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03/06/2019

How To Patch and Protect Linux Kernel Zero Day Local Privilege Escalation Vulnerability CVE-2016-5195 [ 21/Oct/2016 ]



A very serious security problem has been found in the Linux kernel. A 0-day local privilege escalation vulnerability has existed for eleven years since 2005. This bug affects all sort of of Android or Linux kernel to escalate privileges. Any user can become root in less than 5 seconds. The bug has existed since Linux kernel version 2.6.22+. How do I fix this problem?


This bug is named as Dirty COW (CVE-2016-5195) is a privilege escalation vulnerability in the Linux Kernel. Exploitation of this bug does not leave any trace of anything abnormal happening to the logs. So you can not detect if someone has exploited this against your server.

What is CVE-2016-5195 bug?

From the project:

A race condition was found in the way the Linux kernel’s memory subsystem handled the copy-on-write (COW) breakage of private read-only memory mappings. An unprivileged local user could use this flaw to gain write access to otherwise read-only memory mappings and thus increase their privileges on the system.

A nasty bug for sure. Any local users can write to any file they can read, and present since at least Linux kernel version 2.6.22. Linus Torvalds explained:

This is an ancient bug that was actually attempted to be fixed once (badly) by me eleven years ago in commit 4ceb5db9757a (“Fix get_user_pages() race for write access”) but that was then undone due to problems on s390 by commit f33ea7f404e5 (“fix get_user_pages bug”).

In the meantime, the s390 situation has long been fixed, and we can now fix it by checking the pte_dirty() bit properly (and do it better). The s390 dirty bit was implemented in abf09bed3cce (“s390/mm: implement software dirty bits”) which made it into v3.9. Earlier kernels will have to look at the page state itself.

Also, the VM has become more scalable, and what used a purely theoretical race back then has become easier to trigger.

To fix it, we introduce a new internal FOLL_COW flag to mark the “yes, we already did a COW” rather than play racy games with FOLL_WRITE that is very fundamental, and then use the pte dirty flag to validate that the FOLL_COW flag is still valid.

A list of affected Linux distros (including VMs and containers that share the same kernel)

  1. Red Hat Enterprise Linux 7.x
  2. Red Hat Enterprise Linux 6.x
  3. Red Hat Enterprise Linux 5.x
  4. CentOS Linux 7.x
  5. CentOS Linux 6.x
  6. CentOS Linux 5.x
  7. Debian Linux wheezy
  8. Debian Linux jessie
  9. Debian Linux stretch
  10. Debian Linux sid
  11. Ubuntu Linux precise (LTS 12.04)
  12. Ubuntu Linux trusty
  13. Ubuntu Linux xenial (LTS 16.04)
  14. Ubuntu Linux yakkety
  15. Ubuntu Linux vivid/ubuntu-core
  16. SUSE Linux Enterprise 11 and 12.
  17. Openwrt

How do I fix CVE-2016-5195 on Linux?

Type the commands as per your Linux distro. You need to reboot the box. Before you apply patch, note down your current kernel version:
$ uname -a
$ uname -mrs

Sample outputs:

Linux 3.13.0-95-generic x86_64

Debian or Ubuntu Linux

$ sudo apt-get update && sudo apt-get upgrade && sudo apt-get dist-upgrade
Reboot the server:
$ sudo reboot

Related: Ubuntu Linux users can hotfix this Linux kernel bug without rebooting the server.

RHEL / CentOS Linux 5.x/6.x/7.x

$ sudo yum update
$ sudo reboot

RHEL / CentOS Linux 4.x

$ sudo up2date -u
$ sudo reboot

Suse Enterprise Linux or Opensuse Linux

To apply all needed patches to the system type:
# zypper patch
# reboot

Verification

You need to make sure your version number has changed:
$ uname -a
$ uname -r
$ uname -mrs

Determine if your system is vulnerable

For RHEL/CentOS Linux, use the following script:
$ wget https://access.redhat.com/sites/default/files/rh-cve-2016-5195_2.sh
$ bash rh-cve-2016-5195_2.sh

For all other distro try PoC (proof of concept exploit code)

Grab the PoC:
$ wget https://raw.githubusercontent.com/dirtycow/dirtycow.github.io/master/dirtyc0w.c
Run it as follows. First be root:
$ sudo -s
# echo this is not a test > foo

Run it as normal user:
$ gcc -lpthread dirtyc0w.c -o dirtyc0w
### ***[ If you get an error while compiling code, try ***] ###
$ gcc -pthread dirtyc0w.c -o dirtyc0w
$ ./dirtyc0w foo m00000000000000000
mmap 56123000
madvise 0
procselfmem 1800000000

$ cat foo
m00000000000000000

References:

Posted by: SXI ADMIN

The author is the creator of nixCraft and a seasoned sysadmin, DevOps engineer, and a trainer for the Linux operating system/Unix shell scripting. Get the latest tutorials on SysAdmin, Linux/Unix and open source topics via RSS/XML feed or weekly email newsletter.

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