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Based on kernel version 4.15. Page generated on 2018-01-29 10:00 EST.

1	=========
2	Livepatch
3	=========
5	This document outlines basic information about kernel livepatching.
7	Table of Contents:
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
25	1. Motivation
26	=============
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.
36	2. Kprobes, Ftrace, Livepatching
37	================================
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:
43	  + The kernel probes are the most generic. The code can be redirected by
44	    putting a breakpoint instruction instead of any instruction.
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.
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.
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.
63	3. Consistency model
64	====================
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.
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.  (This can be done by setting the
76	'immediate' flag in the klp_patch struct.)
78	But there are more complex fixes. For example, a patch might change
79	ordering of locking in multiple functions at the same time. Or a patch
80	might exchange meaning of some temporary structures and update
81	all the relevant functions. In this case, the affected unit
82	(thread, whole kernel) need to start using all new versions of
83	the functions at the same time. Also the switch must happen only
84	when it is safe to do so, e.g. when the affected locks are released
85	or no data are stored in the modified structures at the moment.
87	The theory about how to apply functions a safe way is rather complex.
88	The aim is to define a so-called consistency model. It attempts to define
89	conditions when the new implementation could be used so that the system
90	stays consistent.
92	Livepatch has a consistency model which is a hybrid of kGraft and
93	kpatch:  it uses kGraft's per-task consistency and syscall barrier
94	switching combined with kpatch's stack trace switching.  There are also
95	a number of fallback options which make it quite flexible.
97	Patches are applied on a per-task basis, when the task is deemed safe to
98	switch over.  When a patch is enabled, livepatch enters into a
99	transition state where tasks are converging to the patched state.
100	Usually this transition state can complete in a few seconds.  The same
101	sequence occurs when a patch is disabled, except the tasks converge from
102	the patched state to the unpatched state.
104	An interrupt handler inherits the patched state of the task it
105	interrupts.  The same is true for forked tasks: the child inherits the
106	patched state of the parent.
108	Livepatch uses several complementary approaches to determine when it's
109	safe to patch tasks:
111	1. The first and most effective approach is stack checking of sleeping
112	   tasks.  If no affected functions are on the stack of a given task,
113	   the task is patched.  In most cases this will patch most or all of
114	   the tasks on the first try.  Otherwise it'll keep trying
115	   periodically.  This option is only available if the architecture has
116	   reliable stacks (HAVE_RELIABLE_STACKTRACE).
118	2. The second approach, if needed, is kernel exit switching.  A
119	   task is switched when it returns to user space from a system call, a
120	   user space IRQ, or a signal.  It's useful in the following cases:
122	   a) Patching I/O-bound user tasks which are sleeping on an affected
123	      function.  In this case you have to send SIGSTOP and SIGCONT to
124	      force it to exit the kernel and be patched.
125	   b) Patching CPU-bound user tasks.  If the task is highly CPU-bound
126	      then it will get patched the next time it gets interrupted by an
127	      IRQ.
128	   c) In the future it could be useful for applying patches for
129	      architectures which don't yet have HAVE_RELIABLE_STACKTRACE.  In
130	      this case you would have to signal most of the tasks on the
131	      system.  However this isn't supported yet because there's
132	      currently no way to patch kthreads without
135	3. For idle "swapper" tasks, since they don't ever exit the kernel, they
136	   instead have a klp_update_patch_state() call in the idle loop which
137	   allows them to be patched before the CPU enters the idle state.
139	   (Note there's not yet such an approach for kthreads.)
141	All the above approaches may be skipped by setting the 'immediate' flag
142	in the 'klp_patch' struct, which will disable per-task consistency and
143	patch all tasks immediately.  This can be useful if the patch doesn't
144	change any function or data semantics.  Note that, even with this flag
145	set, it's possible that some tasks may still be running with an old
146	version of the function, until that function returns.
148	There's also an 'immediate' flag in the 'klp_func' struct which allows
149	you to specify that certain functions in the patch can be applied
150	without per-task consistency.  This might be useful if you want to patch
151	a common function like schedule(), and the function change doesn't need
152	consistency but the rest of the patch does.
154	For architectures which don't have HAVE_RELIABLE_STACKTRACE, the user
155	must set patch->immediate which causes all tasks to be patched
156	immediately.  This option should be used with care, only when the patch
157	doesn't change any function or data semantics.
159	In the future, architectures which don't have HAVE_RELIABLE_STACKTRACE
160	may be allowed to use per-task consistency if we can come up with
161	another way to patch kthreads.
163	The /sys/kernel/livepatch/<patch>/transition file shows whether a patch
164	is in transition.  Only a single patch (the topmost patch on the stack)
165	can be in transition at a given time.  A patch can remain in transition
166	indefinitely, if any of the tasks are stuck in the initial patch state.
168	A transition can be reversed and effectively canceled by writing the
169	opposite value to the /sys/kernel/livepatch/<patch>/enabled file while
170	the transition is in progress.  Then all the tasks will attempt to
171	converge back to the original patch state.
173	There's also a /proc/<pid>/patch_state file which can be used to
174	determine which tasks are blocking completion of a patching operation.
175	If a patch is in transition, this file shows 0 to indicate the task is
176	unpatched and 1 to indicate it's patched.  Otherwise, if no patch is in
177	transition, it shows -1.  Any tasks which are blocking the transition
178	can be signaled with SIGSTOP and SIGCONT to force them to change their
179	patched state.
182	3.1 Adding consistency model support to new architectures
183	---------------------------------------------------------
185	For adding consistency model support to new architectures, there are a
186	few options:
188	1) Add CONFIG_HAVE_RELIABLE_STACKTRACE.  This means porting objtool, and
189	   for non-DWARF unwinders, also making sure there's a way for the stack
190	   tracing code to detect interrupts on the stack.
192	2) Alternatively, ensure that every kthread has a call to
193	   klp_update_patch_state() in a safe location.  Kthreads are typically
194	   in an infinite loop which does some action repeatedly.  The safe
195	   location to switch the kthread's patch state would be at a designated
196	   point in the loop where there are no locks taken and all data
197	   structures are in a well-defined state.
199	   The location is clear when using workqueues or the kthread worker
200	   API.  These kthreads process independent actions in a generic loop.
202	   It's much more complicated with kthreads which have a custom loop.
203	   There the safe location must be carefully selected on a case-by-case
204	   basis.
206	   In that case, arches without HAVE_RELIABLE_STACKTRACE would still be
207	   able to use the non-stack-checking parts of the consistency model:
209	   a) patching user tasks when they cross the kernel/user space
210	      boundary; and
212	   b) patching kthreads and idle tasks at their designated patch points.
214	   This option isn't as good as option 1 because it requires signaling
215	   user tasks and waking kthreads to patch them.  But it could still be
216	   a good backup option for those architectures which don't have
217	   reliable stack traces yet.
219	In the meantime, patches for such architectures can bypass the
220	consistency model by setting klp_patch.immediate to true.  This option
221	is perfectly fine for patches which don't change the semantics of the
222	patched functions.  In practice, this is usable for ~90% of security
223	fixes.  Use of this option also means the patch can't be unloaded after
224	it has been disabled.
227	4. Livepatch module
228	===================
230	Livepatches are distributed using kernel modules, see
231	samples/livepatch/livepatch-sample.c.
233	The module includes a new implementation of functions that we want
234	to replace. In addition, it defines some structures describing the
235	relation between the original and the new implementation. Then there
236	is code that makes the kernel start using the new code when the livepatch
237	module is loaded. Also there is code that cleans up before the
238	livepatch module is removed. All this is explained in more details in
239	the next sections.
242	4.1. New functions
243	------------------
245	New versions of functions are typically just copied from the original
246	sources. A good practice is to add a prefix to the names so that they
247	can be distinguished from the original ones, e.g. in a backtrace. Also
248	they can be declared as static because they are not called directly
249	and do not need the global visibility.
251	The patch contains only functions that are really modified. But they
252	might want to access functions or data from the original source file
253	that may only be locally accessible. This can be solved by a special
254	relocation section in the generated livepatch module, see
255	Documentation/livepatch/module-elf-format.txt for more details.
258	4.2. Metadata
259	-------------
261	The patch is described by several structures that split the information
262	into three levels:
264	  + struct klp_func is defined for each patched function. It describes
265	    the relation between the original and the new implementation of a
266	    particular function.
268	    The structure includes the name, as a string, of the original function.
269	    The function address is found via kallsyms at runtime.
271	    Then it includes the address of the new function. It is defined
272	    directly by assigning the function pointer. Note that the new
273	    function is typically defined in the same source file.
275	    As an optional parameter, the symbol position in the kallsyms database can
276	    be used to disambiguate functions of the same name. This is not the
277	    absolute position in the database, but rather the order it has been found
278	    only for a particular object ( vmlinux or a kernel module ). Note that
279	    kallsyms allows for searching symbols according to the object name.
281	    There's also an 'immediate' flag which, when set, patches the
282	    function immediately, bypassing the consistency model safety checks.
284	  + struct klp_object defines an array of patched functions (struct
285	    klp_func) in the same object. Where the object is either vmlinux
286	    (NULL) or a module name.
288	    The structure helps to group and handle functions for each object
289	    together. Note that patched modules might be loaded later than
290	    the patch itself and the relevant functions might be patched
291	    only when they are available.
294	  + struct klp_patch defines an array of patched objects (struct
295	    klp_object).
297	    This structure handles all patched functions consistently and eventually,
298	    synchronously. The whole patch is applied only when all patched
299	    symbols are found. The only exception are symbols from objects
300	    (kernel modules) that have not been loaded yet.
302	    Setting the 'immediate' flag applies the patch to all tasks
303	    immediately, bypassing the consistency model safety checks.
305	    For more details on how the patch is applied on a per-task basis,
306	    see the "Consistency model" section.
309	4.3. Livepatch module handling
310	------------------------------
312	The usual behavior is that the new functions will get used when
313	the livepatch module is loaded. For this, the module init() function
314	has to register the patch (struct klp_patch) and enable it. See the
315	section "Livepatch life-cycle" below for more details about these
316	two operations.
318	Module removal is only safe when there are no users of the underlying
319	functions. The immediate consistency model is not able to detect this. The
320	code just redirects the functions at the very beginning and it does not
321	check if the functions are in use. In other words, it knows when the
322	functions get called but it does not know when the functions return.
323	Therefore it cannot be decided when the livepatch module can be safely
324	removed. This is solved by a hybrid consistency model. When the system is
325	transitioned to a new patch state (patched/unpatched) it is guaranteed that
326	no task sleeps or runs in the old code.
329	5. Livepatch life-cycle
330	=======================
332	Livepatching defines four basic operations that define the life cycle of each
333	live patch: registration, enabling, disabling and unregistration.  There are
334	several reasons why it is done this way.
336	First, the patch is applied only when all patched symbols for already
337	loaded objects are found. The error handling is much easier if this
338	check is done before particular functions get redirected.
340	Second, the immediate consistency model does not guarantee that anyone is not
341	sleeping in the new code after the patch is reverted. This means that the new
342	code needs to stay around "forever". If the code is there, one could apply it
343	again. Therefore it makes sense to separate the operations that might be done
344	once and those that need to be repeated when the patch is enabled (applied)
345	again.
347	Third, it might take some time until the entire system is migrated
348	when a more complex consistency model is used. The patch revert might
349	block the livepatch module removal for too long. Therefore it is useful
350	to revert the patch using a separate operation that might be called
351	explicitly. But it does not make sense to remove all information
352	until the livepatch module is really removed.
355	5.1. Registration
356	-----------------
358	Each patch first has to be registered using klp_register_patch(). This makes
359	the patch known to the livepatch framework. Also it does some preliminary
360	computing and checks.
362	In particular, the patch is added into the list of known patches. The
363	addresses of the patched functions are found according to their names.
364	The special relocations, mentioned in the section "New functions", are
365	applied. The relevant entries are created under
366	/sys/kernel/livepatch/<name>. The patch is rejected when any operation
367	fails.
370	5.2. Enabling
371	-------------
373	Registered patches might be enabled either by calling klp_enable_patch() or
374	by writing '1' to /sys/kernel/livepatch/<name>/enabled. The system will
375	start using the new implementation of the patched functions at this stage.
377	When a patch is enabled, livepatch enters into a transition state where
378	tasks are converging to the patched state.  This is indicated by a value
379	of '1' in /sys/kernel/livepatch/<name>/transition.  Once all tasks have
380	been patched, the 'transition' value changes to '0'.  For more
381	information about this process, see the "Consistency model" section.
383	If an original function is patched for the first time, a function
384	specific struct klp_ops is created and an universal ftrace handler is
385	registered.
387	Functions might be patched multiple times. The ftrace handler is registered
388	only once for the given function. Further patches just add an entry to the
389	list (see field `func_stack`) of the struct klp_ops. The last added
390	entry is chosen by the ftrace handler and becomes the active function
391	replacement.
393	Note that the patches might be enabled in a different order than they were
394	registered.
397	5.3. Disabling
398	--------------
400	Enabled patches might get disabled either by calling klp_disable_patch() or
401	by writing '0' to /sys/kernel/livepatch/<name>/enabled. At this stage
402	either the code from the previously enabled patch or even the original
403	code gets used.
405	When a patch is disabled, livepatch enters into a transition state where
406	tasks are converging to the unpatched state.  This is indicated by a
407	value of '1' in /sys/kernel/livepatch/<name>/transition.  Once all tasks
408	have been unpatched, the 'transition' value changes to '0'.  For more
409	information about this process, see the "Consistency model" section.
411	Here all the functions (struct klp_func) associated with the to-be-disabled
412	patch are removed from the corresponding struct klp_ops. The ftrace handler
413	is unregistered and the struct klp_ops is freed when the func_stack list
414	becomes empty.
416	Patches must be disabled in exactly the reverse order in which they were
417	enabled. It makes the problem and the implementation much easier.
420	5.4. Unregistration
421	-------------------
423	Disabled patches might be unregistered by calling klp_unregister_patch().
424	This can be done only when the patch is disabled and the code is no longer
425	used. It must be called before the livepatch module gets unloaded.
427	At this stage, all the relevant sys-fs entries are removed and the patch
428	is removed from the list of known patches.
431	6. Sysfs
432	========
434	Information about the registered patches can be found under
435	/sys/kernel/livepatch. The patches could be enabled and disabled
436	by writing there.
438	See Documentation/ABI/testing/sysfs-kernel-livepatch for more details.
441	7. Limitations
442	==============
444	The current Livepatch implementation has several limitations:
447	  + The patch must not change the semantic of the patched functions.
449	    The current implementation guarantees only that either the old
450	    or the new function is called. The functions are patched one
451	    by one. It means that the patch must _not_ change the semantic
452	    of the function.
455	  + Data structures can not be patched.
457	    There is no support to version data structures or anyhow migrate
458	    one structure into another. Also the simple consistency model does
459	    not allow to switch more functions atomically.
461	    Once there is more complex consistency mode, it will be possible to
462	    use some workarounds. For example, it will be possible to use a hole
463	    for a new member because the data structure is aligned. Or it will
464	    be possible to use an existing member for something else.
466	    There are no plans to add more generic support for modified structures
467	    at the moment.
470	  + Only functions that can be traced could be patched.
472	    Livepatch is based on the dynamic ftrace. In particular, functions
473	    implementing ftrace or the livepatch ftrace handler could not be
474	    patched. Otherwise, the code would end up in an infinite loop. A
475	    potential mistake is prevented by marking the problematic functions
476	    by "notrace".
480	  + Livepatch works reliably only when the dynamic ftrace is located at
481	    the very beginning of the function.
483	    The function need to be redirected before the stack or the function
484	    parameters are modified in any way. For example, livepatch requires
485	    using -fentry gcc compiler option on x86_64.
487	    One exception is the PPC port. It uses relative addressing and TOC.
488	    Each function has to handle TOC and save LR before it could call
489	    the ftrace handler. This operation has to be reverted on return.
490	    Fortunately, the generic ftrace code has the same problem and all
491	    this is handled on the ftrace level.
494	  + Kretprobes using the ftrace framework conflict with the patched
495	    functions.
497	    Both kretprobes and livepatches use a ftrace handler that modifies
498	    the return address. The first user wins. Either the probe or the patch
499	    is rejected when the handler is already in use by the other.
502	  + Kprobes in the original function are ignored when the code is
503	    redirected to the new implementation.
505	    There is a work in progress to add warnings about this situation.
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