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1/*
2 * transition.c - Kernel Live Patching transition functions
3 *
4 * Copyright (C) 2015-2016 Josh Poimboeuf <jpoimboe@redhat.com>
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
36static bool klp_forced = false;
37
38/*
39 * This work can be performed periodically to finish patching or unpatching any
40 * "straggler" tasks which failed to transition in the first attempt.
41 */
42static void klp_transition_work_fn(struct work_struct *work)
43{
44 mutex_lock(&klp_mutex);
45
46 if (klp_transition_patch)
47 klp_try_complete_transition();
48
49 mutex_unlock(&klp_mutex);
50}
51static DECLARE_DELAYED_WORK(klp_transition_work, klp_transition_work_fn);
52
53/*
54 * This function is just a stub to implement a hard force
55 * of synchronize_sched(). This requires synchronizing
56 * tasks even in userspace and idle.
57 */
58static void klp_sync(struct work_struct *work)
59{
60}
61
62/*
63 * We allow to patch also functions where RCU is not watching,
64 * e.g. before user_exit(). We can not rely on the RCU infrastructure
65 * to do the synchronization. Instead hard force the sched synchronization.
66 *
67 * This approach allows to use RCU functions for manipulating func_stack
68 * safely.
69 */
70static void klp_synchronize_transition(void)
71{
72 schedule_on_each_cpu(klp_sync);
73}
74
75/*
76 * The transition to the target patch state is complete. Clean up the data
77 * structures.
78 */
79static void klp_complete_transition(void)
80{
81 struct klp_object *obj;
82 struct klp_func *func;
83 struct task_struct *g, *task;
84 unsigned int cpu;
85
86 pr_debug("'%s': completing %s transition\n",
87 klp_transition_patch->mod->name,
88 klp_target_state == KLP_PATCHED ? "patching" : "unpatching");
89
90 if (klp_target_state == KLP_UNPATCHED) {
91 /*
92 * All tasks have transitioned to KLP_UNPATCHED so we can now
93 * remove the new functions from the func_stack.
94 */
95 klp_unpatch_objects(klp_transition_patch);
96
97 /*
98 * Make sure klp_ftrace_handler() can no longer see functions
99 * from this patch on the ops->func_stack. Otherwise, after
100 * func->transition gets cleared, the handler may choose a
101 * removed function.
102 */
103 klp_synchronize_transition();
104 }
105
106 klp_for_each_object(klp_transition_patch, obj)
107 klp_for_each_func(obj, func)
108 func->transition = false;
109
110 /* Prevent klp_ftrace_handler() from seeing KLP_UNDEFINED state */
111 if (klp_target_state == KLP_PATCHED)
112 klp_synchronize_transition();
113
114 read_lock(&tasklist_lock);
115 for_each_process_thread(g, task) {
116 WARN_ON_ONCE(test_tsk_thread_flag(task, TIF_PATCH_PENDING));
117 task->patch_state = KLP_UNDEFINED;
118 }
119 read_unlock(&tasklist_lock);
120
121 for_each_possible_cpu(cpu) {
122 task = idle_task(cpu);
123 WARN_ON_ONCE(test_tsk_thread_flag(task, TIF_PATCH_PENDING));
124 task->patch_state = KLP_UNDEFINED;
125 }
126
127 klp_for_each_object(klp_transition_patch, obj) {
128 if (!klp_is_object_loaded(obj))
129 continue;
130 if (klp_target_state == KLP_PATCHED)
131 klp_post_patch_callback(obj);
132 else if (klp_target_state == KLP_UNPATCHED)
133 klp_post_unpatch_callback(obj);
134 }
135
136 pr_notice("'%s': %s complete\n", klp_transition_patch->mod->name,
137 klp_target_state == KLP_PATCHED ? "patching" : "unpatching");
138
139 /*
140 * klp_forced set implies unbounded increase of module's ref count if
141 * the module is disabled/enabled in a loop.
142 */
143 if (!klp_forced && klp_target_state == KLP_UNPATCHED)
144 module_put(klp_transition_patch->mod);
145
146 klp_target_state = KLP_UNDEFINED;
147 klp_transition_patch = NULL;
148}
149
150/*
151 * This is called in the error path, to cancel a transition before it has
152 * started, i.e. klp_init_transition() has been called but
153 * klp_start_transition() hasn't. If the transition *has* been started,
154 * klp_reverse_transition() should be used instead.
155 */
156void klp_cancel_transition(void)
157{
158 if (WARN_ON_ONCE(klp_target_state != KLP_PATCHED))
159 return;
160
161 pr_debug("'%s': canceling patching transition, going to unpatch\n",
162 klp_transition_patch->mod->name);
163
164 klp_target_state = KLP_UNPATCHED;
165 klp_complete_transition();
166}
167
168/*
169 * Switch the patched state of the task to the set of functions in the target
170 * patch state.
171 *
172 * NOTE: If task is not 'current', the caller must ensure the task is inactive.
173 * Otherwise klp_ftrace_handler() might read the wrong 'patch_state' value.
174 */
175void klp_update_patch_state(struct task_struct *task)
176{
177 /*
178 * A variant of synchronize_sched() is used to allow patching functions
179 * where RCU is not watching, see klp_synchronize_transition().
180 */
181 preempt_disable_notrace();
182
183 /*
184 * This test_and_clear_tsk_thread_flag() call also serves as a read
185 * barrier (smp_rmb) for two cases:
186 *
187 * 1) Enforce the order of the TIF_PATCH_PENDING read and the
188 * klp_target_state read. The corresponding write barrier is in
189 * klp_init_transition().
190 *
191 * 2) Enforce the order of the TIF_PATCH_PENDING read and a future read
192 * of func->transition, if klp_ftrace_handler() is called later on
193 * the same CPU. See __klp_disable_patch().
194 */
195 if (test_and_clear_tsk_thread_flag(task, TIF_PATCH_PENDING))
196 task->patch_state = READ_ONCE(klp_target_state);
197
198 preempt_enable_notrace();
199}
200
201/*
202 * Determine whether the given stack trace includes any references to a
203 * to-be-patched or to-be-unpatched function.
204 */
205static int klp_check_stack_func(struct klp_func *func,
206 struct stack_trace *trace)
207{
208 unsigned long func_addr, func_size, address;
209 struct klp_ops *ops;
210 int i;
211
212 for (i = 0; i < trace->nr_entries; i++) {
213 address = trace->entries[i];
214
215 if (klp_target_state == KLP_UNPATCHED) {
216 /*
217 * Check for the to-be-unpatched function
218 * (the func itself).
219 */
220 func_addr = (unsigned long)func->new_func;
221 func_size = func->new_size;
222 } else {
223 /*
224 * Check for the to-be-patched function
225 * (the previous func).
226 */
227 ops = klp_find_ops(func->old_addr);
228
229 if (list_is_singular(&ops->func_stack)) {
230 /* original function */
231 func_addr = func->old_addr;
232 func_size = func->old_size;
233 } else {
234 /* previously patched function */
235 struct klp_func *prev;
236
237 prev = list_next_entry(func, stack_node);
238 func_addr = (unsigned long)prev->new_func;
239 func_size = prev->new_size;
240 }
241 }
242
243 if (address >= func_addr && address < func_addr + func_size)
244 return -EAGAIN;
245 }
246
247 return 0;
248}
249
250/*
251 * Determine whether it's safe to transition the task to the target patch state
252 * by looking for any to-be-patched or to-be-unpatched functions on its stack.
253 */
254static int klp_check_stack(struct task_struct *task, char *err_buf)
255{
256 static unsigned long entries[MAX_STACK_ENTRIES];
257 struct stack_trace trace;
258 struct klp_object *obj;
259 struct klp_func *func;
260 int ret;
261
262 trace.skip = 0;
263 trace.nr_entries = 0;
264 trace.max_entries = MAX_STACK_ENTRIES;
265 trace.entries = entries;
266 ret = save_stack_trace_tsk_reliable(task, &trace);
267 WARN_ON_ONCE(ret == -ENOSYS);
268 if (ret) {
269 snprintf(err_buf, STACK_ERR_BUF_SIZE,
270 "%s: %s:%d has an unreliable stack\n",
271 __func__, task->comm, task->pid);
272 return ret;
273 }
274
275 klp_for_each_object(klp_transition_patch, obj) {
276 if (!obj->patched)
277 continue;
278 klp_for_each_func(obj, func) {
279 ret = klp_check_stack_func(func, &trace);
280 if (ret) {
281 snprintf(err_buf, STACK_ERR_BUF_SIZE,
282 "%s: %s:%d is sleeping on function %s\n",
283 __func__, task->comm, task->pid,
284 func->old_name);
285 return ret;
286 }
287 }
288 }
289
290 return 0;
291}
292
293/*
294 * Try to safely switch a task to the target patch state. If it's currently
295 * running, or it's sleeping on a to-be-patched or to-be-unpatched function, or
296 * if the stack is unreliable, return false.
297 */
298static bool klp_try_switch_task(struct task_struct *task)
299{
300 struct rq *rq;
301 struct rq_flags flags;
302 int ret;
303 bool success = false;
304 char err_buf[STACK_ERR_BUF_SIZE];
305
306 err_buf[0] = '\0';
307
308 /* check if this task has already switched over */
309 if (task->patch_state == klp_target_state)
310 return true;
311
312 /*
313 * For arches which don't have reliable stack traces, we have to rely
314 * on other methods (e.g., switching tasks at kernel exit).
315 */
316 if (!klp_have_reliable_stack())
317 return false;
318
319 /*
320 * Now try to check the stack for any to-be-patched or to-be-unpatched
321 * functions. If all goes well, switch the task to the target patch
322 * state.
323 */
324 rq = task_rq_lock(task, &flags);
325
326 if (task_running(rq, task) && task != current) {
327 snprintf(err_buf, STACK_ERR_BUF_SIZE,
328 "%s: %s:%d is running\n", __func__, task->comm,
329 task->pid);
330 goto done;
331 }
332
333 ret = klp_check_stack(task, err_buf);
334 if (ret)
335 goto done;
336
337 success = true;
338
339 clear_tsk_thread_flag(task, TIF_PATCH_PENDING);
340 task->patch_state = klp_target_state;
341
342done:
343 task_rq_unlock(rq, task, &flags);
344
345 /*
346 * Due to console deadlock issues, pr_debug() can't be used while
347 * holding the task rq lock. Instead we have to use a temporary buffer
348 * and print the debug message after releasing the lock.
349 */
350 if (err_buf[0] != '\0')
351 pr_debug("%s", err_buf);
352
353 return success;
354
355}
356
357/*
358 * Try to switch all remaining tasks to the target patch state by walking the
359 * stacks of sleeping tasks and looking for any to-be-patched or
360 * to-be-unpatched functions. If such functions are found, the task can't be
361 * switched yet.
362 *
363 * If any tasks are still stuck in the initial patch state, schedule a retry.
364 */
365void klp_try_complete_transition(void)
366{
367 unsigned int cpu;
368 struct task_struct *g, *task;
369 bool complete = true;
370
371 WARN_ON_ONCE(klp_target_state == KLP_UNDEFINED);
372
373 /*
374 * Try to switch the tasks to the target patch state by walking their
375 * stacks and looking for any to-be-patched or to-be-unpatched
376 * functions. If such functions are found on a stack, or if the stack
377 * is deemed unreliable, the task can't be switched yet.
378 *
379 * Usually this will transition most (or all) of the tasks on a system
380 * unless the patch includes changes to a very common function.
381 */
382 read_lock(&tasklist_lock);
383 for_each_process_thread(g, task)
384 if (!klp_try_switch_task(task))
385 complete = false;
386 read_unlock(&tasklist_lock);
387
388 /*
389 * Ditto for the idle "swapper" tasks.
390 */
391 get_online_cpus();
392 for_each_possible_cpu(cpu) {
393 task = idle_task(cpu);
394 if (cpu_online(cpu)) {
395 if (!klp_try_switch_task(task))
396 complete = false;
397 } else if (task->patch_state != klp_target_state) {
398 /* offline idle tasks can be switched immediately */
399 clear_tsk_thread_flag(task, TIF_PATCH_PENDING);
400 task->patch_state = klp_target_state;
401 }
402 }
403 put_online_cpus();
404
405 if (!complete) {
406 /*
407 * Some tasks weren't able to be switched over. Try again
408 * later and/or wait for other methods like kernel exit
409 * switching.
410 */
411 schedule_delayed_work(&klp_transition_work,
412 round_jiffies_relative(HZ));
413 return;
414 }
415
416 /* we're done, now cleanup the data structures */
417 klp_complete_transition();
418}
419
420/*
421 * Start the transition to the specified target patch state so tasks can begin
422 * switching to it.
423 */
424void klp_start_transition(void)
425{
426 struct task_struct *g, *task;
427 unsigned int cpu;
428
429 WARN_ON_ONCE(klp_target_state == KLP_UNDEFINED);
430
431 pr_notice("'%s': starting %s transition\n",
432 klp_transition_patch->mod->name,
433 klp_target_state == KLP_PATCHED ? "patching" : "unpatching");
434
435 /*
436 * Mark all normal tasks as needing a patch state update. They'll
437 * switch either in klp_try_complete_transition() or as they exit the
438 * kernel.
439 */
440 read_lock(&tasklist_lock);
441 for_each_process_thread(g, task)
442 if (task->patch_state != klp_target_state)
443 set_tsk_thread_flag(task, TIF_PATCH_PENDING);
444 read_unlock(&tasklist_lock);
445
446 /*
447 * Mark all idle tasks as needing a patch state update. They'll switch
448 * either in klp_try_complete_transition() or at the idle loop switch
449 * point.
450 */
451 for_each_possible_cpu(cpu) {
452 task = idle_task(cpu);
453 if (task->patch_state != klp_target_state)
454 set_tsk_thread_flag(task, TIF_PATCH_PENDING);
455 }
456}
457
458/*
459 * Initialize the global target patch state and all tasks to the initial patch
460 * state, and initialize all function transition states to true in preparation
461 * for patching or unpatching.
462 */
463void klp_init_transition(struct klp_patch *patch, int state)
464{
465 struct task_struct *g, *task;
466 unsigned int cpu;
467 struct klp_object *obj;
468 struct klp_func *func;
469 int initial_state = !state;
470
471 WARN_ON_ONCE(klp_target_state != KLP_UNDEFINED);
472
473 klp_transition_patch = patch;
474
475 /*
476 * Set the global target patch state which tasks will switch to. This
477 * has no effect until the TIF_PATCH_PENDING flags get set later.
478 */
479 klp_target_state = state;
480
481 pr_debug("'%s': initializing %s transition\n", patch->mod->name,
482 klp_target_state == KLP_PATCHED ? "patching" : "unpatching");
483
484 /*
485 * Initialize all tasks to the initial patch state to prepare them for
486 * switching to the target state.
487 */
488 read_lock(&tasklist_lock);
489 for_each_process_thread(g, task) {
490 WARN_ON_ONCE(task->patch_state != KLP_UNDEFINED);
491 task->patch_state = initial_state;
492 }
493 read_unlock(&tasklist_lock);
494
495 /*
496 * Ditto for the idle "swapper" tasks.
497 */
498 for_each_possible_cpu(cpu) {
499 task = idle_task(cpu);
500 WARN_ON_ONCE(task->patch_state != KLP_UNDEFINED);
501 task->patch_state = initial_state;
502 }
503
504 /*
505 * Enforce the order of the task->patch_state initializations and the
506 * func->transition updates to ensure that klp_ftrace_handler() doesn't
507 * see a func in transition with a task->patch_state of KLP_UNDEFINED.
508 *
509 * Also enforce the order of the klp_target_state write and future
510 * TIF_PATCH_PENDING writes to ensure klp_update_patch_state() doesn't
511 * set a task->patch_state to KLP_UNDEFINED.
512 */
513 smp_wmb();
514
515 /*
516 * Set the func transition states so klp_ftrace_handler() will know to
517 * switch to the transition logic.
518 *
519 * When patching, the funcs aren't yet in the func_stack and will be
520 * made visible to the ftrace handler shortly by the calls to
521 * klp_patch_object().
522 *
523 * When unpatching, the funcs are already in the func_stack and so are
524 * already visible to the ftrace handler.
525 */
526 klp_for_each_object(patch, obj)
527 klp_for_each_func(obj, func)
528 func->transition = true;
529}
530
531/*
532 * This function can be called in the middle of an existing transition to
533 * reverse the direction of the target patch state. This can be done to
534 * effectively cancel an existing enable or disable operation if there are any
535 * tasks which are stuck in the initial patch state.
536 */
537void klp_reverse_transition(void)
538{
539 unsigned int cpu;
540 struct task_struct *g, *task;
541
542 pr_debug("'%s': reversing transition from %s\n",
543 klp_transition_patch->mod->name,
544 klp_target_state == KLP_PATCHED ? "patching to unpatching" :
545 "unpatching to patching");
546
547 klp_transition_patch->enabled = !klp_transition_patch->enabled;
548
549 klp_target_state = !klp_target_state;
550
551 /*
552 * Clear all TIF_PATCH_PENDING flags to prevent races caused by
553 * klp_update_patch_state() running in parallel with
554 * klp_start_transition().
555 */
556 read_lock(&tasklist_lock);
557 for_each_process_thread(g, task)
558 clear_tsk_thread_flag(task, TIF_PATCH_PENDING);
559 read_unlock(&tasklist_lock);
560
561 for_each_possible_cpu(cpu)
562 clear_tsk_thread_flag(idle_task(cpu), TIF_PATCH_PENDING);
563
564 /* Let any remaining calls to klp_update_patch_state() complete */
565 klp_synchronize_transition();
566
567 klp_start_transition();
568}
569
570/* Called from copy_process() during fork */
571void klp_copy_process(struct task_struct *child)
572{
573 child->patch_state = current->patch_state;
574
575 /* TIF_PATCH_PENDING gets copied in setup_thread_stack() */
576}
577
578/*
579 * Sends a fake signal to all non-kthread tasks with TIF_PATCH_PENDING set.
580 * Kthreads with TIF_PATCH_PENDING set are woken up. Only admin can request this
581 * action currently.
582 */
583void klp_send_signals(void)
584{
585 struct task_struct *g, *task;
586
587 pr_notice("signaling remaining tasks\n");
588
589 read_lock(&tasklist_lock);
590 for_each_process_thread(g, task) {
591 if (!klp_patch_pending(task))
592 continue;
593
594 /*
595 * There is a small race here. We could see TIF_PATCH_PENDING
596 * set and decide to wake up a kthread or send a fake signal.
597 * Meanwhile the task could migrate itself and the action
598 * would be meaningless. It is not serious though.
599 */
600 if (task->flags & PF_KTHREAD) {
601 /*
602 * Wake up a kthread which sleeps interruptedly and
603 * still has not been migrated.
604 */
605 wake_up_state(task, TASK_INTERRUPTIBLE);
606 } else {
607 /*
608 * Send fake signal to all non-kthread tasks which are
609 * still not migrated.
610 */
611 spin_lock_irq(&task->sighand->siglock);
612 signal_wake_up(task, 0);
613 spin_unlock_irq(&task->sighand->siglock);
614 }
615 }
616 read_unlock(&tasklist_lock);
617}
618
619/*
620 * Drop TIF_PATCH_PENDING of all tasks on admin's request. This forces an
621 * existing transition to finish.
622 *
623 * NOTE: klp_update_patch_state(task) requires the task to be inactive or
624 * 'current'. This is not the case here and the consistency model could be
625 * broken. Administrator, who is the only one to execute the
626 * klp_force_transitions(), has to be aware of this.
627 */
628void klp_force_transition(void)
629{
630 struct task_struct *g, *task;
631 unsigned int cpu;
632
633 pr_warn("forcing remaining tasks to the patched state\n");
634
635 read_lock(&tasklist_lock);
636 for_each_process_thread(g, task)
637 klp_update_patch_state(task);
638 read_unlock(&tasklist_lock);
639
640 for_each_possible_cpu(cpu)
641 klp_update_patch_state(idle_task(cpu));
642
643 klp_forced = true;
644}