Your uname -rv reveals your current kernel as:

3.2.0-87-generic #125-Ubuntu SMP Fri Jun 19 08:25:10 UTC 2015

The date is June 19, 2015 – over one year old!

According to this: What is dirty COW bug that allows local user to gain administrative access? you should be using 3.2.0-113 I’m not sure how many kernel upgrades you’ve missed, but probably lots over 16 months (subtracting 87 from 113 = 26).

Since everyone and their dog in the Linux world has heard about Dirty COW by now you should put version 3.2.0-113 in ASAP. Until you find out why your Kernel hasn’t been upgrading you can install the kernel manually. You didn’t use uname -a so it’s unknown if you are on 32-bit or 64-bit but I’ll give the instructions for both.

Install Dirty COW fix for Ubuntu 32-Bit Generic

Go to: precise-updates/i386/linux-image-3.2.0-113 and select the link for your country to download the file. Your browser may give you a warning message. In Chrome click Keep.

The file is usually downloaded to your ~/Downloads directory but your browser can be user-configured for any location. We’ll assume it’s ~/Downloads so type the following:

cd ~/Downloads
sudo dpkg -i linux-image-3.2.0-113-generic_3.2.0-113.155_i386.deb
rm linux-image-3.2.0-113-generic_3.2.0-113.155_i386.deb
sudo reboot

Install Dirty COW fix for Ubuntu 64-Bit Generic

Go to: precise-updates/amd64/linux-image-3.2.0-113 and select the link for your country to download the file. Your browser may give you a warning message. In Chrome click Keep.

The file is usually downloaded to your ~/Downloads directory but your browser can be user-configured for any location. We’ll assume it’s ~/Downloads so type the following:

cd ~/Downloads
sudo dpkg -i linux-image-3.2.0-113-generic_3.2.0-113.155_amd64.deb
rm linux-image-3.2.0-113-generic_3.2.0-113.155_amd64.deb
sudo reboot

After rebooting type uname -a to confirm you are on Kernel version 3.2.0-113.