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1 | ========= |
2 | Livepatch | |
3 | ========= | |
4 | ||
5 | This document outlines basic information about kernel livepatching. | |
6 | ||
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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 | |
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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 | ||
89e33ea7 | 43 | - The kernel probes are the most generic. The code can be redirected by |
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44 | putting a breakpoint instruction instead of any instruction. |
45 | ||
89e33ea7 | 46 | - The function tracer calls the code from a predefined location that is |
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47 | close to the function entry point. This location is generated by the |
48 | compiler using the '-pg' gcc option. | |
49 | ||
89e33ea7 | 50 | - Livepatching typically needs to redirect the code at the very beginning |
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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 | |
d0807da7 | 75 | be updated independently one by one. |
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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 | |
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89 | stays consistent. |
90 | ||
91 | Livepatch has a consistency model which is a hybrid of kGraft and | |
92 | kpatch: it uses kGraft's per-task consistency and syscall barrier | |
93 | switching combined with kpatch's stack trace switching. There are also | |
94 | a number of fallback options which make it quite flexible. | |
95 | ||
96 | Patches are applied on a per-task basis, when the task is deemed safe to | |
97 | switch over. When a patch is enabled, livepatch enters into a | |
98 | transition state where tasks are converging to the patched state. | |
99 | Usually this transition state can complete in a few seconds. The same | |
100 | sequence occurs when a patch is disabled, except the tasks converge from | |
101 | the patched state to the unpatched state. | |
102 | ||
103 | An interrupt handler inherits the patched state of the task it | |
104 | interrupts. The same is true for forked tasks: the child inherits the | |
105 | patched state of the parent. | |
106 | ||
107 | Livepatch uses several complementary approaches to determine when it's | |
108 | safe to patch tasks: | |
109 | ||
110 | 1. 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 | ||
117 | 2. 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. | |
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127 | |
128 | 3. 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 | ||
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134 | Architectures which don't have HAVE_RELIABLE_STACKTRACE solely rely on |
135 | the second approach. It's highly likely that some tasks may still be | |
136 | running with an old version of the function, until that function | |
137 | returns. In this case you would have to signal the tasks. This | |
138 | especially applies to kthreads. They may not be woken up and would need | |
139 | to be forced. See below for more information. | |
d83a7cb3 | 140 | |
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141 | Unless we can come up with another way to patch kthreads, architectures |
142 | without HAVE_RELIABLE_STACKTRACE are not considered fully supported by | |
143 | the kernel livepatching. | |
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144 | |
145 | The /sys/kernel/livepatch/<patch>/transition file shows whether a patch | |
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146 | is in transition. Only a single patch can be in transition at a given |
147 | time. A patch can remain in transition indefinitely, if any of the tasks | |
148 | are stuck in the initial patch state. | |
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149 | |
150 | A transition can be reversed and effectively canceled by writing the | |
151 | opposite value to the /sys/kernel/livepatch/<patch>/enabled file while | |
152 | the transition is in progress. Then all the tasks will attempt to | |
153 | converge back to the original patch state. | |
154 | ||
155 | There's also a /proc/<pid>/patch_state file which can be used to | |
156 | determine which tasks are blocking completion of a patching operation. | |
157 | If a patch is in transition, this file shows 0 to indicate the task is | |
158 | unpatched and 1 to indicate it's patched. Otherwise, if no patch is in | |
159 | transition, it shows -1. Any tasks which are blocking the transition | |
160 | can be signaled with SIGSTOP and SIGCONT to force them to change their | |
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161 | patched state. This may be harmful to the system though. Sending a fake signal |
162 | to all remaining blocking tasks is a better alternative. No proper signal is | |
163 | actually delivered (there is no data in signal pending structures). Tasks are | |
164 | interrupted or woken up, and forced to change their patched state. The fake | |
165 | signal is automatically sent every 15 seconds. | |
d83a7cb3 | 166 | |
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167 | Administrator can also affect a transition through |
168 | /sys/kernel/livepatch/<patch>/force attribute. Writing 1 there clears | |
169 | TIF_PATCH_PENDING flag of all tasks and thus forces the tasks to the patched | |
170 | state. Important note! The force attribute is intended for cases when the | |
171 | transition gets stuck for a long time because of a blocking task. Administrator | |
172 | is expected to collect all necessary data (namely stack traces of such blocking | |
173 | tasks) and request a clearance from a patch distributor to force the transition. | |
174 | Unauthorized usage may cause harm to the system. It depends on the nature of the | |
175 | patch, which functions are (un)patched, and which functions the blocking tasks | |
176 | are sleeping in (/proc/<pid>/stack may help here). Removal (rmmod) of patch | |
177 | modules is permanently disabled when the force feature is used. It cannot be | |
178 | guaranteed there is no task sleeping in such module. It implies unbounded | |
179 | reference count if a patch module is disabled and enabled in a loop. | |
180 | ||
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181 | Moreover, the usage of force may also affect future applications of live |
182 | patches and cause even more harm to the system. Administrator should first | |
183 | consider to simply cancel a transition (see above). If force is used, reboot | |
184 | should be planned and no more live patches applied. | |
185 | ||
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186 | 3.1 Adding consistency model support to new architectures |
187 | --------------------------------------------------------- | |
188 | ||
189 | For adding consistency model support to new architectures, there are a | |
190 | few options: | |
191 | ||
192 | 1) 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 | ||
196 | 2) 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 | ||
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223 | |
224 | 4. Livepatch module | |
225 | =================== | |
226 | ||
227 | Livepatches are distributed using kernel modules, see | |
228 | samples/livepatch/livepatch-sample.c. | |
229 | ||
230 | The module includes a new implementation of functions that we want | |
231 | to replace. In addition, it defines some structures describing the | |
232 | relation between the original and the new implementation. Then there | |
233 | is code that makes the kernel start using the new code when the livepatch | |
234 | module is loaded. Also there is code that cleans up before the | |
235 | livepatch module is removed. All this is explained in more details in | |
236 | the next sections. | |
237 | ||
238 | ||
239 | 4.1. New functions | |
240 | ------------------ | |
241 | ||
242 | New versions of functions are typically just copied from the original | |
243 | sources. A good practice is to add a prefix to the names so that they | |
244 | can be distinguished from the original ones, e.g. in a backtrace. Also | |
245 | they can be declared as static because they are not called directly | |
246 | and do not need the global visibility. | |
247 | ||
248 | The patch contains only functions that are really modified. But they | |
249 | might want to access functions or data from the original source file | |
250 | that may only be locally accessible. This can be solved by a special | |
251 | relocation section in the generated livepatch module, see | |
89e33ea7 | 252 | Documentation/livepatch/module-elf-format.rst for more details. |
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253 | |
254 | ||
255 | 4.2. Metadata | |
d83a7cb3 | 256 | ------------- |
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257 | |
258 | The patch is described by several structures that split the information | |
259 | into three levels: | |
260 | ||
89e33ea7 | 261 | - struct klp_func is defined for each patched function. It describes |
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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 | ||
89e33ea7 | 278 | - struct klp_object defines an array of patched functions (struct |
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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 | ||
89e33ea7 | 288 | - struct klp_patch defines an array of patched objects (struct |
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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 | |
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294 | (kernel modules) that have not been loaded yet. |
295 | ||
d83a7cb3 JP |
296 | For more details on how the patch is applied on a per-task basis, |
297 | see the "Consistency model" section. | |
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298 | |
299 | ||
5e4e3844 PM |
300 | 5. Livepatch life-cycle |
301 | ======================= | |
302 | ||
e1452b60 JB |
303 | Livepatching can be described by five basic operations: |
304 | loading, enabling, replacing, disabling, removing. | |
305 | ||
306 | Where the replacing and the disabling operations are mutually | |
307 | exclusive. They have the same result for the given patch but | |
308 | not for the system. | |
5e4e3844 | 309 | |
5e4e3844 | 310 | |
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311 | 5.1. Loading |
312 | ------------ | |
5e4e3844 | 313 | |
958ef1e3 PM |
314 | The only reasonable way is to enable the patch when the livepatch kernel |
315 | module is being loaded. For this, klp_enable_patch() has to be called | |
316 | in the module_init() callback. There are two main reasons: | |
5e4e3844 | 317 | |
958ef1e3 | 318 | First, only the module has an easy access to the related struct klp_patch. |
5e4e3844 | 319 | |
958ef1e3 PM |
320 | Second, the error code might be used to refuse loading the module when |
321 | the patch cannot get enabled. | |
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322 | |
323 | ||
324 | 5.2. Enabling | |
325 | ------------- | |
326 | ||
958ef1e3 PM |
327 | The livepatch gets enabled by calling klp_enable_patch() from |
328 | the module_init() callback. The system will start using the new | |
329 | implementation of the patched functions at this stage. | |
5e4e3844 | 330 | |
958ef1e3 PM |
331 | First, the addresses of the patched functions are found according to their |
332 | names. The special relocations, mentioned in the section "New functions", | |
333 | are applied. The relevant entries are created under | |
334 | /sys/kernel/livepatch/<name>. The patch is rejected when any above | |
335 | operation fails. | |
d83a7cb3 | 336 | |
958ef1e3 PM |
337 | Second, livepatch enters into a transition state where tasks are converging |
338 | to the patched state. If an original function is patched for the first | |
339 | time, a function specific struct klp_ops is created and an universal | |
89e33ea7 | 340 | ftrace handler is registered\ [#]_. This stage is indicated by a value of '1' |
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341 | in /sys/kernel/livepatch/<name>/transition. For more information about |
342 | this process, see the "Consistency model" section. | |
5e4e3844 | 343 | |
958ef1e3 PM |
344 | Finally, once all tasks have been patched, the 'transition' value changes |
345 | to '0'. | |
5e4e3844 | 346 | |
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347 | .. [#] |
348 | ||
349 | Note that functions might be patched multiple times. The ftrace handler | |
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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. | |
5e4e3844 | 354 | |
d67a5372 PM |
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 | ||
5e4e3844 | 359 | |
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360 | 5.3. Replacing |
361 | -------------- | |
362 | ||
363 | All enabled patches might get replaced by a cumulative patch that | |
364 | has the .replace flag set. | |
365 | ||
366 | Once the new patch is enabled and the 'transition' finishes then | |
367 | all the functions (struct klp_func) associated with the replaced | |
368 | patches are removed from the corresponding struct klp_ops. Also | |
369 | the ftrace handler is unregistered and the struct klp_ops is | |
370 | freed when the related function is not modified by the new patch | |
371 | and func_stack list becomes empty. | |
372 | ||
89e33ea7 | 373 | See Documentation/livepatch/cumulative-patches.rst for more details. |
c4e6874f | 374 | |
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375 | |
376 | 5.4. Disabling | |
5e4e3844 PM |
377 | -------------- |
378 | ||
958ef1e3 PM |
379 | Enabled patches might get disabled by writing '0' to |
380 | /sys/kernel/livepatch/<name>/enabled. | |
5e4e3844 | 381 | |
958ef1e3 PM |
382 | First, livepatch enters into a transition state where tasks are converging |
383 | to the unpatched state. The system starts using either the code from | |
384 | the previously enabled patch or even the original one. This stage is | |
385 | indicated by a value of '1' in /sys/kernel/livepatch/<name>/transition. | |
386 | For more information about this process, see the "Consistency model" | |
387 | section. | |
d83a7cb3 | 388 | |
958ef1e3 PM |
389 | Second, once all tasks have been unpatched, the 'transition' value changes |
390 | to '0'. All the functions (struct klp_func) associated with the to-be-disabled | |
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391 | patch are removed from the corresponding struct klp_ops. The ftrace handler |
392 | is unregistered and the struct klp_ops is freed when the func_stack list | |
393 | becomes empty. | |
394 | ||
958ef1e3 | 395 | Third, the sysfs interface is destroyed. |
5e4e3844 | 396 | |
5e4e3844 | 397 | |
e1452b60 | 398 | 5.5. Removing |
958ef1e3 | 399 | ------------- |
5e4e3844 | 400 | |
958ef1e3 PM |
401 | Module removal is only safe when there are no users of functions provided |
402 | by the module. This is the reason why the force feature permanently | |
403 | disables the removal. Only when the system is successfully transitioned | |
404 | to a new patch state (patched/unpatched) without being forced it is | |
405 | guaranteed that no task sleeps or runs in the old code. | |
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406 | |
407 | ||
408 | 6. Sysfs | |
409 | ======== | |
410 | ||
411 | Information about the registered patches can be found under | |
412 | /sys/kernel/livepatch. The patches could be enabled and disabled | |
413 | by writing there. | |
414 | ||
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415 | /sys/kernel/livepatch/<patch>/force attributes allow administrator to affect a |
416 | patching operation. | |
43347d56 | 417 | |
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418 | See Documentation/ABI/testing/sysfs-kernel-livepatch for more details. |
419 | ||
420 | ||
421 | 7. Limitations | |
422 | ============== | |
423 | ||
424 | The current Livepatch implementation has several limitations: | |
425 | ||
89e33ea7 | 426 | - Only functions that can be traced could be patched. |
5e4e3844 PM |
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 | ||
5e4e3844 | 435 | |
89e33ea7 | 436 | - Livepatch works reliably only when the dynamic ftrace is located at |
5e4e3844 PM |
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 | |
8da9704c | 447 | this is handled on the ftrace level. |
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448 | |
449 | ||
89e33ea7 | 450 | - Kretprobes using the ftrace framework conflict with the patched |
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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 | ||
89e33ea7 | 458 | - Kprobes in the original function are ignored when the code is |
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459 | redirected to the new implementation. |
460 | ||
461 | There is a work in progress to add warnings about this situation. |