posix_acl: Clear SGID bit when setting file permissions

When file permissions are modified via chmod(2) and the user is not in
the owning group or capable of CAP_FSETID, the setgid bit is cleared in
inode_change_ok().  Setting a POSIX ACL via setxattr(2) sets the file
permissions as well as the new ACL, but doesn't clear the setgid bit in
a similar way; this allows to bypass the check in chmod(2).  Fix that.

References: CVE-2016-7097
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
int hfsplus_init_posix_acl(struct inode *inode, struct inode *dir)
{
	int err = 0;
	struct posix_acl *default_acl, *acl;

	hfs_dbg(ACL_MOD,
		"[%s]: ino %lu, dir->ino %lu\n",
		__func__, inode->i_ino, dir->i_ino);

	if (S_ISLNK(inode->i_mode))
		return 0;

	err = posix_acl_create(dir, &inode->i_mode, &default_acl, &acl);
	if (err)
		return err;

	if (default_acl) {
		err = hfsplus_set_posix_acl(inode, default_acl,
					    ACL_TYPE_DEFAULT);
		posix_acl_release(default_acl);
	}

	if (acl) {
		if (!err)
			err = hfsplus_set_posix_acl(inode, acl,
						    ACL_TYPE_ACCESS);
		posix_acl_release(acl);
	}
	return err;
}

int hfsplus_init_posix_acl(struct inode *inode, struct inode *dir)
{
	int err = 0;
	struct posix_acl *default_acl, *acl;

	hfs_dbg(ACL_MOD,
		"[%s]: ino %lu, dir->ino %lu\n",
		__func__, inode->i_ino, dir->i_ino);

	if (S_ISLNK(inode->i_mode))
		return 0;

	err = posix_acl_create(dir, &inode->i_mode, &default_acl, &acl);
	if (err)
		return err;

	if (default_acl) {
		err = hfsplus_set_posix_acl(inode, default_acl,
					    ACL_TYPE_DEFAULT);
		posix_acl_release(default_acl);
	}

	if (acl) {
		if (!err)
			err = hfsplus_set_posix_acl(inode, acl,
						    ACL_TYPE_ACCESS);
		posix_acl_release(acl);
	}
	return err;
}
