=========
Livepatch
=========

This document outlines basic information about kernel livepatching.

Table of Contents:

1. Motivation
2. Kprobes, Ftrace, Livepatching
3. Consistency model
4. Livepatch module
   4.1. New functions
   4.2. Metadata
   4.3. Livepatch module handling
5. Livepatch life-cycle
   5.1. Registration
   5.2. Enabling
   5.3. Disabling
   5.4. Unregistration
6. Sysfs
7. Limitations


1. Motivation
=============

There are many situations where users are reluctant to reboot a system. It may
be because their system is performing complex scientific computations or under
heavy load during peak usage. In addition to keeping systems up and running,
users want to also have a stable and secure system. Livepatching gives users
both by allowing for function calls to be redirected; thus, fixing critical
functions without a system reboot.


2. Kprobes, Ftrace, Livepatching
================================

There are multiple mechanisms in the Linux kernel that are directly related
to redirection of code execution; namely: kernel probes, function tracing,
and livepatching:

  + The kernel probes are the most generic. The code can be redirected by
    putting a breakpoint instruction instead of any instruction.

  + The function tracer calls the code from a predefined location that is
    close to the function entry point. This location is generated by the
    compiler using the '-pg' gcc option.

  + Livepatching typically needs to redirect the code at the very beginning
    of the function entry before the function parameters or the stack
    are in any way modified.

All three approaches need to modify the existing code at runtime. Therefore
they need to be aware of each other and not step over each other's toes.
Most of these problems are solved by using the dynamic ftrace framework as
a base. A Kprobe is registered as a ftrace handler when the function entry
is probed, see CONFIG_KPROBES_ON_FTRACE. Also an alternative function from
a live patch is called with the help of a custom ftrace handler. But there are
some limitations, see below.


3. Consistency model
====================

Functions are there for a reason. They take some input parameters, get or
release locks, read, process, and even write some data in a defined way,
have return values. In other words, each function has a defined semantic.

Many fixes do not change the semantic of the modified functions. For
example, they add a NULL pointer or a boundary check, fix a race by adding
a missing memory barrier, or add some locking around a critical section.
Most of these changes are self contained and the function presents itself
the same way to the rest of the system. In this case, the functions might
be updated independently one by one.

But there are more complex fixes. For example, a patch might change
ordering of locking in multiple functions at the same time. Or a patch
might exchange meaning of some temporary structures and update
all the relevant functions. In this case, the affected unit
(thread, whole kernel) need to start using all new versions of
the functions at the same time. Also the switch must happen only
when it is safe to do so, e.g. when the affected locks are released
or no data are stored in the modified structures at the moment.

The theory about how to apply functions a safe way is rather complex.
The aim is to define a so-called consistency model. It attempts to define
conditions when the new implementation could be used so that the system
stays consistent. The theory is not yet finished. See the discussion at
https://lkml.kernel.org/r/20141107140458.GA21774@suse.cz

The current consistency model is very simple. It guarantees that either
the old or the new function is called. But various functions get redirected
one by one without any synchronization.

In other words, the current implementation _never_ modifies the behavior
in the middle of the call. It is because it does _not_ rewrite the entire
function in the memory. Instead, the function gets redirected at the
very beginning. But this redirection is used immediately even when
some other functions from the same patch have not been redirected yet.

See also the section "Limitations" below.


4. Livepatch module
===================

Livepatches are distributed using kernel modules, see
samples/livepatch/livepatch-sample.c.

The module includes a new implementation of functions that we want
to replace. In addition, it defines some structures describing the
relation between the original and the new implementation. Then there
is code that makes the kernel start using the new code when the livepatch
module is loaded. Also there is code that cleans up before the
livepatch module is removed. All this is explained in more details in
the next sections.


4.1. New functions
------------------

New versions of functions are typically just copied from the original
sources. A good practice is to add a prefix to the names so that they
can be distinguished from the original ones, e.g. in a backtrace. Also
they can be declared as static because they are not called directly
and do not need the global visibility.

The patch contains only functions that are really modified. But they
might want to access functions or data from the original source file
that may only be locally accessible. This can be solved by a special
relocation section in the generated livepatch module, see
Documentation/livepatch/module-elf-format.txt for more details.


4.2. Metadata
------------

The patch is described by several structures that split the information
into three levels:

  + struct klp_func is defined for each patched function. It describes
    the relation between the original and the new implementation of a
    particular function.

    The structure includes the name, as a string, of the original function.
    The function address is found via kallsyms at runtime.

    Then it includes the address of the new function. It is defined
    directly by assigning the function pointer. Note that the new
    function is typically defined in the same source file.

    As an optional parameter, the symbol position in the kallsyms database can
    be used to disambiguate functions of the same name. This is not the
    absolute position in the database, but rather the order it has been found
    only for a particular object ( vmlinux or a kernel module ). Note that
    kallsyms allows for searching symbols according to the object name.

  + struct klp_object defines an array of patched functions (struct
    klp_func) in the same object. Where the object is either vmlinux
    (NULL) or a module name.

    The structure helps to group and handle functions for each object
    together. Note that patched modules might be loaded later than
    the patch itself and the relevant functions might be patched
    only when they are available.


  + struct klp_patch defines an array of patched objects (struct
    klp_object).

    This structure handles all patched functions consistently and eventually,
    synchronously. The whole patch is applied only when all patched
    symbols are found. The only exception are symbols from objects
    (kernel modules) that have not been loaded yet. Also if a more complex
    consistency model is supported then a selected unit (thread,
    kernel as a whole) will see the new code from the entire patch
    only when it is in a safe state.


4.3. Livepatch module handling
------------------------------

The usual behavior is that the new functions will get used when
the livepatch module is loaded. For this, the module init() function
has to register the patch (struct klp_patch) and enable it. See the
section "Livepatch life-cycle" below for more details about these
two operations.

Module removal is only safe when there are no users of the underlying
functions.  The immediate consistency model is not able to detect this;
therefore livepatch modules cannot be removed. See "Limitations" below.

5. Livepatch life-cycle
=======================

Livepatching defines four basic operations that define the life cycle of each
live patch: registration, enabling, disabling and unregistration.  There are
several reasons why it is done this way.

First, the patch is applied only when all patched symbols for already
loaded objects are found. The error handling is much easier if this
check is done before particular functions get redirected.

Second, the immediate consistency model does not guarantee that anyone is not
sleeping in the new code after the patch is reverted. This means that the new
code needs to stay around "forever". If the code is there, one could apply it
again. Therefore it makes sense to separate the operations that might be done
once and those that need to be repeated when the patch is enabled (applied)
again.

Third, it might take some time until the entire system is migrated
when a more complex consistency model is used. The patch revert might
block the livepatch module removal for too long. Therefore it is useful
to revert the patch using a separate operation that might be called
explicitly. But it does not make sense to remove all information
until the livepatch module is really removed.


5.1. Registration
-----------------

Each patch first has to be registered using klp_register_patch(). This makes
the patch known to the livepatch framework. Also it does some preliminary
computing and checks.

In particular, the patch is added into the list of known patches. The
addresses of the patched functions are found according to their names.
The special relocations, mentioned in the section "New functions", are
applied. The relevant entries are created under
/sys/kernel/livepatch/<name>. The patch is rejected when any operation
fails.


5.2. Enabling
-------------

Registered patches might be enabled either by calling klp_enable_patch() or
by writing '1' to /sys/kernel/livepatch/<name>/enabled. The system will
start using the new implementation of the patched functions at this stage.

In particular, if an original function is patched for the first time, a
function specific struct klp_ops is created and an universal ftrace handler
is registered.

Functions might be patched multiple times. The ftrace handler is registered
only once for the given function. Further patches just add an entry to the
list (see field `func_stack`) of the struct klp_ops. The last added
entry is chosen by the ftrace handler and becomes the active function
replacement.

Note that the patches might be enabled in a different order than they were
registered.


5.3. Disabling
--------------

Enabled patches might get disabled either by calling klp_disable_patch() or
by writing '0' to /sys/kernel/livepatch/<name>/enabled. At this stage
either the code from the previously enabled patch or even the original
code gets used.

Here all the functions (struct klp_func) associated with the to-be-disabled
patch are removed from the corresponding struct klp_ops. The ftrace handler
is unregistered and the struct klp_ops is freed when the func_stack list
becomes empty.

Patches must be disabled in exactly the reverse order in which they were
enabled. It makes the problem and the implementation much easier.


5.4. Unregistration
-------------------

Disabled patches might be unregistered by calling klp_unregister_patch().
This can be done only when the patch is disabled and the code is no longer
used. It must be called before the livepatch module gets unloaded.

At this stage, all the relevant sys-fs entries are removed and the patch
is removed from the list of known patches.


6. Sysfs
========

Information about the registered patches can be found under
/sys/kernel/livepatch. The patches could be enabled and disabled
by writing there.

See Documentation/ABI/testing/sysfs-kernel-livepatch for more details.


7. Limitations
==============

The current Livepatch implementation has several limitations:


  + The patch must not change the semantic of the patched functions.

    The current implementation guarantees only that either the old
    or the new function is called. The functions are patched one
    by one. It means that the patch must _not_ change the semantic
    of the function.


  + Data structures can not be patched.

    There is no support to version data structures or anyhow migrate
    one structure into another. Also the simple consistency model does
    not allow to switch more functions atomically.

    Once there is more complex consistency mode, it will be possible to
    use some workarounds. For example, it will be possible to use a hole
    for a new member because the data structure is aligned. Or it will
    be possible to use an existing member for something else.

    There are no plans to add more generic support for modified structures
    at the moment.


  + Only functions that can be traced could be patched.

    Livepatch is based on the dynamic ftrace. In particular, functions
    implementing ftrace or the livepatch ftrace handler could not be
    patched. Otherwise, the code would end up in an infinite loop. A
    potential mistake is prevented by marking the problematic functions
    by "notrace".


  + Anything inlined into __schedule() can not be patched.

    The switch_to macro is inlined into __schedule(). It switches the
    context between two processes in the middle of the macro. It does
    not save RIP in x86_64 version (contrary to 32-bit version). Instead,
    the currently used __schedule()/switch_to() handles both processes.

    Now, let's have two different tasks. One calls the original
    __schedule(), its registers are stored in a defined order and it
    goes to sleep in the switch_to macro and some other task is restored
    using the original __schedule(). Then there is the second task which
    calls patched__schedule(), it goes to sleep there and the first task
    is picked by the patched__schedule(). Its RSP is restored and now
    the registers should be restored as well. But the order is different
    in the new patched__schedule(), so...

    There is work in progress to remove this limitation.


  + Livepatch modules can not be removed.

    The current implementation just redirects the functions at the very
    beginning. It does not check if the functions are in use. In other
    words, it knows when the functions get called but it does not
    know when the functions return. Therefore it can not decide when
    the livepatch module can be safely removed.

    This will get most likely solved once a more complex consistency model
    is supported. The idea is that a safe state for patching should also
    mean a safe state for removing the patch.

    Note that the patch itself might get disabled by writing zero
    to /sys/kernel/livepatch/<patch>/enabled. It causes that the new
    code will not longer get called. But it does not guarantee
    that anyone is not sleeping anywhere in the new code.


  + Livepatch works reliably only when the dynamic ftrace is located at
    the very beginning of the function.

    The function need to be redirected before the stack or the function
    parameters are modified in any way. For example, livepatch requires
    using -fentry gcc compiler option on x86_64.

    One exception is the PPC port. It uses relative addressing and TOC.
    Each function has to handle TOC and save LR before it could call
    the ftrace handler. This operation has to be reverted on return.
    Fortunately, the generic ftrace code has the same problem and all
    this is is handled on the ftrace level.


  + Kretprobes using the ftrace framework conflict with the patched
    functions.

    Both kretprobes and livepatches use a ftrace handler that modifies
    the return address. The first user wins. Either the probe or the patch
    is rejected when the handler is already in use by the other.


  + Kprobes in the original function are ignored when the code is
    redirected to the new implementation.

    There is a work in progress to add warnings about this situation.
