How To Patch Running Linux Kernel Source Tree

Posted on in Categories Howto, Linux, Linux distribution, RedHat/Fedora Linux, Security, Sys admin, Tips last updated February 14, 2008

Yesterday, I wrote about a serious Linux kernel bug and fix. However, few readers like to know about patching running Linux kernel. Patching production kernel is a risky business. Following procedure will help you to fix the problem.

Step # 1: Make sure your product is affected

First find out if your product is affected by reported exploit. For example, vmsplice() but only affects RHEL 5.x but RHEL 4.x,3.x, and 2.1.x are not affected at all. You can always obtain this information by visiting vendors bug reporting system called bugzilla. Also make sure bug affects your architectures. For example, a bug may only affect 64 bit or 32 bit platform.

Step # 2: Apply patch

You better apply and test patch in a test environment. Please note that some vendors such as Redhat and Suse modifies or backports kernel. So it is good idea to apply patch to their kernel source code tree. Otherwise you can always grab and apply patch to latest kernel version.

Step # 3: How do I apply kernel patch?

WARNING! These instructions require having the skills of a sysadmin. Personally, I avoid recompiling any kernel unless absolutely necessary. Most our production boxes (over 1400+) are powered by mix of RHEL 4 and 5. Wrong kernel option can disable hardware or may not boot system at all. If you don’t understand the internal kernel dependencies don’t try this on a production box.

Change directory to your kernel source code:
# cd linux-2.6.xx.yy
Download and save patch file as fix.vmsplice.exploit.patch:
# cat fix.vmsplice.exploit.patch
Output:

--- a/fs/splice.c
+++ b/fs/splice.c
@@ -1234,7 +1234,7 @@ static int get_iovec_page_array(const struct iovec __user *iov,
                if (unlikely(!len))
                        break;
                error = -EFAULT;
-               if (unlikely(!base))
+               if (!access_ok(VERIFY_READ, base, len))
                        break;
 
                /*

Now apply patch using patch command, enter:
# patch < fix.vmsplice.exploit.patch -p1
Now recompile and install Linux kernel.

I hope this quick and dirty guide will save someones time. On a related note Erek has unofficial patched RPMs for CentOS / RHEL distros.

Howto: Build Linux Kernel Module Against Installed Kernel w/o Full Kernel Source Tree

Posted on in Categories CentOS, Debian Linux, FAQ, File system, GNU/Open source, Howto, kernel, Linux, Linux distribution, Open source coding, RedHat/Fedora Linux, Tips, Ubuntu Linux last updated November 4, 2008

Recently I received a question via email:

How do I build Linux kernel module against installed or running Linux kernel? Do I need to install new kernel source tree from kernel.org?

How to: Compile Linux kernel 2.6

Posted on in Categories CentOS, Debian Linux, GNU/Open source, Howto, Linux last updated May 5, 2012

Compiling custom kernel has its own advantages and disadvantages. However, new Linux user / admin find it difficult to compile Linux kernel. Compiling kernel needs to understand few things and then just type couple of commands. This step by step howto covers compiling Linux kernel version 2.6.xx under Debian GNU Linux. However, instructions remains the same for any other distribution except for apt-get command.

Step # 1 Get Latest Linux kernel code

Visit http://kernel.org/ and download the latest source code. File name would be linux-x.y.z.tar.bz2, where x.y.z is actual version number. For example file inux-2.6.25.tar.bz2 represents 2.6.25 kernel version. Use wget command to download kernel source code:
$ cd /tmp
$ wget http://www.kernel.org/pub/linux/kernel/v2.6/linux-x.y.z.tar.bz2

Note: Replace x.y.z with actual version number.

Step # 2 Extract tar (.tar.bz3) file

Type the following command:
# tar -xjvf linux-2.6.25.tar.bz2 -C /usr/src
# cd /usr/src

Step # 3 Configure kernel

Before you configure kernel make sure you have development tools (gcc compilers and related tools) are installed on your system. If gcc compiler and tools are not installed then use apt-get command under Debian Linux to install development tools.
# apt-get install gcc

Now you can start kernel configuration by typing any one of the command:

  • $ make menuconfig – Text based color menus, radiolists & dialogs. This option also useful on remote server if you wanna compile kernel remotely.
  • $ make xconfig – X windows (Qt) based configuration tool, works best under KDE desktop
  • $ make gconfig – X windows (Gtk) based configuration tool, works best under Gnome Dekstop.

For example make menuconfig command launches following screen:
$ make menuconfig

You have to select different options as per your need. Each configuration option has HELP button associated with it so select help button to get help.

Step # 4 Compile kernel

Start compiling to create a compressed kernel image, enter:
$ make
Start compiling to kernel modules:
$ make modules

Install kernel modules (become a root user, use su command):
$ su -
# make modules_install

Step # 5 Install kernel

So far we have compiled kernel and installed kernel modules. It is time to install kernel itself.
# make install

It will install three files into /boot directory as well as modification to your kernel grub configuration file:

  • System.map-2.6.25
  • config-2.6.25
  • vmlinuz-2.6.25

Step # 6: Create an initrd image

Type the following command at a shell prompt:
# cd /boot
# mkinitrd -o initrd.img-2.6.25 2.6.25

initrd images contains device driver which needed to load rest of the operating system later on. Not all computer requires initrd, but it is safe to create one.

Step # 7 Modify Grub configuration file – /boot/grub/menu.lst

Open file using vi:
# vi /boot/grub/menu.lst

title           Debian GNU/Linux, kernel 2.6.25 Default
root            (hd0,0)
kernel          /boot/vmlinuz root=/dev/hdb1 ro
initrd          /boot/initrd.img-2.6.25
savedefault
boot

Remember to setup correct root=/dev/hdXX device. Save and close the file. If you think editing and writing all lines by hand is too much for you, try out update-grub command to update the lines for each kernel in /boot/grub/menu.lst file. Just type the command:
# update-grub
Neat. Huh?

Step # 8 : Reboot computer and boot into your new kernel

Just issue reboot command:
# reboot
For more information see:

  • Our Exploring Linux kernel article and Compiling Linux Kernel module only.
  • Official README file has more information on kernel and software requirement to compile it. This file is kernel source directory tree.
  • Documentation/ directory has interesting kernel documentation for you in kernel source tree.