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1=========
2Livepatch
3=========
4
5This 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 5. Livepatch life-cycle
16 5.1. Loading
17 5.2. Enabling
18 5.3. Replacing
19 5.4. Disabling
20 5.5. Removing
21 6. Sysfs
22 7. Limitations
23
24
251. Motivation
26=============
27
28There are many situations where users are reluctant to reboot a system. It may
29be because their system is performing complex scientific computations or under
30heavy load during peak usage. In addition to keeping systems up and running,
31users want to also have a stable and secure system. Livepatching gives users
32both by allowing for function calls to be redirected; thus, fixing critical
33functions without a system reboot.
34
35
362. Kprobes, Ftrace, Livepatching
37================================
38
39There are multiple mechanisms in the Linux kernel that are directly related
40to redirection of code execution; namely: kernel probes, function tracing,
41and 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
54All three approaches need to modify the existing code at runtime. Therefore
55they need to be aware of each other and not step over each other's toes.
56Most of these problems are solved by using the dynamic ftrace framework as
57a base. A Kprobe is registered as a ftrace handler when the function entry
58is probed, see CONFIG_KPROBES_ON_FTRACE. Also an alternative function from
59a live patch is called with the help of a custom ftrace handler. But there are
60some limitations, see below.
61
62
633. Consistency model
64====================
65
66Functions are there for a reason. They take some input parameters, get or
67release locks, read, process, and even write some data in a defined way,
68have return values. In other words, each function has a defined semantic.
69
70Many fixes do not change the semantic of the modified functions. For
71example, they add a NULL pointer or a boundary check, fix a race by adding
72a missing memory barrier, or add some locking around a critical section.
73Most of these changes are self contained and the function presents itself
74the same way to the rest of the system. In this case, the functions might
75be updated independently one by one.
76
77But there are more complex fixes. For example, a patch might change
78ordering of locking in multiple functions at the same time. Or a patch
79might exchange meaning of some temporary structures and update
80all the relevant functions. In this case, the affected unit
81(thread, whole kernel) need to start using all new versions of
82the functions at the same time. Also the switch must happen only
83when it is safe to do so, e.g. when the affected locks are released
84or no data are stored in the modified structures at the moment.
85
86The theory about how to apply functions a safe way is rather complex.
87The aim is to define a so-called consistency model. It attempts to define
88conditions when the new implementation could be used so that the system
89stays consistent.
90
91Livepatch has a consistency model which is a hybrid of kGraft and
92kpatch: it uses kGraft's per-task consistency and syscall barrier
93switching combined with kpatch's stack trace switching. There are also
94a number of fallback options which make it quite flexible.
95
96Patches are applied on a per-task basis, when the task is deemed safe to
97switch over. When a patch is enabled, livepatch enters into a
98transition state where tasks are converging to the patched state.
99Usually this transition state can complete in a few seconds. The same
100sequence occurs when a patch is disabled, except the tasks converge from
101the patched state to the unpatched state.
102
103An interrupt handler inherits the patched state of the task it
104interrupts. The same is true for forked tasks: the child inherits the
105patched state of the parent.
106
107Livepatch uses several complementary approaches to determine when it's
108safe to patch tasks:
109
1101. The first and most effective approach is stack checking of sleeping
111 tasks. If no affected functions are on the stack of a given task,
112 the task is patched. In most cases this will patch most or all of
113 the tasks on the first try. Otherwise it'll keep trying
114 periodically. This option is only available if the architecture has
115 reliable stacks (HAVE_RELIABLE_STACKTRACE).
116
1172. The second approach, if needed, is kernel exit switching. A
118 task is switched when it returns to user space from a system call, a
119 user space IRQ, or a signal. It's useful in the following cases:
120
121 a) Patching I/O-bound user tasks which are sleeping on an affected
122 function. In this case you have to send SIGSTOP and SIGCONT to
123 force it to exit the kernel and be patched.
124 b) Patching CPU-bound user tasks. If the task is highly CPU-bound
125 then it will get patched the next time it gets interrupted by an
126 IRQ.
127
1283. For idle "swapper" tasks, since they don't ever exit the kernel, they
129 instead have a klp_update_patch_state() call in the idle loop which
130 allows them to be patched before the CPU enters the idle state.
131
132 (Note there's not yet such an approach for kthreads.)
133
134Architectures which don't have HAVE_RELIABLE_STACKTRACE solely rely on
135the second approach. It's highly likely that some tasks may still be
136running with an old version of the function, until that function
137returns. In this case you would have to signal the tasks. This
138especially applies to kthreads. They may not be woken up and would need
139to be forced. See below for more information.
140
141Unless we can come up with another way to patch kthreads, architectures
142without HAVE_RELIABLE_STACKTRACE are not considered fully supported by
143the kernel livepatching.
144
145The /sys/kernel/livepatch/<patch>/transition file shows whether a patch
146is in transition. Only a single patch can be in transition at a given
147time. A patch can remain in transition indefinitely, if any of the tasks
148are stuck in the initial patch state.
149
150A transition can be reversed and effectively canceled by writing the
151opposite value to the /sys/kernel/livepatch/<patch>/enabled file while
152the transition is in progress. Then all the tasks will attempt to
153converge back to the original patch state.
154
155There's also a /proc/<pid>/patch_state file which can be used to
156determine which tasks are blocking completion of a patching operation.
157If a patch is in transition, this file shows 0 to indicate the task is
158unpatched and 1 to indicate it's patched. Otherwise, if no patch is in
159transition, it shows -1. Any tasks which are blocking the transition
160can be signaled with SIGSTOP and SIGCONT to force them to change their
161patched state. This may be harmful to the system though. Sending a fake signal
162to all remaining blocking tasks is a better alternative. No proper signal is
163actually delivered (there is no data in signal pending structures). Tasks are
164interrupted or woken up, and forced to change their patched state. The fake
165signal is automatically sent every 15 seconds.
166
167Administrator can also affect a transition through
168/sys/kernel/livepatch/<patch>/force attribute. Writing 1 there clears
169TIF_PATCH_PENDING flag of all tasks and thus forces the tasks to the patched
170state. Important note! The force attribute is intended for cases when the
171transition gets stuck for a long time because of a blocking task. Administrator
172is expected to collect all necessary data (namely stack traces of such blocking
173tasks) and request a clearance from a patch distributor to force the transition.
174Unauthorized usage may cause harm to the system. It depends on the nature of the
175patch, which functions are (un)patched, and which functions the blocking tasks
176are sleeping in (/proc/<pid>/stack may help here). Removal (rmmod) of patch
177modules is permanently disabled when the force feature is used. It cannot be
178guaranteed there is no task sleeping in such module. It implies unbounded
179reference count if a patch module is disabled and enabled in a loop.
180
181Moreover, the usage of force may also affect future applications of live
182patches and cause even more harm to the system. Administrator should first
183consider to simply cancel a transition (see above). If force is used, reboot
184should be planned and no more live patches applied.
185
1863.1 Adding consistency model support to new architectures
187---------------------------------------------------------
188
189For adding consistency model support to new architectures, there are a
190few options:
191
1921) Add CONFIG_HAVE_RELIABLE_STACKTRACE. This means porting objtool, and
193 for non-DWARF unwinders, also making sure there's a way for the stack
194 tracing code to detect interrupts on the stack.
195
1962) Alternatively, ensure that every kthread has a call to
197 klp_update_patch_state() in a safe location. Kthreads are typically
198 in an infinite loop which does some action repeatedly. The safe
199 location to switch the kthread's patch state would be at a designated
200 point in the loop where there are no locks taken and all data
201 structures are in a well-defined state.
202
203 The location is clear when using workqueues or the kthread worker
204 API. These kthreads process independent actions in a generic loop.
205
206 It's much more complicated with kthreads which have a custom loop.
207 There the safe location must be carefully selected on a case-by-case
208 basis.
209
210 In that case, arches without HAVE_RELIABLE_STACKTRACE would still be
211 able to use the non-stack-checking parts of the consistency model:
212
213 a) patching user tasks when they cross the kernel/user space
214 boundary; and
215
216 b) patching kthreads and idle tasks at their designated patch points.
217
218 This option isn't as good as option 1 because it requires signaling
219 user tasks and waking kthreads to patch them. But it could still be
220 a good backup option for those architectures which don't have
221 reliable stack traces yet.
222
223
2244. Livepatch module
225===================
226
227Livepatches are distributed using kernel modules, see
228samples/livepatch/livepatch-sample.c.
229
230The module includes a new implementation of functions that we want
231to replace. In addition, it defines some structures describing the
232relation between the original and the new implementation. Then there
233is code that makes the kernel start using the new code when the livepatch
234module is loaded. Also there is code that cleans up before the
235livepatch module is removed. All this is explained in more details in
236the next sections.
237
238
2394.1. New functions
240------------------
241
242New versions of functions are typically just copied from the original
243sources. A good practice is to add a prefix to the names so that they
244can be distinguished from the original ones, e.g. in a backtrace. Also
245they can be declared as static because they are not called directly
246and do not need the global visibility.
247
248The patch contains only functions that are really modified. But they
249might want to access functions or data from the original source file
250that may only be locally accessible. This can be solved by a special
251relocation section in the generated livepatch module, see
252Documentation/livepatch/module-elf-format.rst for more details.
253
254
2554.2. Metadata
256-------------
257
258The patch is described by several structures that split the information
259into three levels:
260
261 - struct klp_func is defined for each patched function. It describes
262 the relation between the original and the new implementation of a
263 particular function.
264
265 The structure includes the name, as a string, of the original function.
266 The function address is found via kallsyms at runtime.
267
268 Then it includes the address of the new function. It is defined
269 directly by assigning the function pointer. Note that the new
270 function is typically defined in the same source file.
271
272 As an optional parameter, the symbol position in the kallsyms database can
273 be used to disambiguate functions of the same name. This is not the
274 absolute position in the database, but rather the order it has been found
275 only for a particular object ( vmlinux or a kernel module ). Note that
276 kallsyms allows for searching symbols according to the object name.
277
278 - struct klp_object defines an array of patched functions (struct
279 klp_func) in the same object. Where the object is either vmlinux
280 (NULL) or a module name.
281
282 The structure helps to group and handle functions for each object
283 together. Note that patched modules might be loaded later than
284 the patch itself and the relevant functions might be patched
285 only when they are available.
286
287
288 - struct klp_patch defines an array of patched objects (struct
289 klp_object).
290
291 This structure handles all patched functions consistently and eventually,
292 synchronously. The whole patch is applied only when all patched
293 symbols are found. The only exception are symbols from objects
294 (kernel modules) that have not been loaded yet.
295
296 For more details on how the patch is applied on a per-task basis,
297 see the "Consistency model" section.
298
299
3005. Livepatch life-cycle
301=======================
302
303Livepatching can be described by five basic operations:
304loading, enabling, replacing, disabling, removing.
305
306Where the replacing and the disabling operations are mutually
307exclusive. They have the same result for the given patch but
308not for the system.
309
310
3115.1. Loading
312------------
313
314The only reasonable way is to enable the patch when the livepatch kernel
315module is being loaded. For this, klp_enable_patch() has to be called
316in the module_init() callback. There are two main reasons:
317
318First, only the module has an easy access to the related struct klp_patch.
319
320Second, the error code might be used to refuse loading the module when
321the patch cannot get enabled.
322
323
3245.2. Enabling
325-------------
326
327The livepatch gets enabled by calling klp_enable_patch() from
328the module_init() callback. The system will start using the new
329implementation of the patched functions at this stage.
330
331First, the addresses of the patched functions are found according to their
332names. The special relocations, mentioned in the section "New functions",
333are applied. The relevant entries are created under
334/sys/kernel/livepatch/<name>. The patch is rejected when any above
335operation fails.
336
337Second, livepatch enters into a transition state where tasks are converging
338to the patched state. If an original function is patched for the first
339time, a function specific struct klp_ops is created and an universal
340ftrace handler is registered\ [#]_. This stage is indicated by a value of '1'
341in /sys/kernel/livepatch/<name>/transition. For more information about
342this process, see the "Consistency model" section.
343
344Finally, once all tasks have been patched, the 'transition' value changes
345to '0'.
346
347.. [#]
348
349 Note that functions might be patched multiple times. The ftrace handler
350 is registered only once for a given function. Further patches just add
351 an entry to the list (see field `func_stack`) of the struct klp_ops.
352 The right implementation is selected by the ftrace handler, see
353 the "Consistency model" section.
354
355 That said, it is highly recommended to use cumulative livepatches
356 because they help keeping the consistency of all changes. In this case,
357 functions might be patched two times only during the transition period.
358
359
3605.3. Replacing
361--------------
362
363All enabled patches might get replaced by a cumulative patch that
364has the .replace flag set.
365
366Once the new patch is enabled and the 'transition' finishes then
367all the functions (struct klp_func) associated with the replaced
368patches are removed from the corresponding struct klp_ops. Also
369the ftrace handler is unregistered and the struct klp_ops is
370freed when the related function is not modified by the new patch
371and func_stack list becomes empty.
372
373See Documentation/livepatch/cumulative-patches.rst for more details.
374
375
3765.4. Disabling
377--------------
378
379Enabled patches might get disabled by writing '0' to
380/sys/kernel/livepatch/<name>/enabled.
381
382First, livepatch enters into a transition state where tasks are converging
383to the unpatched state. The system starts using either the code from
384the previously enabled patch or even the original one. This stage is
385indicated by a value of '1' in /sys/kernel/livepatch/<name>/transition.
386For more information about this process, see the "Consistency model"
387section.
388
389Second, once all tasks have been unpatched, the 'transition' value changes
390to '0'. All the functions (struct klp_func) associated with the to-be-disabled
391patch are removed from the corresponding struct klp_ops. The ftrace handler
392is unregistered and the struct klp_ops is freed when the func_stack list
393becomes empty.
394
395Third, the sysfs interface is destroyed.
396
397
3985.5. Removing
399-------------
400
401Module removal is only safe when there are no users of functions provided
402by the module. This is the reason why the force feature permanently
403disables the removal. Only when the system is successfully transitioned
404to a new patch state (patched/unpatched) without being forced it is
405guaranteed that no task sleeps or runs in the old code.
406
407
4086. Sysfs
409========
410
411Information about the registered patches can be found under
412/sys/kernel/livepatch. The patches could be enabled and disabled
413by writing there.
414
415/sys/kernel/livepatch/<patch>/force attributes allow administrator to affect a
416patching operation.
417
418See Documentation/ABI/testing/sysfs-kernel-livepatch for more details.
419
420
4217. Limitations
422==============
423
424The current Livepatch implementation has several limitations:
425
426 - Only functions that can be traced could be patched.
427
428 Livepatch is based on the dynamic ftrace. In particular, functions
429 implementing ftrace or the livepatch ftrace handler could not be
430 patched. Otherwise, the code would end up in an infinite loop. A
431 potential mistake is prevented by marking the problematic functions
432 by "notrace".
433
434
435
436 - Livepatch works reliably only when the dynamic ftrace is located at
437 the very beginning of the function.
438
439 The function need to be redirected before the stack or the function
440 parameters are modified in any way. For example, livepatch requires
441 using -fentry gcc compiler option on x86_64.
442
443 One exception is the PPC port. It uses relative addressing and TOC.
444 Each function has to handle TOC and save LR before it could call
445 the ftrace handler. This operation has to be reverted on return.
446 Fortunately, the generic ftrace code has the same problem and all
447 this is handled on the ftrace level.
448
449
450 - Kretprobes using the ftrace framework conflict with the patched
451 functions.
452
453 Both kretprobes and livepatches use a ftrace handler that modifies
454 the return address. The first user wins. Either the probe or the patch
455 is rejected when the handler is already in use by the other.
456
457
458 - Kprobes in the original function are ignored when the code is
459 redirected to the new implementation.
460
461 There is a work in progress to add warnings about this situation.
1=========
2Livepatch
3=========
4
5This document outlines basic information about kernel livepatching.
6
7.. Table of Contents:
8
9.. contents:: :local:
10
11
121. Motivation
13=============
14
15There are many situations where users are reluctant to reboot a system. It may
16be because their system is performing complex scientific computations or under
17heavy load during peak usage. In addition to keeping systems up and running,
18users want to also have a stable and secure system. Livepatching gives users
19both by allowing for function calls to be redirected; thus, fixing critical
20functions without a system reboot.
21
22
232. Kprobes, Ftrace, Livepatching
24================================
25
26There are multiple mechanisms in the Linux kernel that are directly related
27to redirection of code execution; namely: kernel probes, function tracing,
28and livepatching:
29
30 - The kernel probes are the most generic. The code can be redirected by
31 putting a breakpoint instruction instead of any instruction.
32
33 - The function tracer calls the code from a predefined location that is
34 close to the function entry point. This location is generated by the
35 compiler using the '-pg' gcc option.
36
37 - Livepatching typically needs to redirect the code at the very beginning
38 of the function entry before the function parameters or the stack
39 are in any way modified.
40
41All three approaches need to modify the existing code at runtime. Therefore
42they need to be aware of each other and not step over each other's toes.
43Most of these problems are solved by using the dynamic ftrace framework as
44a base. A Kprobe is registered as a ftrace handler when the function entry
45is probed, see CONFIG_KPROBES_ON_FTRACE. Also an alternative function from
46a live patch is called with the help of a custom ftrace handler. But there are
47some limitations, see below.
48
49
503. Consistency model
51====================
52
53Functions are there for a reason. They take some input parameters, get or
54release locks, read, process, and even write some data in a defined way,
55have return values. In other words, each function has a defined semantic.
56
57Many fixes do not change the semantic of the modified functions. For
58example, they add a NULL pointer or a boundary check, fix a race by adding
59a missing memory barrier, or add some locking around a critical section.
60Most of these changes are self contained and the function presents itself
61the same way to the rest of the system. In this case, the functions might
62be updated independently one by one.
63
64But there are more complex fixes. For example, a patch might change
65ordering of locking in multiple functions at the same time. Or a patch
66might exchange meaning of some temporary structures and update
67all the relevant functions. In this case, the affected unit
68(thread, whole kernel) need to start using all new versions of
69the functions at the same time. Also the switch must happen only
70when it is safe to do so, e.g. when the affected locks are released
71or no data are stored in the modified structures at the moment.
72
73The theory about how to apply functions a safe way is rather complex.
74The aim is to define a so-called consistency model. It attempts to define
75conditions when the new implementation could be used so that the system
76stays consistent.
77
78Livepatch has a consistency model which is a hybrid of kGraft and
79kpatch: it uses kGraft's per-task consistency and syscall barrier
80switching combined with kpatch's stack trace switching. There are also
81a number of fallback options which make it quite flexible.
82
83Patches are applied on a per-task basis, when the task is deemed safe to
84switch over. When a patch is enabled, livepatch enters into a
85transition state where tasks are converging to the patched state.
86Usually this transition state can complete in a few seconds. The same
87sequence occurs when a patch is disabled, except the tasks converge from
88the patched state to the unpatched state.
89
90An interrupt handler inherits the patched state of the task it
91interrupts. The same is true for forked tasks: the child inherits the
92patched state of the parent.
93
94Livepatch uses several complementary approaches to determine when it's
95safe to patch tasks:
96
971. The first and most effective approach is stack checking of sleeping
98 tasks. If no affected functions are on the stack of a given task,
99 the task is patched. In most cases this will patch most or all of
100 the tasks on the first try. Otherwise it'll keep trying
101 periodically. This option is only available if the architecture has
102 reliable stacks (HAVE_RELIABLE_STACKTRACE).
103
1042. The second approach, if needed, is kernel exit switching. A
105 task is switched when it returns to user space from a system call, a
106 user space IRQ, or a signal. It's useful in the following cases:
107
108 a) Patching I/O-bound user tasks which are sleeping on an affected
109 function. In this case you have to send SIGSTOP and SIGCONT to
110 force it to exit the kernel and be patched.
111 b) Patching CPU-bound user tasks. If the task is highly CPU-bound
112 then it will get patched the next time it gets interrupted by an
113 IRQ.
114
1153. For idle "swapper" tasks, since they don't ever exit the kernel, they
116 instead have a klp_update_patch_state() call in the idle loop which
117 allows them to be patched before the CPU enters the idle state.
118
119 (Note there's not yet such an approach for kthreads.)
120
121Architectures which don't have HAVE_RELIABLE_STACKTRACE solely rely on
122the second approach. It's highly likely that some tasks may still be
123running with an old version of the function, until that function
124returns. In this case you would have to signal the tasks. This
125especially applies to kthreads. They may not be woken up and would need
126to be forced. See below for more information.
127
128Unless we can come up with another way to patch kthreads, architectures
129without HAVE_RELIABLE_STACKTRACE are not considered fully supported by
130the kernel livepatching.
131
132The /sys/kernel/livepatch/<patch>/transition file shows whether a patch
133is in transition. Only a single patch can be in transition at a given
134time. A patch can remain in transition indefinitely, if any of the tasks
135are stuck in the initial patch state.
136
137A transition can be reversed and effectively canceled by writing the
138opposite value to the /sys/kernel/livepatch/<patch>/enabled file while
139the transition is in progress. Then all the tasks will attempt to
140converge back to the original patch state.
141
142There's also a /proc/<pid>/patch_state file which can be used to
143determine which tasks are blocking completion of a patching operation.
144If a patch is in transition, this file shows 0 to indicate the task is
145unpatched and 1 to indicate it's patched. Otherwise, if no patch is in
146transition, it shows -1. Any tasks which are blocking the transition
147can be signaled with SIGSTOP and SIGCONT to force them to change their
148patched state. This may be harmful to the system though. Sending a fake signal
149to all remaining blocking tasks is a better alternative. No proper signal is
150actually delivered (there is no data in signal pending structures). Tasks are
151interrupted or woken up, and forced to change their patched state. The fake
152signal is automatically sent every 15 seconds.
153
154Administrator can also affect a transition through
155/sys/kernel/livepatch/<patch>/force attribute. Writing 1 there clears
156TIF_PATCH_PENDING flag of all tasks and thus forces the tasks to the patched
157state. Important note! The force attribute is intended for cases when the
158transition gets stuck for a long time because of a blocking task. Administrator
159is expected to collect all necessary data (namely stack traces of such blocking
160tasks) and request a clearance from a patch distributor to force the transition.
161Unauthorized usage may cause harm to the system. It depends on the nature of the
162patch, which functions are (un)patched, and which functions the blocking tasks
163are sleeping in (/proc/<pid>/stack may help here). Removal (rmmod) of patch
164modules is permanently disabled when the force feature is used. It cannot be
165guaranteed there is no task sleeping in such module. It implies unbounded
166reference count if a patch module is disabled and enabled in a loop.
167
168Moreover, the usage of force may also affect future applications of live
169patches and cause even more harm to the system. Administrator should first
170consider to simply cancel a transition (see above). If force is used, reboot
171should be planned and no more live patches applied.
172
1733.1 Adding consistency model support to new architectures
174---------------------------------------------------------
175
176For adding consistency model support to new architectures, there are a
177few options:
178
1791) Add CONFIG_HAVE_RELIABLE_STACKTRACE. This means porting objtool, and
180 for non-DWARF unwinders, also making sure there's a way for the stack
181 tracing code to detect interrupts on the stack.
182
1832) Alternatively, ensure that every kthread has a call to
184 klp_update_patch_state() in a safe location. Kthreads are typically
185 in an infinite loop which does some action repeatedly. The safe
186 location to switch the kthread's patch state would be at a designated
187 point in the loop where there are no locks taken and all data
188 structures are in a well-defined state.
189
190 The location is clear when using workqueues or the kthread worker
191 API. These kthreads process independent actions in a generic loop.
192
193 It's much more complicated with kthreads which have a custom loop.
194 There the safe location must be carefully selected on a case-by-case
195 basis.
196
197 In that case, arches without HAVE_RELIABLE_STACKTRACE would still be
198 able to use the non-stack-checking parts of the consistency model:
199
200 a) patching user tasks when they cross the kernel/user space
201 boundary; and
202
203 b) patching kthreads and idle tasks at their designated patch points.
204
205 This option isn't as good as option 1 because it requires signaling
206 user tasks and waking kthreads to patch them. But it could still be
207 a good backup option for those architectures which don't have
208 reliable stack traces yet.
209
210
2114. Livepatch module
212===================
213
214Livepatches are distributed using kernel modules, see
215samples/livepatch/livepatch-sample.c.
216
217The module includes a new implementation of functions that we want
218to replace. In addition, it defines some structures describing the
219relation between the original and the new implementation. Then there
220is code that makes the kernel start using the new code when the livepatch
221module is loaded. Also there is code that cleans up before the
222livepatch module is removed. All this is explained in more details in
223the next sections.
224
225
2264.1. New functions
227------------------
228
229New versions of functions are typically just copied from the original
230sources. A good practice is to add a prefix to the names so that they
231can be distinguished from the original ones, e.g. in a backtrace. Also
232they can be declared as static because they are not called directly
233and do not need the global visibility.
234
235The patch contains only functions that are really modified. But they
236might want to access functions or data from the original source file
237that may only be locally accessible. This can be solved by a special
238relocation section in the generated livepatch module, see
239Documentation/livepatch/module-elf-format.rst for more details.
240
241
2424.2. Metadata
243-------------
244
245The patch is described by several structures that split the information
246into three levels:
247
248 - struct klp_func is defined for each patched function. It describes
249 the relation between the original and the new implementation of a
250 particular function.
251
252 The structure includes the name, as a string, of the original function.
253 The function address is found via kallsyms at runtime.
254
255 Then it includes the address of the new function. It is defined
256 directly by assigning the function pointer. Note that the new
257 function is typically defined in the same source file.
258
259 As an optional parameter, the symbol position in the kallsyms database can
260 be used to disambiguate functions of the same name. This is not the
261 absolute position in the database, but rather the order it has been found
262 only for a particular object ( vmlinux or a kernel module ). Note that
263 kallsyms allows for searching symbols according to the object name.
264
265 - struct klp_object defines an array of patched functions (struct
266 klp_func) in the same object. Where the object is either vmlinux
267 (NULL) or a module name.
268
269 The structure helps to group and handle functions for each object
270 together. Note that patched modules might be loaded later than
271 the patch itself and the relevant functions might be patched
272 only when they are available.
273
274
275 - struct klp_patch defines an array of patched objects (struct
276 klp_object).
277
278 This structure handles all patched functions consistently and eventually,
279 synchronously. The whole patch is applied only when all patched
280 symbols are found. The only exception are symbols from objects
281 (kernel modules) that have not been loaded yet.
282
283 For more details on how the patch is applied on a per-task basis,
284 see the "Consistency model" section.
285
286
2875. Livepatch life-cycle
288=======================
289
290Livepatching can be described by five basic operations:
291loading, enabling, replacing, disabling, removing.
292
293Where the replacing and the disabling operations are mutually
294exclusive. They have the same result for the given patch but
295not for the system.
296
297
2985.1. Loading
299------------
300
301The only reasonable way is to enable the patch when the livepatch kernel
302module is being loaded. For this, klp_enable_patch() has to be called
303in the module_init() callback. There are two main reasons:
304
305First, only the module has an easy access to the related struct klp_patch.
306
307Second, the error code might be used to refuse loading the module when
308the patch cannot get enabled.
309
310
3115.2. Enabling
312-------------
313
314The livepatch gets enabled by calling klp_enable_patch() from
315the module_init() callback. The system will start using the new
316implementation of the patched functions at this stage.
317
318First, the addresses of the patched functions are found according to their
319names. The special relocations, mentioned in the section "New functions",
320are applied. The relevant entries are created under
321/sys/kernel/livepatch/<name>. The patch is rejected when any above
322operation fails.
323
324Second, livepatch enters into a transition state where tasks are converging
325to the patched state. If an original function is patched for the first
326time, a function specific struct klp_ops is created and an universal
327ftrace handler is registered\ [#]_. This stage is indicated by a value of '1'
328in /sys/kernel/livepatch/<name>/transition. For more information about
329this process, see the "Consistency model" section.
330
331Finally, once all tasks have been patched, the 'transition' value changes
332to '0'.
333
334.. [#]
335
336 Note that functions might be patched multiple times. The ftrace handler
337 is registered only once for a given function. Further patches just add
338 an entry to the list (see field `func_stack`) of the struct klp_ops.
339 The right implementation is selected by the ftrace handler, see
340 the "Consistency model" section.
341
342 That said, it is highly recommended to use cumulative livepatches
343 because they help keeping the consistency of all changes. In this case,
344 functions might be patched two times only during the transition period.
345
346
3475.3. Replacing
348--------------
349
350All enabled patches might get replaced by a cumulative patch that
351has the .replace flag set.
352
353Once the new patch is enabled and the 'transition' finishes then
354all the functions (struct klp_func) associated with the replaced
355patches are removed from the corresponding struct klp_ops. Also
356the ftrace handler is unregistered and the struct klp_ops is
357freed when the related function is not modified by the new patch
358and func_stack list becomes empty.
359
360See Documentation/livepatch/cumulative-patches.rst for more details.
361
362
3635.4. Disabling
364--------------
365
366Enabled patches might get disabled by writing '0' to
367/sys/kernel/livepatch/<name>/enabled.
368
369First, livepatch enters into a transition state where tasks are converging
370to the unpatched state. The system starts using either the code from
371the previously enabled patch or even the original one. This stage is
372indicated by a value of '1' in /sys/kernel/livepatch/<name>/transition.
373For more information about this process, see the "Consistency model"
374section.
375
376Second, once all tasks have been unpatched, the 'transition' value changes
377to '0'. All the functions (struct klp_func) associated with the to-be-disabled
378patch are removed from the corresponding struct klp_ops. The ftrace handler
379is unregistered and the struct klp_ops is freed when the func_stack list
380becomes empty.
381
382Third, the sysfs interface is destroyed.
383
384
3855.5. Removing
386-------------
387
388Module removal is only safe when there are no users of functions provided
389by the module. This is the reason why the force feature permanently
390disables the removal. Only when the system is successfully transitioned
391to a new patch state (patched/unpatched) without being forced it is
392guaranteed that no task sleeps or runs in the old code.
393
394
3956. Sysfs
396========
397
398Information about the registered patches can be found under
399/sys/kernel/livepatch. The patches could be enabled and disabled
400by writing there.
401
402/sys/kernel/livepatch/<patch>/force attributes allow administrator to affect a
403patching operation.
404
405See Documentation/ABI/testing/sysfs-kernel-livepatch for more details.
406
407
4087. Limitations
409==============
410
411The current Livepatch implementation has several limitations:
412
413 - Only functions that can be traced could be patched.
414
415 Livepatch is based on the dynamic ftrace. In particular, functions
416 implementing ftrace or the livepatch ftrace handler could not be
417 patched. Otherwise, the code would end up in an infinite loop. A
418 potential mistake is prevented by marking the problematic functions
419 by "notrace".
420
421
422
423 - Livepatch works reliably only when the dynamic ftrace is located at
424 the very beginning of the function.
425
426 The function need to be redirected before the stack or the function
427 parameters are modified in any way. For example, livepatch requires
428 using -fentry gcc compiler option on x86_64.
429
430 One exception is the PPC port. It uses relative addressing and TOC.
431 Each function has to handle TOC and save LR before it could call
432 the ftrace handler. This operation has to be reverted on return.
433 Fortunately, the generic ftrace code has the same problem and all
434 this is handled on the ftrace level.
435
436
437 - Kretprobes using the ftrace framework conflict with the patched
438 functions.
439
440 Both kretprobes and livepatches use a ftrace handler that modifies
441 the return address. The first user wins. Either the probe or the patch
442 is rejected when the handler is already in use by the other.
443
444
445 - Kprobes in the original function are ignored when the code is
446 redirected to the new implementation.
447
448 There is a work in progress to add warnings about this situation.