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.