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Based on kernel version 4.9. Page generated on 2016-12-21 14:35 EST.

1	=========
2	Livepatch
3	=========
4	
5	This document outlines basic information about kernel livepatching.
6	
7	Table of Contents:
8	
9	1. Motivation
10	2. Kprobes, Ftrace, Livepatching
11	3. Consistency model
12	4. Livepatch module
13	   4.1. New functions
14	   4.2. Metadata
15	   4.3. Livepatch module handling
16	5. Livepatch life-cycle
17	   5.1. Registration
18	   5.2. Enabling
19	   5.3. Disabling
20	   5.4. Unregistration
21	6. Sysfs
22	7. Limitations
23	
24	
25	1. Motivation
26	=============
27	
28	There are many situations where users are reluctant to reboot a system. It may
29	be because their system is performing complex scientific computations or under
30	heavy load during peak usage. In addition to keeping systems up and running,
31	users want to also have a stable and secure system. Livepatching gives users
32	both by allowing for function calls to be redirected; thus, fixing critical
33	functions without a system reboot.
34	
35	
36	2. Kprobes, Ftrace, Livepatching
37	================================
38	
39	There are multiple mechanisms in the Linux kernel that are directly related
40	to redirection of code execution; namely: kernel probes, function tracing,
41	and livepatching:
42	
43	  + The kernel probes are the most generic. The code can be redirected by
44	    putting a breakpoint instruction instead of any instruction.
45	
46	  + The function tracer calls the code from a predefined location that is
47	    close to the function entry point. This location is generated by the
48	    compiler using the '-pg' gcc option.
49	
50	  + Livepatching typically needs to redirect the code at the very beginning
51	    of the function entry before the function parameters or the stack
52	    are in any way modified.
53	
54	All three approaches need to modify the existing code at runtime. Therefore
55	they need to be aware of each other and not step over each other's toes.
56	Most of these problems are solved by using the dynamic ftrace framework as
57	a base. A Kprobe is registered as a ftrace handler when the function entry
58	is probed, see CONFIG_KPROBES_ON_FTRACE. Also an alternative function from
59	a live patch is called with the help of a custom ftrace handler. But there are
60	some limitations, see below.
61	
62	
63	3. Consistency model
64	====================
65	
66	Functions are there for a reason. They take some input parameters, get or
67	release locks, read, process, and even write some data in a defined way,
68	have return values. In other words, each function has a defined semantic.
69	
70	Many fixes do not change the semantic of the modified functions. For
71	example, they add a NULL pointer or a boundary check, fix a race by adding
72	a missing memory barrier, or add some locking around a critical section.
73	Most of these changes are self contained and the function presents itself
74	the same way to the rest of the system. In this case, the functions might
75	be updated independently one by one.
76	
77	But there are more complex fixes. For example, a patch might change
78	ordering of locking in multiple functions at the same time. Or a patch
79	might exchange meaning of some temporary structures and update
80	all the relevant functions. In this case, the affected unit
81	(thread, whole kernel) need to start using all new versions of
82	the functions at the same time. Also the switch must happen only
83	when it is safe to do so, e.g. when the affected locks are released
84	or no data are stored in the modified structures at the moment.
85	
86	The theory about how to apply functions a safe way is rather complex.
87	The aim is to define a so-called consistency model. It attempts to define
88	conditions when the new implementation could be used so that the system
89	stays consistent. The theory is not yet finished. See the discussion at
90	http://thread.gmane.org/gmane.linux.kernel/1823033/focus=1828189
91	
92	The current consistency model is very simple. It guarantees that either
93	the old or the new function is called. But various functions get redirected
94	one by one without any synchronization.
95	
96	In other words, the current implementation _never_ modifies the behavior
97	in the middle of the call. It is because it does _not_ rewrite the entire
98	function in the memory. Instead, the function gets redirected at the
99	very beginning. But this redirection is used immediately even when
100	some other functions from the same patch have not been redirected yet.
101	
102	See also the section "Limitations" below.
103	
104	
105	4. Livepatch module
106	===================
107	
108	Livepatches are distributed using kernel modules, see
109	samples/livepatch/livepatch-sample.c.
110	
111	The module includes a new implementation of functions that we want
112	to replace. In addition, it defines some structures describing the
113	relation between the original and the new implementation. Then there
114	is code that makes the kernel start using the new code when the livepatch
115	module is loaded. Also there is code that cleans up before the
116	livepatch module is removed. All this is explained in more details in
117	the next sections.
118	
119	
120	4.1. New functions
121	------------------
122	
123	New versions of functions are typically just copied from the original
124	sources. A good practice is to add a prefix to the names so that they
125	can be distinguished from the original ones, e.g. in a backtrace. Also
126	they can be declared as static because they are not called directly
127	and do not need the global visibility.
128	
129	The patch contains only functions that are really modified. But they
130	might want to access functions or data from the original source file
131	that may only be locally accessible. This can be solved by a special
132	relocation section in the generated livepatch module, see
133	Documentation/livepatch/module-elf-format.txt for more details.
134	
135	
136	4.2. Metadata
137	------------
138	
139	The patch is described by several structures that split the information
140	into three levels:
141	
142	  + struct klp_func is defined for each patched function. It describes
143	    the relation between the original and the new implementation of a
144	    particular function.
145	
146	    The structure includes the name, as a string, of the original function.
147	    The function address is found via kallsyms at runtime.
148	
149	    Then it includes the address of the new function. It is defined
150	    directly by assigning the function pointer. Note that the new
151	    function is typically defined in the same source file.
152	
153	    As an optional parameter, the symbol position in the kallsyms database can
154	    be used to disambiguate functions of the same name. This is not the
155	    absolute position in the database, but rather the order it has been found
156	    only for a particular object ( vmlinux or a kernel module ). Note that
157	    kallsyms allows for searching symbols according to the object name.
158	
159	  + struct klp_object defines an array of patched functions (struct
160	    klp_func) in the same object. Where the object is either vmlinux
161	    (NULL) or a module name.
162	
163	    The structure helps to group and handle functions for each object
164	    together. Note that patched modules might be loaded later than
165	    the patch itself and the relevant functions might be patched
166	    only when they are available.
167	
168	
169	  + struct klp_patch defines an array of patched objects (struct
170	    klp_object).
171	
172	    This structure handles all patched functions consistently and eventually,
173	    synchronously. The whole patch is applied only when all patched
174	    symbols are found. The only exception are symbols from objects
175	    (kernel modules) that have not been loaded yet. Also if a more complex
176	    consistency model is supported then a selected unit (thread,
177	    kernel as a whole) will see the new code from the entire patch
178	    only when it is in a safe state.
179	
180	
181	4.3. Livepatch module handling
182	------------------------------
183	
184	The usual behavior is that the new functions will get used when
185	the livepatch module is loaded. For this, the module init() function
186	has to register the patch (struct klp_patch) and enable it. See the
187	section "Livepatch life-cycle" below for more details about these
188	two operations.
189	
190	Module removal is only safe when there are no users of the underlying
191	functions.  The immediate consistency model is not able to detect this;
192	therefore livepatch modules cannot be removed. See "Limitations" below.
193	
194	5. Livepatch life-cycle
195	=======================
196	
197	Livepatching defines four basic operations that define the life cycle of each
198	live patch: registration, enabling, disabling and unregistration.  There are
199	several reasons why it is done this way.
200	
201	First, the patch is applied only when all patched symbols for already
202	loaded objects are found. The error handling is much easier if this
203	check is done before particular functions get redirected.
204	
205	Second, the immediate consistency model does not guarantee that anyone is not
206	sleeping in the new code after the patch is reverted. This means that the new
207	code needs to stay around "forever". If the code is there, one could apply it
208	again. Therefore it makes sense to separate the operations that might be done
209	once and those that need to be repeated when the patch is enabled (applied)
210	again.
211	
212	Third, it might take some time until the entire system is migrated
213	when a more complex consistency model is used. The patch revert might
214	block the livepatch module removal for too long. Therefore it is useful
215	to revert the patch using a separate operation that might be called
216	explicitly. But it does not make sense to remove all information
217	until the livepatch module is really removed.
218	
219	
220	5.1. Registration
221	-----------------
222	
223	Each patch first has to be registered using klp_register_patch(). This makes
224	the patch known to the livepatch framework. Also it does some preliminary
225	computing and checks.
226	
227	In particular, the patch is added into the list of known patches. The
228	addresses of the patched functions are found according to their names.
229	The special relocations, mentioned in the section "New functions", are
230	applied. The relevant entries are created under
231	/sys/kernel/livepatch/<name>. The patch is rejected when any operation
232	fails.
233	
234	
235	5.2. Enabling
236	-------------
237	
238	Registered patches might be enabled either by calling klp_enable_patch() or
239	by writing '1' to /sys/kernel/livepatch/<name>/enabled. The system will
240	start using the new implementation of the patched functions at this stage.
241	
242	In particular, if an original function is patched for the first time, a
243	function specific struct klp_ops is created and an universal ftrace handler
244	is registered.
245	
246	Functions might be patched multiple times. The ftrace handler is registered
247	only once for the given function. Further patches just add an entry to the
248	list (see field `func_stack`) of the struct klp_ops. The last added
249	entry is chosen by the ftrace handler and becomes the active function
250	replacement.
251	
252	Note that the patches might be enabled in a different order than they were
253	registered.
254	
255	
256	5.3. Disabling
257	--------------
258	
259	Enabled patches might get disabled either by calling klp_disable_patch() or
260	by writing '0' to /sys/kernel/livepatch/<name>/enabled. At this stage
261	either the code from the previously enabled patch or even the original
262	code gets used.
263	
264	Here all the functions (struct klp_func) associated with the to-be-disabled
265	patch are removed from the corresponding struct klp_ops. The ftrace handler
266	is unregistered and the struct klp_ops is freed when the func_stack list
267	becomes empty.
268	
269	Patches must be disabled in exactly the reverse order in which they were
270	enabled. It makes the problem and the implementation much easier.
271	
272	
273	5.4. Unregistration
274	-------------------
275	
276	Disabled patches might be unregistered by calling klp_unregister_patch().
277	This can be done only when the patch is disabled and the code is no longer
278	used. It must be called before the livepatch module gets unloaded.
279	
280	At this stage, all the relevant sys-fs entries are removed and the patch
281	is removed from the list of known patches.
282	
283	
284	6. Sysfs
285	========
286	
287	Information about the registered patches can be found under
288	/sys/kernel/livepatch. The patches could be enabled and disabled
289	by writing there.
290	
291	See Documentation/ABI/testing/sysfs-kernel-livepatch for more details.
292	
293	
294	7. Limitations
295	==============
296	
297	The current Livepatch implementation has several limitations:
298	
299	
300	  + The patch must not change the semantic of the patched functions.
301	
302	    The current implementation guarantees only that either the old
303	    or the new function is called. The functions are patched one
304	    by one. It means that the patch must _not_ change the semantic
305	    of the function.
306	
307	
308	  + Data structures can not be patched.
309	
310	    There is no support to version data structures or anyhow migrate
311	    one structure into another. Also the simple consistency model does
312	    not allow to switch more functions atomically.
313	
314	    Once there is more complex consistency mode, it will be possible to
315	    use some workarounds. For example, it will be possible to use a hole
316	    for a new member because the data structure is aligned. Or it will
317	    be possible to use an existing member for something else.
318	
319	    There are no plans to add more generic support for modified structures
320	    at the moment.
321	
322	
323	  + Only functions that can be traced could be patched.
324	
325	    Livepatch is based on the dynamic ftrace. In particular, functions
326	    implementing ftrace or the livepatch ftrace handler could not be
327	    patched. Otherwise, the code would end up in an infinite loop. A
328	    potential mistake is prevented by marking the problematic functions
329	    by "notrace".
330	
331	
332	  + Anything inlined into __schedule() can not be patched.
333	
334	    The switch_to macro is inlined into __schedule(). It switches the
335	    context between two processes in the middle of the macro. It does
336	    not save RIP in x86_64 version (contrary to 32-bit version). Instead,
337	    the currently used __schedule()/switch_to() handles both processes.
338	
339	    Now, let's have two different tasks. One calls the original
340	    __schedule(), its registers are stored in a defined order and it
341	    goes to sleep in the switch_to macro and some other task is restored
342	    using the original __schedule(). Then there is the second task which
343	    calls patched__schedule(), it goes to sleep there and the first task
344	    is picked by the patched__schedule(). Its RSP is restored and now
345	    the registers should be restored as well. But the order is different
346	    in the new patched__schedule(), so...
347	
348	    There is work in progress to remove this limitation.
349	
350	
351	  + Livepatch modules can not be removed.
352	
353	    The current implementation just redirects the functions at the very
354	    beginning. It does not check if the functions are in use. In other
355	    words, it knows when the functions get called but it does not
356	    know when the functions return. Therefore it can not decide when
357	    the livepatch module can be safely removed.
358	
359	    This will get most likely solved once a more complex consistency model
360	    is supported. The idea is that a safe state for patching should also
361	    mean a safe state for removing the patch.
362	
363	    Note that the patch itself might get disabled by writing zero
364	    to /sys/kernel/livepatch/<patch>/enabled. It causes that the new
365	    code will not longer get called. But it does not guarantee
366	    that anyone is not sleeping anywhere in the new code.
367	
368	
369	  + Livepatch works reliably only when the dynamic ftrace is located at
370	    the very beginning of the function.
371	
372	    The function need to be redirected before the stack or the function
373	    parameters are modified in any way. For example, livepatch requires
374	    using -fentry gcc compiler option on x86_64.
375	
376	    One exception is the PPC port. It uses relative addressing and TOC.
377	    Each function has to handle TOC and save LR before it could call
378	    the ftrace handler. This operation has to be reverted on return.
379	    Fortunately, the generic ftrace code has the same problem and all
380	    this is is handled on the ftrace level.
381	
382	
383	  + Kretprobes using the ftrace framework conflict with the patched
384	    functions.
385	
386	    Both kretprobes and livepatches use a ftrace handler that modifies
387	    the return address. The first user wins. Either the probe or the patch
388	    is rejected when the handler is already in use by the other.
389	
390	
391	  + Kprobes in the original function are ignored when the code is
392	    redirected to the new implementation.
393	
394	    There is a work in progress to add warnings about this situation.
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