关于Linux TCP "SACK PANIC" 远程拒绝服务漏洞的修复
Linux 内核被曝存在TCP “SACK PANIC” 远程拒绝服务漏洞（漏洞编号：CVE-2019-11477,CVE-2019-11478,CVE-2019-11479），攻击者可利用该漏洞远程攻击目标服务器，导致系统崩溃或无法提供服务
漏洞详情
近日，腾讯云安全中心情报平台监测到 Netflix 信息安全团队研究员Jonathan Looney发现 Linux 以及 FreeBSD 等系统内核上存在严重远程DoS漏洞，攻击者可利用该漏洞构造并发送特定的 SACK 序列请求到目标服务器导致服务器崩溃或拒绝服务。
影响版本
目前已知受影响版本如下：
- FreeBSD 12（使用到 RACK TCP 协议栈）
- CentOS 5（Redhat 官方已停止支持，不再提供补丁）
- CentOS 6
- CentOS 7
- Ubuntu 18.04 LTS
- Ubuntu 16.04 LTS
- Ubuntu 19.04
- Ubuntu 18.10
安全版本
各大Linux发行厂商已发布内核修复补丁，没漏洞的内核版本如下：
- CentOS 6 ：2.6.32-754.15.3
- CentOS 7 ：3.10.0-957.21.3
- Ubuntu 18.04 LTS：4.15.0-52.56
- Ubuntu 16.04 LTS：4.4.0-151.178
修复方法
请参照上述【安全版本】升级您的 Linux 服务器内核，参考操作如下：
CentOS
推荐方案：【CentOS 6/7 系列用户】
yum clean all && yum makecache，进行软件源更新；yum update kernel -y，更新当前内核版本;reboot，更新后重启系统生效;uname -a，检查当前版本是否为上述【安全版本】，如果是，则说明修复成功。
Ubuntu
推荐方案：【Ubuntu 16.04/18.04 LTS 系列用户】
sudo apt-get update && sudo apt-get install linux-image-generic，进行软件源更新并安装最新内核版本；sudo reboot，更新后重启系统生效；uname -a，检查当前版本是否为【安全版本】，如果是，则说明修复成功。
Temp
临时缓解方案：如用户不方便重启进行内核补丁更新，可选择如下方式禁用内核 SACK配置防范漏洞利用（可能会对网络性能产生一定影响），运行如下命令即可：
echo 'net.ipv4.tcp_sack = 0' >> /etc/sysctl.conf，禁用 SACK 配置；sysctl -p，重载配置，使其生效。
参考
- 官方通告：https://github.com/Netflix/security-bulletins/blob/master/advisories/third-party/2019-001.md
- 社区参考：https://www.openwall.com/lists/oss-security/2019/06/17/5
- 红帽公告：https://access.redhat.com/security/vulnerabilities/tcpsack
- 腾讯公告：https://cloud.tencent.com/announce/detail/622
附录
我服务器是CentOS，所以以其为示例：
# 进行软件源更新
[root@VM_0_3_centos ~]# yum clean all && yum makecache
Loaded plugins: fastestmirror, langpacks
Cleaning repos: epel extras librehat-shadowsocks os updates
Cleaning up list of fastest mirrors
Loaded plugins: fastestmirror, langpacks
Determining fastest mirrors
epel | 5.3 kB 00:00:00
extras | 3.4 kB 00:00:00
librehat-shadowsocks | 3.0 kB 00:00:00
os | 3.6 kB 00:00:00
updates | 3.4 kB 00:00:00
(1/22): epel/7/x86_64/group_gz | 88 kB 00:00:01
(2/22): epel/7/x86_64/updateinfo | 978 kB 00:00:01
(3/22): epel/7/x86_64/prestodelta | 717 B 00:00:00
(4/22): epel/7/x86_64/filelists_db | 11 MB 00:00:06
(5/22): epel/7/x86_64/primary_db | 6.8 MB 00:00:15
(6/22): epel/7/x86_64/updateinfo_zck | 1.4 MB 00:00:00
(6/22): epel/7/x86_64/updateinfo_zck | 1.4 MB 00:00:00
(6/22): epel/7/x86_64/updateinfo_zck | 1.4 MB 00:00:00
(6/22): epel/7/x86_64/updateinfo_zck | 1.4 MB 00:00:00
(6/22): epel/7/x86_64/updateinfo_zck | 1.4 MB 00:00:00
(7/22): extras/7/x86_64/prestodelta | 65 kB 00:00:01
(8/22): extras/7/x86_64/primary_db | 205 kB 00:00:01
(9/22): extras/7/x86_64/other_db | 127 kB 00:00:00
(10/22): librehat-shadowsocks/x86_64/filelists_db | 17 kB 00:00:00
(11/22): librehat-shadowsocks/x86_64/primary_db | 23 kB 00:00:00
(12/22): librehat-shadowsocks/x86_64/other_db | 10 kB 00:00:00
(13/22): os/7/x86_64/group_gz | 166 kB 00:00:00
(14/22): os/7/x86_64/primary_db | 6.0 MB 00:00:03
(15/22): os/7/x86_64/filelists_db | 7.1 MB 00:00:05
(16/22): os/7/x86_64/other_db | 2.6 MB 00:00:00
(17/22): updates/7/x86_64/prestodelta | 797 kB 00:00:00
(18/22): updates/7/x86_64/primary_db | 6.4 MB 00:00:01
(19/22): updates/7/x86_64/other_db | 641 kB 00:00:00
(20/22): updates/7/x86_64/filelists_db | 4.4 MB 00:00:05
epel/7/x86_64/other_db
http://mirrors.tencentyun.com/epel/7/x86_64/repodata/24838144a5b86bb0cd90b22255258aeed11691115ae3c35463e471cbae0f2ab9-other.sqlite.bz2: [Errno 12] Timeout on http://mirrors.tencentyun.com/epel/7/x86_64/repodata/24838144a5b86bb0cd90b22255258aeed11691115ae3c35463e471cbae0f2ab9-other.sqlite.bz2: (28, 'Operation too slow. Less than 1000 bytes/sec transferred the last 30 seconds')
Trying other mirror.
(21/22): extras/7/x86_64/filelists_db | 246 kB 00:00:26
(22/22): epel/7/x86_64/other_db | 3.2 MB 00:00:12
Metadata Cache Created
# 更新内核
[root@VM_0_3_centos ~]# yum update kernel -y
Loaded plugins: fastestmirror, langpacks
Loading mirror speeds from cached hostfile
Resolving Dependencies
--> Running transaction check
---> Package kernel.x86_64 0:3.10.0-957.21.3.el7 will be installed
--> Finished Dependency Resolution
Dependencies Resolved
=======================================================================================================================================================================
Package Arch Version Repository Size
=======================================================================================================================================================================
Installing:
kernel x86_64 3.10.0-957.21.3.el7 updates 48 M
Transaction Summary
=======================================================================================================================================================================
Install 1 Package
Total download size: 48 M
Installed size: 63 M
Downloading packages:
Delta RPMs disabled because /usr/bin/applydeltarpm not installed.
kernel-3.10.0-957.21.3.el7.x86_64.rpm | 48 MB 00:00:11
Running transaction check
Running transaction test
Transaction test succeeded
Running transaction
Installing : kernel-3.10.0-957.21.3.el7.x86_64 1/1
Verifying : kernel-3.10.0-957.21.3.el7.x86_64 1/1
Installed:
kernel.x86_64 0:3.10.0-957.21.3.el7
Complete!
# 重启
[root@VM_0_3_centos ~]# reboot
# 查看内核版本是否为最新的`3.10.0-957.21.3`
[root@VM_0_3_centos ~]# uname -a
Linux VM_0_3_centos 3.10.0-957.21.3.el7.x86_64 #1 SMP Tue Jun 18 16:35:19 UTC 2019 x86_64 x86_64 x86_64 GNU/Linux
