livepatch: Add some basic livepatch documentation

=========

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

http://thread.gmane.org/gmane.linux.kernel/1823033/focus=1828189

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.

F:	include/linux/livepatch.h

F:	arch/x86/include/asm/livepatch.h

F:	arch/x86/kernel/livepatch.c

F:	Documentation/livepatch/

F:	Documentation/ABI/testing/sysfs-kernel-livepatch

F:	samples/livepatch/

L:	live-patching@vger.kernel.org