kpatch: dynamic kernel patching

kpatch is a Linux dynamic kernel patching infrastructure which allows you to patch a running kernel without rebooting or restarting any processes. It enables sysadmins to apply critical security patches to the kernel immediately, without having to wait for long-running tasks to complete, for users to log off, or for scheduled reboot windows. It gives more control over uptime without sacrificing security or stability.

WARNING: Use with caution! Kernel crashes, spontaneous reboots, and data loss may occur!

Here's a video of kpatch in action:

And a few more:

• https://www.youtube.com/watch?v=rN0sFjrJQfU
• https://www.youtube.com/watch?v=Mftc80KyjA4

Table of contents

• Supported Architectures
• Installation
  • Prerequisites
    • Fedora, RHEL, CentOS
    • Oracle Linux 7
    • Ubuntu
    • Debian 9 (Stretch)
    • Debian 8 (Jessie)
    • Debian 7 (Lenny)
    • Gentoo
  • Build
  • Install
• Quick start
• Patch Author Guide
• How it works
  • kpatch-build
  • Patching
• Limitations
• Frequently Asked Questions
• Get involved
• License

Supported Architectures

• x86-64
• ppc64le
• arm64
• s390

Installation

Prerequisites

Before starting, see Supported Architectures and check if your device's architecture is supported.

Fedora, RHEL, CentOS

NOTE: You'll need about 15GB of free disk space for the kpatch-build cache in ~/.kpatch and for ccache.

Install the dependencies for compiling kpatch and running kpatch-build:

source test/integration/lib.sh
# Will request root privileges
kpatch_dependencies

Oracle Linux 7

NOTE: You'll need about 15GB of free disk space for the kpatch-build cache in ~/.kpatch and for ccache.

Install the dependencies for compiling kpatch:

UNAME=$(uname -r)
sudo yum install gcc kernel-devel-${UNAME%.*} elfutils elfutils-devel

Install the dependencies for the "kpatch-build" command:

sudo yum install pesign yum-utils zlib-devel \
  binutils-devel newt-devel python-devel perl-ExtUtils-Embed \
  audit-libs numactl-devel pciutils-devel bison patchutils

# enable ol7_optional_latest repo
sudo yum-config-manager --enable ol7_optional_latest

sudo yum-builddep kernel-${UNAME%.*}

# manually install kernel debuginfo packages
rpm -ivh https://oss.oracle.com/ol7/debuginfo/kernel-debuginfo-$(uname -r).rpm
rpm -ivh https://oss.oracle.com/ol7/debuginfo/kernel-debuginfo-common-x86_64-$(uname -r).rpm

# optional, but highly recommended - enable EPEL 7
sudo yum install ccache
ccache --max-size=5G

Ubuntu

NOTE: You'll need about 15GB of free disk space for the kpatch-build cache in ~/.kpatch and for ccache.

Install the dependencies for compiling kpatch and running kpatch-build

source test/integration/lib.sh
# required on ppc64le
# e.g., on Ubuntu 18.04 for gcc-7.3
apt-get install gcc-7-plugin-dev
# Will request root privileges
kpatch_dependencies

Debian 9 (Stretch)

Since Stretch the stock kernel can be used without changes, however the version of kpatch in Stretch is too old so you still need to build it manually. Follow the instructions for Debian Jessie (next section) but skip building a custom kernel/rebooting.

Debian 8 (Jessie)

NOTE: You'll need about 15GB of free disk space for the kpatch-build cache in ~/.kpatch and for ccache.

Install the dependencies for compiling kpatch:

apt-get install make gcc libelf-dev build-essential

Install and prepare the kernel sources:

apt-get install linux-source-$(uname -r)
cd /usr/src && tar xvf linux-source-$(uname -r).tar.xz && ln -s linux-source-$(uname -r) linux && cd linux
cp /boot/config-$(uname -r) .config
for OPTION in CONFIG_KALLSYMS_ALL CONFIG_FUNCTION_TRACER ; do sed -i "s/# $OPTION is not set/$OPTION=y/g" .config ; done
sed -i "s/^SUBLEVEL.*/SUBLEVEL =/" Makefile
make -j`getconf _NPROCESSORS_CONF` deb-pkg KDEB_PKGVERSION=$(uname -r).9-1

Install the kernel packages and reboot

dpkg -i /usr/src/*.deb
reboot

Install the dependencies for the "kpatch-build" command:

apt-get install dpkg-dev
apt-get build-dep linux

# required on ppc64le
# e.g., on stretch for gcc-6.3
apt-get install gcc-6-plugin-dev

# optional, but highly recommended
apt-get install ccache
ccache --max-size=5G

Debian 7 (Lenny)

NOTE: You'll need about 15GB of free disk space for the kpatch-build cache in ~/.kpatch and for ccache.

Add backports repositories:

echo "deb http://http.debian.net/debian wheezy-backports main" > /etc/apt/sources.list.d/wheezy-backports.list
echo "deb http://packages.incloudus.com backports-incloudus main" > /etc/apt/sources.list.d/incloudus.list
wget http://packages.incloudus.com/incloudus/incloudus.pub -O- | apt-key add -
aptitude update

Install the linux kernel, symbols and gcc 4.9:

aptitude install -t wheezy-backports -y initramfs-tools
aptitude install -y gcc gcc-4.9 g++-4.9 linux-image-3.14 linux-image-3.14-dbg

Configure gcc 4.9 as the default gcc compiler:

update-alternatives --install /usr/bin/gcc gcc /usr/bin/gcc-4.7 20
update-alternatives --install /usr/bin/gcc gcc /usr/bin/gcc-4.9 50
update-alternatives --install /usr/bin/g++ g++ /usr/bin/g++-4.7 20
update-alternatives --install /usr/bin/g++ g++ /usr/bin/g++-4.9 50

Install kpatch and these dependencies:

aptitude install kpatch

Configure ccache (installed by kpatch package):

ccache --max-size=5G

Gentoo

NOTE: You'll need about 15GB of free disk space for the kpatch-build cache in ~/.kpatch and for ccache.

Install Kpatch and Kpatch dependencies:

emerge --ask sys-kernel/kpatch

Install ccache (optional):

emerge --ask dev-util/ccache

Configure ccache:

ccache --max-size=5G

Build

Compile kpatch:

make

Install

OPTIONAL: Install kpatch to /usr/local:

sudo make install

Alternatively, the kpatch and kpatch-build scripts can be run directly from the git tree.

Quick start

NOTE: While kpatch is designed to work with any recent Linux kernel on any distribution, the kpatch-build command has ONLY been tested and confirmed to work on Fedora 20 and later, RHEL 7, Oracle Linux 7, CentOS 7 and Ubuntu 14.04.

First, make a source code patch against the kernel tree using diff, git, or quilt.

As a contrived example, let's patch /proc/meminfo to show VmallocChunk in ALL CAPS so we can see it better:

$ cat meminfo-string.patch
Index: src/fs/proc/meminfo.c
===================================================================
--- src.orig/fs/proc/meminfo.c
+++ src/fs/proc/meminfo.c
@@ -95,7 +95,7 @@ static int meminfo_proc_show(struct seq_
 		"Committed_AS:   %8lu kB\n"
 		"VmallocTotal:   %8lu kB\n"
 		"VmallocUsed:    %8lu kB\n"
-		"VmallocChunk:   %8lu kB\n"
+		"VMALLOCCHUNK:   %8lu kB\n"
 #ifdef CONFIG_MEMORY_FAILURE
 		"HardwareCorrupted: %5lu kB\n"
 #endif

Build the patch module:

$ kpatch-build -t vmlinux meminfo-string.patch
Using cache at /home/jpoimboe/.kpatch/3.13.10-200.fc20.x86_64/src
Testing patch file
checking file fs/proc/meminfo.c
Building original kernel
Building patched kernel
Detecting changed objects
Rebuilding changed objects
Extracting new and modified ELF sections
meminfo.o: changed function: meminfo_proc_show
Building patch module: kpatch-meminfo-string.ko
SUCCESS

NOTE: The -t vmlinux option is used to tell kpatch-build to only look for changes in the vmlinux base kernel image, which is much faster than also compiling all the kernel modules. If your patch affects a kernel module, you can either omit this option to build everything, and have kpatch-build detect which modules changed, or you can specify the affected kernel build targets with multiple -t options.

That outputs a patch module named kpatch-meminfo-string.ko in the current directory. Now apply it to the running kernel:

$ sudo kpatch load kpatch-meminfo-string.ko
loading core module: /usr/local/lib/modules/3.13.10-200.fc20.x86_64/kpatch/kpatch.ko
loading patch module: kpatch-meminfo-string.ko

Done! The kernel is now patched.

$ grep -i chunk /proc/meminfo
VMALLOCCHUNK:   34359337092 kB

Patch Author Guide

Unfortunately, live patching isn't always as easy as the previous example, and can have some major pitfalls if you're not careful. To learn more about how to properly create live patches, see the Patch Author Guide.

How it works

kpatch works at a function granularity: old functions are replaced with new ones. It has four main components:

• kpatch-build: a collection of tools which convert a source diff patch to a patch module. They work by compiling the kernel both with and without the source patch, comparing the binaries, and generating a patch module which includes new binary versions of the functions to be replaced.

• patch module: a kernel module (.ko file) which includes the replacement functions and metadata about the original functions.

• kpatch core module: a kernel module (.ko file) which provides an interface for the patch modules to register new functions for replacement. It uses the kernel ftrace subsystem to hook into the original function's mcount call instruction, so that a call to the original function is redirected to the replacement function.

• kpatch utility: a command-line tool which allows a user to manage a collection of patch modules. One or more patch modules may be configured to load at boot time, so that a system can remain patched even after a reboot into the same version of the kernel.

kpatch-build

The "kpatch-build" command converts a source-level diff patch file to a kernel patch module. Most of its work is performed by the kpatch-build script which uses a utility named create-diff-object to compare changed objects.

The primary steps in kpatch-build are:

• Build the unstripped vmlinux for the kernel
• Patch the source tree
• Rebuild vmlinux and monitor which objects are being rebuilt. These are the "changed objects".
• Recompile each changed object with -ffunction-sections -fdata-sections, resulting in the changed patched objects
• Unpatch the source tree
• Recompile each changed object with -ffunction-sections -fdata-sections, resulting in the changed original objects
• For every changed object, use create-diff-object to do the following:
  • Analyze each original/patched object pair for patchability
  • Add .kpatch.funcs and .rela.kpatch.funcs sections to the output object. The kpatch core module uses this to determine the list of functions that need to be redirected using ftrace.
  • Add .kpatch.dynrelas and .rela.kpatch.dynrelas sections to the output object. This will be used to resolve references to non-included local and non-exported global symbols. These relocations will be resolved by the kpatch core module.
  • Generate the resulting output object containing the new and modified sections
• Link all the output objects into a cumulative object
• Generate the patch module

Patching

The patch modules register with the core module (kpatch.ko). They provide information about original functions that need to be replaced, and corresponding function pointers to the replacement functions.

The core module registers a handler function with ftrace. The handler function is called by ftrace immediately before the original function begins executing. This occurs with the help of the reserved mcount call at the beginning of every function, created by the gcc -mfentry flag. The ftrace handler then modifies the return instruction pointer (IP) address on the stack and returns to ftrace, which then restores the original function's arguments and stack, and "returns" to the new function.

Limitations

• Patches which modify init functions (annotated with __init) are not supported. kpatch-build will return an error if the patch attempts to do so.

• Patches which modify statically allocated data are not directly supported. kpatch-build will detect that and return an error. This limitation can be overcome by using callbacks or shadow variables, as described in the Patch Author Guide.

• Patches which change the way a function interacts with dynamically allocated data might be safe, or might not. It isn't possible for kpatch-build to verify the safety of this kind of patch. It's up to the user to understand what the patch does, whether the new functions interact with dynamically allocated data in a different way than the old functions did, and whether it would be safe to atomically apply such a patch to a running kernel.

• Patches which modify functions in vdso are not supported. These run in user-space and ftrace can't hook them.

• Patches which modify functions that are missing a fentry call are not supported. This includes any lib-y targets that are archived into a lib.a library for later linking (for example, lib/string.o).

• Some incompatibilities currently exist between kpatch and usage of ftrace and kprobes. See the Frequently Asked Questions section for more details.

Frequently Asked Questions

Q. What's the relationship between kpatch and the upstream Linux live kernel patching component (livepatch)?

Starting with Linux 4.0, the Linux kernel has livepatch, which is a new converged live kernel patching framework. Livepatch is similar in functionality to the kpatch core module, though it doesn't yet have all the features that kpatch does.

kpatch-build already works with both livepatch and kpatch. If your kernel has CONFIG_LIVEPATCH enabled, it detects that and builds a patch module in the livepatch format. Otherwise it builds a kpatch patch module.

The kpatch script also supports both patch module formats.

Q. Isn't this just a virus/rootkit injection framework?

kpatch uses kernel modules to replace code. It requires the CAP_SYS_MODULE capability. If you already have that capability, then you already have the ability to arbitrarily modify the kernel, with or without kpatch.

Q. How can I detect if somebody has patched the kernel?

When a patch module is loaded, the TAINT_USER or TAINT_LIVEPATCH flag is set. (The latter flag was introduced in Linux version 4.0.) To test for these flags, cat /proc/sys/kernel/tainted and check to see if the value of TAINT_USER (64) or TAINT_LIVEPATCH (32768) has been OR'ed in.

Note that the TAINT_OOT_MODULE flag (4096) will also be set, since the patch module is built outside the Linux kernel source tree.

If your patch module is unsigned, the TAINT_FORCED_MODULE flag (2) will also be set. Starting with Linux 3.15, this will be changed to the more specific TAINT_UNSIGNED_MODULE (8192).

Linux versions starting with 4.9 also support a per-module TAINT_LIVEPATCH taint flag. This can be checked by verifying the output of cat /sys/module/<kpatch module>/taint -- a 'K' character indicates the presence of TAINT_LIVEPATCH.

Q. Will it destabilize my system?

No, as long as the patch is chosen carefully. See the Limitations section above.

Q. Why does kpatch use ftrace to jump to the replacement function instead of adding the jump directly?

ftrace owns the first "call mcount" instruction of every kernel function. In order to keep compatibility with ftrace, we go through ftrace rather than updating the instruction directly. This approach also ensures that the code modification path is reliable, since ftrace has been doing it successfully for years.

Q. Is kpatch compatible with <insert kernel debugging subsystem here>?

We aim to be good kernel citizens and maintain compatibility. A kpatch replacement function is no different than a function loaded by any other kernel module. Each replacement function has its own symbol name and kallsyms entry, so it looks like a normal function to the kernel.

• oops stack traces: Yes. If the replacement function is involved in an oops, the stack trace will show the function and kernel module name of the replacement function, just like any other kernel module function. The oops message will also show the taint flag (see the FAQ "How can I detect if somebody has patched the kernel" for specifics).
• kdump/crash: Yes. Replacement functions are normal functions, so crash will have no issues.
• ftrace: Yes, but certain uses of ftrace which involve opening the /sys/kernel/debug/tracing/trace file or using trace-cmd record can result in a tiny window of time where a patch gets temporarily disabled. Therefore it's a good idea to avoid using ftrace on a patched system until this issue is resolved.
• systemtap/kprobes: Some incompatibilities exist.
  • If you setup a kprobe module at the beginning of a function before loading a kpatch module, and they both affect the same function, kprobes "wins" until the kprobe has been unregistered. This is tracked in issue #47.
  • Setting a kretprobe before loading a kpatch module could be unsafe. See issue #67.
• perf: Yes.
• tracepoints: Patches to a function which uses tracepoints will result in the tracepoints being effectively disabled as long as the patch is applied.

Q. Why not use something like kexec instead?

If you want to avoid a hardware reboot, but are ok with restarting processes, kexec is a good alternative.

Q. If an application can't handle a reboot, it's designed wrong.

That's a good poi... [system reboots]

Q. What changes are needed in other upstream projects?

We hope to make the following changes to other projects:

• kernel:
  • ftrace improvements to close any windows that would allow a patch to be inadvertently disabled

Q. Is it possible to register a function that gets called atomically with stop_machine when the patch module loads and unloads?

We do have plans to implement something like that.

Q. What kernels are supported?

kpatch needs gcc >= 4.8 and Linux >= 3.9.

Q. Is it possible to remove a patch?

Yes. Just run kpatch unload which will disable and unload the patch module and restore the function to its original state.

Q. Can you apply multiple patches?

Yes, but to prevent any unexpected interactions between multiple patch modules, it's recommended that patch upgrades are cumulative, so that each patch is a superset of the previous patch. This can be achieved by combining the new patch with the previous patch using combinediff before running kpatch-build.

Q. Why did kpatch-build detect a changed function that wasn't touched by the source patch?

There could be a variety of reasons for this, such as:

• The patch changed an inline function.
• The compiler decided to inline a changed function, resulting in the outer function getting recompiled. This is common in the case where the inner function is static and is only called once.

Q. How do I patch a function which is always on the stack of at least one task, such as schedule(), sys_poll(), sys_select(), sys_read(), sys_nanosleep(), etc?

• If you're sure it would be safe for the old function and the new function to run simultaneously, use the KPATCH_FORCE_UNSAFE macro to skip the activeness safety check for the function. See kmod/patch/kpatch-macros.h for more details.

Q. Are patching of kernel modules supported?

• Yes.

Q. Can you patch out-of-tree modules?

Yes! There's a few requirements, and the feature is still in its infancy.

1. You need to use the --oot-module flag to specify the version of the module that's currently running on the machine.
2. --sourcedir has to be passed with a directory containing the same version of code as the running module, all set up and ready to build with a make command. For example, some modules need autogen.sh and ./configure to have been run with the appropriate flags to match the currently-running module.
3. If the Module.symvers file for the out-of-tree module doesn't appear in the root of the provided source directory, a symlink needs to be created in that directory that points to its actual location.
4. Usually you'll need to pass the --target flag as well, to specify the proper make target names.
5. This has only been tested for a single out-of-tree module per patch, and not for out-of-tree modules with dependencies on other out-of-tree modules built separately.

Sample invocation

kpatch-build --sourcedir ~/test/ --target default --oot-module /lib/modules/$(uname -r)/extra/test.ko test.patch

Get involved

If you have questions or feedback, join the #kpatch IRC channel on freenode and say hi. We also have a mailing list.

Contributions are very welcome. Feel free to open issues or PRs on github. For big PRs, it's a good idea to discuss them first in github issues or on the mailing list before you write a lot of code.

License

kpatch is under the GPLv2 license.

This program is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; either version 2 of the License, or (at your option) any later version.

This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.

You should have received a copy of the GNU General Public License along with this program; if not, write to the Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.