projects/linux·blob v4.12 · refs

579 lines · 15.5 KiB

1/*
2 * transition.c - Kernel Live Patching transition functions
3 *
4 * Copyright (C) 2015-2016 Josh Poimboeuf <[email protected]>
5 *
6 * This program is free software; you can redistribute it and/or
7 * modify it under the terms of the GNU General Public License
8 * as published by the Free Software Foundation; either version 2
9 * of the License, or (at your option) any later version.
10 *
11 * This program is distributed in the hope that it will be useful,
12 * but WITHOUT ANY WARRANTY; without even the implied warranty of
13 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
14 * GNU General Public License for more details.
15 *
16 * You should have received a copy of the GNU General Public License
17 * along with this program; if not, see <http://www.gnu.org/licenses/>.
18 */
19
20#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
21
22#include <linux/cpu.h>
23#include <linux/stacktrace.h>
24#include "core.h"
25#include "patch.h"
26#include "transition.h"
27#include "../sched/sched.h"
28
29#define MAX_STACK_ENTRIES 100
30#define STACK_ERR_BUF_SIZE 128
31
32struct klp_patch *klp_transition_patch;
33
34static int klp_target_state = KLP_UNDEFINED;
35
36/*
37 * This work can be performed periodically to finish patching or unpatching any
38 * "straggler" tasks which failed to transition in the first attempt.
39 */
40static void klp_transition_work_fn(struct work_struct *work)
41{
42 mutex_lock(&klp_mutex);
43
44 if (klp_transition_patch)
45 klp_try_complete_transition();
46
47 mutex_unlock(&klp_mutex);
48}
49static DECLARE_DELAYED_WORK(klp_transition_work, klp_transition_work_fn);
50
51/*
52 * This function is just a stub to implement a hard force
53 * of synchronize_sched(). This requires synchronizing
54 * tasks even in userspace and idle.
55 */
56static void klp_sync(struct work_struct *work)
57{
58}
59
60/*
61 * We allow to patch also functions where RCU is not watching,
62 * e.g. before user_exit(). We can not rely on the RCU infrastructure
63 * to do the synchronization. Instead hard force the sched synchronization.
64 *
65 * This approach allows to use RCU functions for manipulating func_stack
66 * safely.
67 */
68static void klp_synchronize_transition(void)
69{
70 schedule_on_each_cpu(klp_sync);
71}
72
73/*
74 * The transition to the target patch state is complete. Clean up the data
75 * structures.
76 */
77static void klp_complete_transition(void)
78{
79 struct klp_object *obj;
80 struct klp_func *func;
81 struct task_struct *g, *task;
82 unsigned int cpu;
83 bool immediate_func = false;
84
85 if (klp_target_state == KLP_UNPATCHED) {
86 /*
87 * All tasks have transitioned to KLP_UNPATCHED so we can now
88 * remove the new functions from the func_stack.
89 */
90 klp_unpatch_objects(klp_transition_patch);
91
92 /*
93 * Make sure klp_ftrace_handler() can no longer see functions
94 * from this patch on the ops->func_stack. Otherwise, after
95 * func->transition gets cleared, the handler may choose a
96 * removed function.
97 */
98 klp_synchronize_transition();
99 }
100
101 if (klp_transition_patch->immediate)
102 goto done;
103
104 klp_for_each_object(klp_transition_patch, obj) {
105 klp_for_each_func(obj, func) {
106 func->transition = false;
107 if (func->immediate)
108 immediate_func = true;
109 }
110 }
111
112 if (klp_target_state == KLP_UNPATCHED && !immediate_func)
113 module_put(klp_transition_patch->mod);
114
115 /* Prevent klp_ftrace_handler() from seeing KLP_UNDEFINED state */
116 if (klp_target_state == KLP_PATCHED)
117 klp_synchronize_transition();
118
119 read_lock(&tasklist_lock);
120 for_each_process_thread(g, task) {
121 WARN_ON_ONCE(test_tsk_thread_flag(task, TIF_PATCH_PENDING));
122 task->patch_state = KLP_UNDEFINED;
123 }
124 read_unlock(&tasklist_lock);
125
126 for_each_possible_cpu(cpu) {
127 task = idle_task(cpu);
128 WARN_ON_ONCE(test_tsk_thread_flag(task, TIF_PATCH_PENDING));
129 task->patch_state = KLP_UNDEFINED;
130 }
131
132done:
133 klp_target_state = KLP_UNDEFINED;
134 klp_transition_patch = NULL;
135}
136
137/*
138 * This is called in the error path, to cancel a transition before it has
139 * started, i.e. klp_init_transition() has been called but
140 * klp_start_transition() hasn't. If the transition *has* been started,
141 * klp_reverse_transition() should be used instead.
142 */
143void klp_cancel_transition(void)
144{
145 if (WARN_ON_ONCE(klp_target_state != KLP_PATCHED))
146 return;
147
148 klp_target_state = KLP_UNPATCHED;
149 klp_complete_transition();
150}
151
152/*
153 * Switch the patched state of the task to the set of functions in the target
154 * patch state.
155 *
156 * NOTE: If task is not 'current', the caller must ensure the task is inactive.
157 * Otherwise klp_ftrace_handler() might read the wrong 'patch_state' value.
158 */
159void klp_update_patch_state(struct task_struct *task)
160{
161 /*
162 * A variant of synchronize_sched() is used to allow patching functions
163 * where RCU is not watching, see klp_synchronize_transition().
164 */
165 preempt_disable_notrace();
166
167 /*
168 * This test_and_clear_tsk_thread_flag() call also serves as a read
169 * barrier (smp_rmb) for two cases:
170 *
171 * 1) Enforce the order of the TIF_PATCH_PENDING read and the
172 * klp_target_state read. The corresponding write barrier is in
173 * klp_init_transition().
174 *
175 * 2) Enforce the order of the TIF_PATCH_PENDING read and a future read
176 * of func->transition, if klp_ftrace_handler() is called later on
177 * the same CPU. See __klp_disable_patch().
178 */
179 if (test_and_clear_tsk_thread_flag(task, TIF_PATCH_PENDING))
180 task->patch_state = READ_ONCE(klp_target_state);
181
182 preempt_enable_notrace();
183}
184
185/*
186 * Determine whether the given stack trace includes any references to a
187 * to-be-patched or to-be-unpatched function.
188 */
189static int klp_check_stack_func(struct klp_func *func,
190 struct stack_trace *trace)
191{
192 unsigned long func_addr, func_size, address;
193 struct klp_ops *ops;
194 int i;
195
196 if (func->immediate)
197 return 0;
198
199 for (i = 0; i < trace->nr_entries; i++) {
200 address = trace->entries[i];
201
202 if (klp_target_state == KLP_UNPATCHED) {
203 /*
204 * Check for the to-be-unpatched function
205 * (the func itself).
206 */
207 func_addr = (unsigned long)func->new_func;
208 func_size = func->new_size;
209 } else {
210 /*
211 * Check for the to-be-patched function
212 * (the previous func).
213 */
214 ops = klp_find_ops(func->old_addr);
215
216 if (list_is_singular(&ops->func_stack)) {
217 /* original function */
218 func_addr = func->old_addr;
219 func_size = func->old_size;
220 } else {
221 /* previously patched function */
222 struct klp_func *prev;
223
224 prev = list_next_entry(func, stack_node);
225 func_addr = (unsigned long)prev->new_func;
226 func_size = prev->new_size;
227 }
228 }
229
230 if (address >= func_addr && address < func_addr + func_size)
231 return -EAGAIN;
232 }
233
234 return 0;
235}
236
237/*
238 * Determine whether it's safe to transition the task to the target patch state
239 * by looking for any to-be-patched or to-be-unpatched functions on its stack.
240 */
241static int klp_check_stack(struct task_struct *task, char *err_buf)
242{
243 static unsigned long entries[MAX_STACK_ENTRIES];
244 struct stack_trace trace;
245 struct klp_object *obj;
246 struct klp_func *func;
247 int ret;
248
249 trace.skip = 0;
250 trace.nr_entries = 0;
251 trace.max_entries = MAX_STACK_ENTRIES;
252 trace.entries = entries;
253 ret = save_stack_trace_tsk_reliable(task, &trace);
254 WARN_ON_ONCE(ret == -ENOSYS);
255 if (ret) {
256 snprintf(err_buf, STACK_ERR_BUF_SIZE,
257 "%s: %s:%d has an unreliable stack\n",
258 __func__, task->comm, task->pid);
259 return ret;
260 }
261
262 klp_for_each_object(klp_transition_patch, obj) {
263 if (!obj->patched)
264 continue;
265 klp_for_each_func(obj, func) {
266 ret = klp_check_stack_func(func, &trace);
267 if (ret) {
268 snprintf(err_buf, STACK_ERR_BUF_SIZE,
269 "%s: %s:%d is sleeping on function %s\n",
270 __func__, task->comm, task->pid,
271 func->old_name);
272 return ret;
273 }
274 }
275 }
276
277 return 0;
278}
279
280/*
281 * Try to safely switch a task to the target patch state. If it's currently
282 * running, or it's sleeping on a to-be-patched or to-be-unpatched function, or
283 * if the stack is unreliable, return false.
284 */
285static bool klp_try_switch_task(struct task_struct *task)
286{
287 struct rq *rq;
288 struct rq_flags flags;
289 int ret;
290 bool success = false;
291 char err_buf[STACK_ERR_BUF_SIZE];
292
293 err_buf[0] = '\0';
294
295 /* check if this task has already switched over */
296 if (task->patch_state == klp_target_state)
297 return true;
298
299 /*
300 * For arches which don't have reliable stack traces, we have to rely
301 * on other methods (e.g., switching tasks at kernel exit).
302 */
303 if (!klp_have_reliable_stack())
304 return false;
305
306 /*
307 * Now try to check the stack for any to-be-patched or to-be-unpatched
308 * functions. If all goes well, switch the task to the target patch
309 * state.
310 */
311 rq = task_rq_lock(task, &flags);
312
313 if (task_running(rq, task) && task != current) {
314 snprintf(err_buf, STACK_ERR_BUF_SIZE,
315 "%s: %s:%d is running\n", __func__, task->comm,
316 task->pid);
317 goto done;
318 }
319
320 ret = klp_check_stack(task, err_buf);
321 if (ret)
322 goto done;
323
324 success = true;
325
326 clear_tsk_thread_flag(task, TIF_PATCH_PENDING);
327 task->patch_state = klp_target_state;
328
329done:
330 task_rq_unlock(rq, task, &flags);
331
332 /*
333 * Due to console deadlock issues, pr_debug() can't be used while
334 * holding the task rq lock. Instead we have to use a temporary buffer
335 * and print the debug message after releasing the lock.
336 */
337 if (err_buf[0] != '\0')
338 pr_debug("%s", err_buf);
339
340 return success;
341
342}
343
344/*
345 * Try to switch all remaining tasks to the target patch state by walking the
346 * stacks of sleeping tasks and looking for any to-be-patched or
347 * to-be-unpatched functions. If such functions are found, the task can't be
348 * switched yet.
349 *
350 * If any tasks are still stuck in the initial patch state, schedule a retry.
351 */
352void klp_try_complete_transition(void)
353{
354 unsigned int cpu;
355 struct task_struct *g, *task;
356 bool complete = true;
357
358 WARN_ON_ONCE(klp_target_state == KLP_UNDEFINED);
359
360 /*
361 * If the patch can be applied or reverted immediately, skip the
362 * per-task transitions.
363 */
364 if (klp_transition_patch->immediate)
365 goto success;
366
367 /*
368 * Try to switch the tasks to the target patch state by walking their
369 * stacks and looking for any to-be-patched or to-be-unpatched
370 * functions. If such functions are found on a stack, or if the stack
371 * is deemed unreliable, the task can't be switched yet.
372 *
373 * Usually this will transition most (or all) of the tasks on a system
374 * unless the patch includes changes to a very common function.
375 */
376 read_lock(&tasklist_lock);
377 for_each_process_thread(g, task)
378 if (!klp_try_switch_task(task))
379 complete = false;
380 read_unlock(&tasklist_lock);
381
382 /*
383 * Ditto for the idle "swapper" tasks.
384 */
385 get_online_cpus();
386 for_each_possible_cpu(cpu) {
387 task = idle_task(cpu);
388 if (cpu_online(cpu)) {
389 if (!klp_try_switch_task(task))
390 complete = false;
391 } else if (task->patch_state != klp_target_state) {
392 /* offline idle tasks can be switched immediately */
393 clear_tsk_thread_flag(task, TIF_PATCH_PENDING);
394 task->patch_state = klp_target_state;
395 }
396 }
397 put_online_cpus();
398
399 if (!complete) {
400 /*
401 * Some tasks weren't able to be switched over. Try again
402 * later and/or wait for other methods like kernel exit
403 * switching.
404 */
405 schedule_delayed_work(&klp_transition_work,
406 round_jiffies_relative(HZ));
407 return;
408 }
409
410success:
411 pr_notice("'%s': %s complete\n", klp_transition_patch->mod->name,
412 klp_target_state == KLP_PATCHED ? "patching" : "unpatching");
413
414 /* we're done, now cleanup the data structures */
415 klp_complete_transition();
416}
417
418/*
419 * Start the transition to the specified target patch state so tasks can begin
420 * switching to it.
421 */
422void klp_start_transition(void)
423{
424 struct task_struct *g, *task;
425 unsigned int cpu;
426
427 WARN_ON_ONCE(klp_target_state == KLP_UNDEFINED);
428
429 pr_notice("'%s': %s...\n", klp_transition_patch->mod->name,
430 klp_target_state == KLP_PATCHED ? "patching" : "unpatching");
431
432 /*
433 * If the patch can be applied or reverted immediately, skip the
434 * per-task transitions.
435 */
436 if (klp_transition_patch->immediate)
437 return;
438
439 /*
440 * Mark all normal tasks as needing a patch state update. They'll
441 * switch either in klp_try_complete_transition() or as they exit the
442 * kernel.
443 */
444 read_lock(&tasklist_lock);
445 for_each_process_thread(g, task)
446 if (task->patch_state != klp_target_state)
447 set_tsk_thread_flag(task, TIF_PATCH_PENDING);
448 read_unlock(&tasklist_lock);
449
450 /*
451 * Mark all idle tasks as needing a patch state update. They'll switch
452 * either in klp_try_complete_transition() or at the idle loop switch
453 * point.
454 */
455 for_each_possible_cpu(cpu) {
456 task = idle_task(cpu);
457 if (task->patch_state != klp_target_state)
458 set_tsk_thread_flag(task, TIF_PATCH_PENDING);
459 }
460}
461
462/*
463 * Initialize the global target patch state and all tasks to the initial patch
464 * state, and initialize all function transition states to true in preparation
465 * for patching or unpatching.
466 */
467void klp_init_transition(struct klp_patch *patch, int state)
468{
469 struct task_struct *g, *task;
470 unsigned int cpu;
471 struct klp_object *obj;
472 struct klp_func *func;
473 int initial_state = !state;
474
475 WARN_ON_ONCE(klp_target_state != KLP_UNDEFINED);
476
477 klp_transition_patch = patch;
478
479 /*
480 * Set the global target patch state which tasks will switch to. This
481 * has no effect until the TIF_PATCH_PENDING flags get set later.
482 */
483 klp_target_state = state;
484
485 /*
486 * If the patch can be applied or reverted immediately, skip the
487 * per-task transitions.
488 */
489 if (patch->immediate)
490 return;
491
492 /*
493 * Initialize all tasks to the initial patch state to prepare them for
494 * switching to the target state.
495 */
496 read_lock(&tasklist_lock);
497 for_each_process_thread(g, task) {
498 WARN_ON_ONCE(task->patch_state != KLP_UNDEFINED);
499 task->patch_state = initial_state;
500 }
501 read_unlock(&tasklist_lock);
502
503 /*
504 * Ditto for the idle "swapper" tasks.
505 */
506 for_each_possible_cpu(cpu) {
507 task = idle_task(cpu);
508 WARN_ON_ONCE(task->patch_state != KLP_UNDEFINED);
509 task->patch_state = initial_state;
510 }
511
512 /*
513 * Enforce the order of the task->patch_state initializations and the
514 * func->transition updates to ensure that klp_ftrace_handler() doesn't
515 * see a func in transition with a task->patch_state of KLP_UNDEFINED.
516 *
517 * Also enforce the order of the klp_target_state write and future
518 * TIF_PATCH_PENDING writes to ensure klp_update_patch_state() doesn't
519 * set a task->patch_state to KLP_UNDEFINED.
520 */
521 smp_wmb();
522
523 /*
524 * Set the func transition states so klp_ftrace_handler() will know to
525 * switch to the transition logic.
526 *
527 * When patching, the funcs aren't yet in the func_stack and will be
528 * made visible to the ftrace handler shortly by the calls to
529 * klp_patch_object().
530 *
531 * When unpatching, the funcs are already in the func_stack and so are
532 * already visible to the ftrace handler.
533 */
534 klp_for_each_object(patch, obj)
535 klp_for_each_func(obj, func)
536 func->transition = true;
537}
538
539/*
540 * This function can be called in the middle of an existing transition to
541 * reverse the direction of the target patch state. This can be done to
542 * effectively cancel an existing enable or disable operation if there are any
543 * tasks which are stuck in the initial patch state.
544 */
545void klp_reverse_transition(void)
546{
547 unsigned int cpu;
548 struct task_struct *g, *task;
549
550 klp_transition_patch->enabled = !klp_transition_patch->enabled;
551
552 klp_target_state = !klp_target_state;
553
554 /*
555 * Clear all TIF_PATCH_PENDING flags to prevent races caused by
556 * klp_update_patch_state() running in parallel with
557 * klp_start_transition().
558 */
559 read_lock(&tasklist_lock);
560 for_each_process_thread(g, task)
561 clear_tsk_thread_flag(task, TIF_PATCH_PENDING);
562 read_unlock(&tasklist_lock);
563
564 for_each_possible_cpu(cpu)
565 clear_tsk_thread_flag(idle_task(cpu), TIF_PATCH_PENDING);
566
567 /* Let any remaining calls to klp_update_patch_state() complete */
568 klp_synchronize_transition();
569
570 klp_start_transition();
571}
572
573/* Called from copy_process() during fork */
574void klp_copy_process(struct task_struct *child)
575{
576 child->patch_state = current->patch_state;
577
578 /* TIF_PATCH_PENDING gets copied in setup_thread_stack() */
579}