		err = notify_change(newdentry, &attr, NULL);
+		inode_unlock(newdentry->d_inode);
+		if (err)
+			goto out_cleanup;
+	}
+
 	if (S_ISDIR(stat->mode)) {
 		err = ovl_set_opaque(newdentry);
 		if (err)
@@ -397,7 +412,6 @@ static int ovl_create_or_link(struct dentry *dentry, int mode, dev_t rdev,
 	const struct cred *old_cred;
 	struct cred *override_cred;
 	struct kstat stat = {
-		.mode = mode,
 		.rdev = rdev,
 	};
 
@@ -410,12 +424,15 @@ static int ovl_create_or_link(struct dentry *dentry, int mode, dev_t rdev,
 	if (err)
 		goto out_iput;
 
+	inode_init_owner(inode, dentry->d_parent->d_inode, mode);
+	stat.mode = inode->i_mode;
+
 	old_cred = ovl_override_creds(dentry->d_sb);
 	err = -ENOMEM;
 	override_cred = prepare_creds();
 	if (override_cred) {
-		override_cred->fsuid = old_cred->fsuid;
-		override_cred->fsgid = old_cred->fsgid;
+		override_cred->fsuid = inode->i_uid;
+		override_cred->fsgid = inode->i_gid;
 		put_cred(override_creds(override_cred));
 		put_cred(override_cred);
 
@@ -427,8 +444,14 @@ static int ovl_create_or_link(struct dentry *dentry, int mode, dev_t rdev,
 							link, hardlink);
 	}
 	revert_creds(old_cred);
-	if (!err)
+	if (!err) {
+		struct inode *realinode = d_inode(ovl_dentry_upper(dentry));
+
+		WARN_ON(inode->i_mode != realinode->i_mode);
+		WARN_ON(!uid_eq(inode->i_uid, realinode->i_uid));
+		WARN_ON(!gid_eq(inode->i_gid, realinode->i_gid));
 		inode = NULL;
+	}
 out_iput:
 	iput(inode);
 out: