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 jfs_init_acl(tid_t tid, struct inode *inode, struct inode *dir)
{
	struct posix_acl *default_acl, *acl;
	int rc = 0;

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

	if (default_acl) {
		rc = __jfs_set_acl(tid, inode, ACL_TYPE_DEFAULT, default_acl);
		posix_acl_release(default_acl);
	}

	if (acl) {
		if (!rc)
			rc = __jfs_set_acl(tid, inode, ACL_TYPE_ACCESS, acl);
		posix_acl_release(acl);
	}

	JFS_IP(inode)->mode2 = (JFS_IP(inode)->mode2 & 0xffff0000) |
			       inode->i_mode;

	return rc;
}

int jfs_init_acl(tid_t tid, struct inode *inode, struct inode *dir)
{
	struct posix_acl *default_acl, *acl;
	int rc = 0;

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

	if (default_acl) {
		rc = __jfs_set_acl(tid, inode, ACL_TYPE_DEFAULT, default_acl);
		posix_acl_release(default_acl);
	}

	if (acl) {
		if (!rc)
			rc = __jfs_set_acl(tid, inode, ACL_TYPE_ACCESS, acl);
		posix_acl_release(acl);
	}

	JFS_IP(inode)->mode2 = (JFS_IP(inode)->mode2 & 0xffff0000) |
			       inode->i_mode;

	return rc;
}
