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
• Quick start
• Patch Author Guide
• How it works
  • kpatch-build
• Limitations
• Frequently Asked Questions
• Get involved
• License

Supported Architectures

• x86-64
• ppc64le
• arm64
• s390 upstream prerequisites

Installation

See INSTALL.md.

Quick start

NOTE: While kpatch is designed to work with any recent Linux kernel on any distribution, kpatch-build has specifically been tested and confirmed to work on Fedora and RHEL. It has also been known to work on Oracle Linux, Ubuntu, Debian, and Gentoo.

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 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: livepatch-meminfo-string.ko
SUCCESS

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 patch module: livepatch-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 three 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 livepatch module (.ko file) which includes the replacement functions and metadata about the original functions. Upon loading, it registers itself with the kernel livepatch infrastructure (CONFIG_LIVEPATCH) which does the patching.

• 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

Limitations

• NOTE: Many of these limitations can be worked around with creative solutions. For more details, see the Patch Author Guide.

• 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. 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?

If a patch is currently applied, you can see it in /sys/kernel/livepatch.

Also, if a patch has been previously applied, the TAINT_LIVEPATCH flag is set. To test for these flags, cat /proc/sys/kernel/tainted and check to see if the value of 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_UNSIGNED_MODULE flag (8192) will also be set.

Q. Will it destabilize my system?

No, as long as the patch is created carefully. See the Limitations section above and the Patch Author Guide.

Q. Why not use something like kexec instead?

If you want to avoid a hardware reboot, but are ok with restarting processes or using CRIU, 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 kernels are supported?

kpatch needs gcc >= 4.8 and Linux >= 4.0.

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. It's also recommended to use livepatch atomic "replace" mode, which is the default.

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.
• A bug in kpatch-build's detection of __LINE__ macro usage.

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

Q. What is needed to support a new architecture?

Porting an architecture can be done in three phases:

1. In the kernel, add CONFIG_HAVE_LIVEPATCH support. For some arches this might be as simple as enabling CONFIG_DYNAMIC_FTRACE_WITH REGS. With this support you can do basic live patches like those in samples/livepatch. Livepatch functionality is limited and extra care must be taken to avoid certain pitfalls.
2. Add kpatch-build (create-diff-object) support. This makes it easier to build patches, and avoids some of the pitfalls. For example, https://github.com/dynup/kpatch/pull/1203 added s390x support.
3. Add CONFIG_HAVE_RELIABLE_STACKTRACE and (if needed) objtool support in the kernel. This avoids more pitfalls and enables full livepatch functionality.

Get involved

If you have questions or feedback, join the #kpatch IRC channel on Libera and say hi.

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/discussions or on IRC 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.