Based on kernel version 4.15. Page generated on 2018-01-29 10:00 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. (This can be done by setting the 76 'immediate' flag in the klp_patch struct.) 77 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. 86 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. 91 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. 96 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. 103 104 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. 107 108 Livepatch uses several complementary approaches to determine when it's 109 safe to patch tasks: 110 111 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). 117 118 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: 121 122 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 133 HAVE_RELIABLE_STACKTRACE. 134 135 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. 138 139 (Note there's not yet such an approach for kthreads.) 140 141 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. 147 148 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. 153 154 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. 158 159 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. 162 163 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. 167 168 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. 172 173 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. 180 181 182 3.1 Adding consistency model support to new architectures 183 --------------------------------------------------------- 184 185 For adding consistency model support to new architectures, there are a 186 few options: 187 188 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. 191 192 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. 198 199 The location is clear when using workqueues or the kthread worker 200 API. These kthreads process independent actions in a generic loop. 201 202 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. 205 206 In that case, arches without HAVE_RELIABLE_STACKTRACE would still be 207 able to use the non-stack-checking parts of the consistency model: 208 209 a) patching user tasks when they cross the kernel/user space 210 boundary; and 211 212 b) patching kthreads and idle tasks at their designated patch points. 213 214 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. 218 219 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. 225 226 227 4. Livepatch module 228 =================== 229 230 Livepatches are distributed using kernel modules, see 231 samples/livepatch/livepatch-sample.c. 232 233 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. 240 241 242 4.1. New functions 243 ------------------ 244 245 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. 250 251 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. 256 257 258 4.2. Metadata 259 ------------- 260 261 The patch is described by several structures that split the information 262 into three levels: 263 264 + 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. 267 268 The structure includes the name, as a string, of the original function. 269 The function address is found via kallsyms at runtime. 270 271 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. 274 275 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. 280 281 There's also an 'immediate' flag which, when set, patches the 282 function immediately, bypassing the consistency model safety checks. 283 284 + 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. 287 288 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. 292 293 294 + struct klp_patch defines an array of patched objects (struct 295 klp_object). 296 297 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. 301 302 Setting the 'immediate' flag applies the patch to all tasks 303 immediately, bypassing the consistency model safety checks. 304 305 For more details on how the patch is applied on a per-task basis, 306 see the "Consistency model" section. 307 308 309 4.3. Livepatch module handling 310 ------------------------------ 311 312 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. 317 318 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. 327 328 329 5. Livepatch life-cycle 330 ======================= 331 332 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. 335 336 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. 339 340 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. 346 347 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. 353 354 355 5.1. Registration 356 ----------------- 357 358 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. 361 362 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. 368 369 370 5.2. Enabling 371 ------------- 372 373 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. 376 377 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. 382 383 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. 386 387 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. 392 393 Note that the patches might be enabled in a different order than they were 394 registered. 395 396 397 5.3. Disabling 398 -------------- 399 400 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. 404 405 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. 410 411 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. 415 416 Patches must be disabled in exactly the reverse order in which they were 417 enabled. It makes the problem and the implementation much easier. 418 419 420 5.4. Unregistration 421 ------------------- 422 423 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. 426 427 At this stage, all the relevant sys-fs entries are removed and the patch 428 is removed from the list of known patches. 429 430 431 6. Sysfs 432 ======== 433 434 Information about the registered patches can be found under 435 /sys/kernel/livepatch. The patches could be enabled and disabled 436 by writing there. 437 438 See Documentation/ABI/testing/sysfs-kernel-livepatch for more details. 439 440 441 7. Limitations 442 ============== 443 444 The current Livepatch implementation has several limitations: 445 446 447 + The patch must not change the semantic of the patched functions. 448 449 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. 453 454 455 + Data structures can not be patched. 456 457 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. 460 461 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. 465 466 There are no plans to add more generic support for modified structures 467 at the moment. 468 469 470 + Only functions that can be traced could be patched. 471 472 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". 477 478 479 480 + Livepatch works reliably only when the dynamic ftrace is located at 481 the very beginning of the function. 482 483 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. 486 487 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. 492 493 494 + Kretprobes using the ftrace framework conflict with the patched 495 functions. 496 497 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. 500 501 502 + Kprobes in the original function are ignored when the code is 503 redirected to the new implementation. 504 505 There is a work in progress to add warnings about this situation.