xref: /openbmc/linux/kernel/livepatch/transition.c (revision 42cffe98)
1 // SPDX-License-Identifier: GPL-2.0-or-later
2 /*
3  * transition.c - Kernel Live Patching transition functions
4  *
5  * Copyright (C) 2015-2016 Josh Poimboeuf <jpoimboe@redhat.com>
6  */
7 
8 #define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
9 
10 #include <linux/cpu.h>
11 #include <linux/stacktrace.h>
12 #include <linux/static_call.h>
13 #include "core.h"
14 #include "patch.h"
15 #include "transition.h"
16 
17 #define MAX_STACK_ENTRIES  100
18 static DEFINE_PER_CPU(unsigned long[MAX_STACK_ENTRIES], klp_stack_entries);
19 
20 #define STACK_ERR_BUF_SIZE 128
21 
22 #define SIGNALS_TIMEOUT 15
23 
24 struct klp_patch *klp_transition_patch;
25 
26 static int klp_target_state = KLP_UNDEFINED;
27 
28 static unsigned int klp_signals_cnt;
29 
30 /*
31  * When a livepatch is in progress, enable klp stack checking in
32  * cond_resched().  This helps CPU-bound kthreads get patched.
33  */
34 #if defined(CONFIG_PREEMPT_DYNAMIC) && defined(CONFIG_HAVE_PREEMPT_DYNAMIC_CALL)
35 
36 #define klp_cond_resched_enable() sched_dynamic_klp_enable()
37 #define klp_cond_resched_disable() sched_dynamic_klp_disable()
38 
39 #else /* !CONFIG_PREEMPT_DYNAMIC || !CONFIG_HAVE_PREEMPT_DYNAMIC_CALL */
40 
41 DEFINE_STATIC_KEY_FALSE(klp_sched_try_switch_key);
42 EXPORT_SYMBOL(klp_sched_try_switch_key);
43 
44 #define klp_cond_resched_enable() static_branch_enable(&klp_sched_try_switch_key)
45 #define klp_cond_resched_disable() static_branch_disable(&klp_sched_try_switch_key)
46 
47 #endif /* CONFIG_PREEMPT_DYNAMIC && CONFIG_HAVE_PREEMPT_DYNAMIC_CALL */
48 
49 /*
50  * This work can be performed periodically to finish patching or unpatching any
51  * "straggler" tasks which failed to transition in the first attempt.
52  */
klp_transition_work_fn(struct work_struct * work)53 static void klp_transition_work_fn(struct work_struct *work)
54 {
55 	mutex_lock(&klp_mutex);
56 
57 	if (klp_transition_patch)
58 		klp_try_complete_transition();
59 
60 	mutex_unlock(&klp_mutex);
61 }
62 static DECLARE_DELAYED_WORK(klp_transition_work, klp_transition_work_fn);
63 
64 /*
65  * This function is just a stub to implement a hard force
66  * of synchronize_rcu(). This requires synchronizing
67  * tasks even in userspace and idle.
68  */
klp_sync(struct work_struct * work)69 static void klp_sync(struct work_struct *work)
70 {
71 }
72 
73 /*
74  * We allow to patch also functions where RCU is not watching,
75  * e.g. before user_exit(). We can not rely on the RCU infrastructure
76  * to do the synchronization. Instead hard force the sched synchronization.
77  *
78  * This approach allows to use RCU functions for manipulating func_stack
79  * safely.
80  */
klp_synchronize_transition(void)81 static void klp_synchronize_transition(void)
82 {
83 	schedule_on_each_cpu(klp_sync);
84 }
85 
86 /*
87  * The transition to the target patch state is complete.  Clean up the data
88  * structures.
89  */
klp_complete_transition(void)90 static void klp_complete_transition(void)
91 {
92 	struct klp_object *obj;
93 	struct klp_func *func;
94 	struct task_struct *g, *task;
95 	unsigned int cpu;
96 
97 	pr_debug("'%s': completing %s transition\n",
98 		 klp_transition_patch->mod->name,
99 		 klp_target_state == KLP_PATCHED ? "patching" : "unpatching");
100 
101 	if (klp_transition_patch->replace && klp_target_state == KLP_PATCHED) {
102 		klp_unpatch_replaced_patches(klp_transition_patch);
103 		klp_discard_nops(klp_transition_patch);
104 	}
105 
106 	if (klp_target_state == KLP_UNPATCHED) {
107 		/*
108 		 * All tasks have transitioned to KLP_UNPATCHED so we can now
109 		 * remove the new functions from the func_stack.
110 		 */
111 		klp_unpatch_objects(klp_transition_patch);
112 
113 		/*
114 		 * Make sure klp_ftrace_handler() can no longer see functions
115 		 * from this patch on the ops->func_stack.  Otherwise, after
116 		 * func->transition gets cleared, the handler may choose a
117 		 * removed function.
118 		 */
119 		klp_synchronize_transition();
120 	}
121 
122 	klp_for_each_object(klp_transition_patch, obj)
123 		klp_for_each_func(obj, func)
124 			func->transition = false;
125 
126 	/* Prevent klp_ftrace_handler() from seeing KLP_UNDEFINED state */
127 	if (klp_target_state == KLP_PATCHED)
128 		klp_synchronize_transition();
129 
130 	read_lock(&tasklist_lock);
131 	for_each_process_thread(g, task) {
132 		WARN_ON_ONCE(test_tsk_thread_flag(task, TIF_PATCH_PENDING));
133 		task->patch_state = KLP_UNDEFINED;
134 	}
135 	read_unlock(&tasklist_lock);
136 
137 	for_each_possible_cpu(cpu) {
138 		task = idle_task(cpu);
139 		WARN_ON_ONCE(test_tsk_thread_flag(task, TIF_PATCH_PENDING));
140 		task->patch_state = KLP_UNDEFINED;
141 	}
142 
143 	klp_for_each_object(klp_transition_patch, obj) {
144 		if (!klp_is_object_loaded(obj))
145 			continue;
146 		if (klp_target_state == KLP_PATCHED)
147 			klp_post_patch_callback(obj);
148 		else if (klp_target_state == KLP_UNPATCHED)
149 			klp_post_unpatch_callback(obj);
150 	}
151 
152 	pr_notice("'%s': %s complete\n", klp_transition_patch->mod->name,
153 		  klp_target_state == KLP_PATCHED ? "patching" : "unpatching");
154 
155 	klp_target_state = KLP_UNDEFINED;
156 	klp_transition_patch = NULL;
157 }
158 
159 /*
160  * This is called in the error path, to cancel a transition before it has
161  * started, i.e. klp_init_transition() has been called but
162  * klp_start_transition() hasn't.  If the transition *has* been started,
163  * klp_reverse_transition() should be used instead.
164  */
klp_cancel_transition(void)165 void klp_cancel_transition(void)
166 {
167 	if (WARN_ON_ONCE(klp_target_state != KLP_PATCHED))
168 		return;
169 
170 	pr_debug("'%s': canceling patching transition, going to unpatch\n",
171 		 klp_transition_patch->mod->name);
172 
173 	klp_target_state = KLP_UNPATCHED;
174 	klp_complete_transition();
175 }
176 
177 /*
178  * Switch the patched state of the task to the set of functions in the target
179  * patch state.
180  *
181  * NOTE: If task is not 'current', the caller must ensure the task is inactive.
182  * Otherwise klp_ftrace_handler() might read the wrong 'patch_state' value.
183  */
klp_update_patch_state(struct task_struct * task)184 void klp_update_patch_state(struct task_struct *task)
185 {
186 	/*
187 	 * A variant of synchronize_rcu() is used to allow patching functions
188 	 * where RCU is not watching, see klp_synchronize_transition().
189 	 */
190 	preempt_disable_notrace();
191 
192 	/*
193 	 * This test_and_clear_tsk_thread_flag() call also serves as a read
194 	 * barrier (smp_rmb) for two cases:
195 	 *
196 	 * 1) Enforce the order of the TIF_PATCH_PENDING read and the
197 	 *    klp_target_state read.  The corresponding write barriers are in
198 	 *    klp_init_transition() and klp_reverse_transition().
199 	 *
200 	 * 2) Enforce the order of the TIF_PATCH_PENDING read and a future read
201 	 *    of func->transition, if klp_ftrace_handler() is called later on
202 	 *    the same CPU.  See __klp_disable_patch().
203 	 */
204 	if (test_and_clear_tsk_thread_flag(task, TIF_PATCH_PENDING))
205 		task->patch_state = READ_ONCE(klp_target_state);
206 
207 	preempt_enable_notrace();
208 }
209 
210 /*
211  * Determine whether the given stack trace includes any references to a
212  * to-be-patched or to-be-unpatched function.
213  */
klp_check_stack_func(struct klp_func * func,unsigned long * entries,unsigned int nr_entries)214 static int klp_check_stack_func(struct klp_func *func, unsigned long *entries,
215 				unsigned int nr_entries)
216 {
217 	unsigned long func_addr, func_size, address;
218 	struct klp_ops *ops;
219 	int i;
220 
221 	if (klp_target_state == KLP_UNPATCHED) {
222 		 /*
223 		  * Check for the to-be-unpatched function
224 		  * (the func itself).
225 		  */
226 		func_addr = (unsigned long)func->new_func;
227 		func_size = func->new_size;
228 	} else {
229 		/*
230 		 * Check for the to-be-patched function
231 		 * (the previous func).
232 		 */
233 		ops = klp_find_ops(func->old_func);
234 
235 		if (list_is_singular(&ops->func_stack)) {
236 			/* original function */
237 			func_addr = (unsigned long)func->old_func;
238 			func_size = func->old_size;
239 		} else {
240 			/* previously patched function */
241 			struct klp_func *prev;
242 
243 			prev = list_next_entry(func, stack_node);
244 			func_addr = (unsigned long)prev->new_func;
245 			func_size = prev->new_size;
246 		}
247 	}
248 
249 	for (i = 0; i < nr_entries; i++) {
250 		address = entries[i];
251 
252 		if (address >= func_addr && address < func_addr + func_size)
253 			return -EAGAIN;
254 	}
255 
256 	return 0;
257 }
258 
259 /*
260  * Determine whether it's safe to transition the task to the target patch state
261  * by looking for any to-be-patched or to-be-unpatched functions on its stack.
262  */
klp_check_stack(struct task_struct * task,const char ** oldname)263 static int klp_check_stack(struct task_struct *task, const char **oldname)
264 {
265 	unsigned long *entries = this_cpu_ptr(klp_stack_entries);
266 	struct klp_object *obj;
267 	struct klp_func *func;
268 	int ret, nr_entries;
269 
270 	/* Protect 'klp_stack_entries' */
271 	lockdep_assert_preemption_disabled();
272 
273 	ret = stack_trace_save_tsk_reliable(task, entries, MAX_STACK_ENTRIES);
274 	if (ret < 0)
275 		return -EINVAL;
276 	nr_entries = ret;
277 
278 	klp_for_each_object(klp_transition_patch, obj) {
279 		if (!obj->patched)
280 			continue;
281 		klp_for_each_func(obj, func) {
282 			ret = klp_check_stack_func(func, entries, nr_entries);
283 			if (ret) {
284 				*oldname = func->old_name;
285 				return -EADDRINUSE;
286 			}
287 		}
288 	}
289 
290 	return 0;
291 }
292 
klp_check_and_switch_task(struct task_struct * task,void * arg)293 static int klp_check_and_switch_task(struct task_struct *task, void *arg)
294 {
295 	int ret;
296 
297 	if (task_curr(task) && task != current)
298 		return -EBUSY;
299 
300 	ret = klp_check_stack(task, arg);
301 	if (ret)
302 		return ret;
303 
304 	clear_tsk_thread_flag(task, TIF_PATCH_PENDING);
305 	task->patch_state = klp_target_state;
306 	return 0;
307 }
308 
309 /*
310  * Try to safely switch a task to the target patch state.  If it's currently
311  * running, or it's sleeping on a to-be-patched or to-be-unpatched function, or
312  * if the stack is unreliable, return false.
313  */
klp_try_switch_task(struct task_struct * task)314 static bool klp_try_switch_task(struct task_struct *task)
315 {
316 	const char *old_name;
317 	int ret;
318 
319 	/* check if this task has already switched over */
320 	if (task->patch_state == klp_target_state)
321 		return true;
322 
323 	/*
324 	 * For arches which don't have reliable stack traces, we have to rely
325 	 * on other methods (e.g., switching tasks at kernel exit).
326 	 */
327 	if (!klp_have_reliable_stack())
328 		return false;
329 
330 	/*
331 	 * Now try to check the stack for any to-be-patched or to-be-unpatched
332 	 * functions.  If all goes well, switch the task to the target patch
333 	 * state.
334 	 */
335 	if (task == current)
336 		ret = klp_check_and_switch_task(current, &old_name);
337 	else
338 		ret = task_call_func(task, klp_check_and_switch_task, &old_name);
339 
340 	switch (ret) {
341 	case 0:		/* success */
342 		break;
343 
344 	case -EBUSY:	/* klp_check_and_switch_task() */
345 		pr_debug("%s: %s:%d is running\n",
346 			 __func__, task->comm, task->pid);
347 		break;
348 	case -EINVAL:	/* klp_check_and_switch_task() */
349 		pr_debug("%s: %s:%d has an unreliable stack\n",
350 			 __func__, task->comm, task->pid);
351 		break;
352 	case -EADDRINUSE: /* klp_check_and_switch_task() */
353 		pr_debug("%s: %s:%d is sleeping on function %s\n",
354 			 __func__, task->comm, task->pid, old_name);
355 		break;
356 
357 	default:
358 		pr_debug("%s: Unknown error code (%d) when trying to switch %s:%d\n",
359 			 __func__, ret, task->comm, task->pid);
360 		break;
361 	}
362 
363 	return !ret;
364 }
365 
__klp_sched_try_switch(void)366 void __klp_sched_try_switch(void)
367 {
368 	if (likely(!klp_patch_pending(current)))
369 		return;
370 
371 	/*
372 	 * This function is called from cond_resched() which is called in many
373 	 * places throughout the kernel.  Using the klp_mutex here might
374 	 * deadlock.
375 	 *
376 	 * Instead, disable preemption to prevent racing with other callers of
377 	 * klp_try_switch_task().  Thanks to task_call_func() they won't be
378 	 * able to switch this task while it's running.
379 	 */
380 	preempt_disable();
381 
382 	/*
383 	 * Make sure current didn't get patched between the above check and
384 	 * preempt_disable().
385 	 */
386 	if (unlikely(!klp_patch_pending(current)))
387 		goto out;
388 
389 	/*
390 	 * Enforce the order of the TIF_PATCH_PENDING read above and the
391 	 * klp_target_state read in klp_try_switch_task().  The corresponding
392 	 * write barriers are in klp_init_transition() and
393 	 * klp_reverse_transition().
394 	 */
395 	smp_rmb();
396 
397 	klp_try_switch_task(current);
398 
399 out:
400 	preempt_enable();
401 }
402 EXPORT_SYMBOL(__klp_sched_try_switch);
403 
404 /*
405  * Sends a fake signal to all non-kthread tasks with TIF_PATCH_PENDING set.
406  * Kthreads with TIF_PATCH_PENDING set are woken up.
407  */
klp_send_signals(void)408 static void klp_send_signals(void)
409 {
410 	struct task_struct *g, *task;
411 
412 	if (klp_signals_cnt == SIGNALS_TIMEOUT)
413 		pr_notice("signaling remaining tasks\n");
414 
415 	read_lock(&tasklist_lock);
416 	for_each_process_thread(g, task) {
417 		if (!klp_patch_pending(task))
418 			continue;
419 
420 		/*
421 		 * There is a small race here. We could see TIF_PATCH_PENDING
422 		 * set and decide to wake up a kthread or send a fake signal.
423 		 * Meanwhile the task could migrate itself and the action
424 		 * would be meaningless. It is not serious though.
425 		 */
426 		if (task->flags & PF_KTHREAD) {
427 			/*
428 			 * Wake up a kthread which sleeps interruptedly and
429 			 * still has not been migrated.
430 			 */
431 			wake_up_state(task, TASK_INTERRUPTIBLE);
432 		} else {
433 			/*
434 			 * Send fake signal to all non-kthread tasks which are
435 			 * still not migrated.
436 			 */
437 			set_notify_signal(task);
438 		}
439 	}
440 	read_unlock(&tasklist_lock);
441 }
442 
443 /*
444  * Try to switch all remaining tasks to the target patch state by walking the
445  * stacks of sleeping tasks and looking for any to-be-patched or
446  * to-be-unpatched functions.  If such functions are found, the task can't be
447  * switched yet.
448  *
449  * If any tasks are still stuck in the initial patch state, schedule a retry.
450  */
klp_try_complete_transition(void)451 void klp_try_complete_transition(void)
452 {
453 	unsigned int cpu;
454 	struct task_struct *g, *task;
455 	struct klp_patch *patch;
456 	bool complete = true;
457 
458 	WARN_ON_ONCE(klp_target_state == KLP_UNDEFINED);
459 
460 	/*
461 	 * Try to switch the tasks to the target patch state by walking their
462 	 * stacks and looking for any to-be-patched or to-be-unpatched
463 	 * functions.  If such functions are found on a stack, or if the stack
464 	 * is deemed unreliable, the task can't be switched yet.
465 	 *
466 	 * Usually this will transition most (or all) of the tasks on a system
467 	 * unless the patch includes changes to a very common function.
468 	 */
469 	read_lock(&tasklist_lock);
470 	for_each_process_thread(g, task)
471 		if (!klp_try_switch_task(task))
472 			complete = false;
473 	read_unlock(&tasklist_lock);
474 
475 	/*
476 	 * Ditto for the idle "swapper" tasks.
477 	 */
478 	cpus_read_lock();
479 	for_each_possible_cpu(cpu) {
480 		task = idle_task(cpu);
481 		if (cpu_online(cpu)) {
482 			if (!klp_try_switch_task(task)) {
483 				complete = false;
484 				/* Make idle task go through the main loop. */
485 				wake_up_if_idle(cpu);
486 			}
487 		} else if (task->patch_state != klp_target_state) {
488 			/* offline idle tasks can be switched immediately */
489 			clear_tsk_thread_flag(task, TIF_PATCH_PENDING);
490 			task->patch_state = klp_target_state;
491 		}
492 	}
493 	cpus_read_unlock();
494 
495 	if (!complete) {
496 		if (klp_signals_cnt && !(klp_signals_cnt % SIGNALS_TIMEOUT))
497 			klp_send_signals();
498 		klp_signals_cnt++;
499 
500 		/*
501 		 * Some tasks weren't able to be switched over.  Try again
502 		 * later and/or wait for other methods like kernel exit
503 		 * switching.
504 		 */
505 		schedule_delayed_work(&klp_transition_work,
506 				      round_jiffies_relative(HZ));
507 		return;
508 	}
509 
510 	/* Done!  Now cleanup the data structures. */
511 	klp_cond_resched_disable();
512 	patch = klp_transition_patch;
513 	klp_complete_transition();
514 
515 	/*
516 	 * It would make more sense to free the unused patches in
517 	 * klp_complete_transition() but it is called also
518 	 * from klp_cancel_transition().
519 	 */
520 	if (!patch->enabled)
521 		klp_free_patch_async(patch);
522 	else if (patch->replace)
523 		klp_free_replaced_patches_async(patch);
524 }
525 
526 /*
527  * Start the transition to the specified target patch state so tasks can begin
528  * switching to it.
529  */
klp_start_transition(void)530 void klp_start_transition(void)
531 {
532 	struct task_struct *g, *task;
533 	unsigned int cpu;
534 
535 	WARN_ON_ONCE(klp_target_state == KLP_UNDEFINED);
536 
537 	pr_notice("'%s': starting %s transition\n",
538 		  klp_transition_patch->mod->name,
539 		  klp_target_state == KLP_PATCHED ? "patching" : "unpatching");
540 
541 	/*
542 	 * Mark all normal tasks as needing a patch state update.  They'll
543 	 * switch either in klp_try_complete_transition() or as they exit the
544 	 * kernel.
545 	 */
546 	read_lock(&tasklist_lock);
547 	for_each_process_thread(g, task)
548 		if (task->patch_state != klp_target_state)
549 			set_tsk_thread_flag(task, TIF_PATCH_PENDING);
550 	read_unlock(&tasklist_lock);
551 
552 	/*
553 	 * Mark all idle tasks as needing a patch state update.  They'll switch
554 	 * either in klp_try_complete_transition() or at the idle loop switch
555 	 * point.
556 	 */
557 	for_each_possible_cpu(cpu) {
558 		task = idle_task(cpu);
559 		if (task->patch_state != klp_target_state)
560 			set_tsk_thread_flag(task, TIF_PATCH_PENDING);
561 	}
562 
563 	klp_cond_resched_enable();
564 
565 	klp_signals_cnt = 0;
566 }
567 
568 /*
569  * Initialize the global target patch state and all tasks to the initial patch
570  * state, and initialize all function transition states to true in preparation
571  * for patching or unpatching.
572  */
klp_init_transition(struct klp_patch * patch,int state)573 void klp_init_transition(struct klp_patch *patch, int state)
574 {
575 	struct task_struct *g, *task;
576 	unsigned int cpu;
577 	struct klp_object *obj;
578 	struct klp_func *func;
579 	int initial_state = !state;
580 
581 	WARN_ON_ONCE(klp_target_state != KLP_UNDEFINED);
582 
583 	klp_transition_patch = patch;
584 
585 	/*
586 	 * Set the global target patch state which tasks will switch to.  This
587 	 * has no effect until the TIF_PATCH_PENDING flags get set later.
588 	 */
589 	klp_target_state = state;
590 
591 	pr_debug("'%s': initializing %s transition\n", patch->mod->name,
592 		 klp_target_state == KLP_PATCHED ? "patching" : "unpatching");
593 
594 	/*
595 	 * Initialize all tasks to the initial patch state to prepare them for
596 	 * switching to the target state.
597 	 */
598 	read_lock(&tasklist_lock);
599 	for_each_process_thread(g, task) {
600 		WARN_ON_ONCE(task->patch_state != KLP_UNDEFINED);
601 		task->patch_state = initial_state;
602 	}
603 	read_unlock(&tasklist_lock);
604 
605 	/*
606 	 * Ditto for the idle "swapper" tasks.
607 	 */
608 	for_each_possible_cpu(cpu) {
609 		task = idle_task(cpu);
610 		WARN_ON_ONCE(task->patch_state != KLP_UNDEFINED);
611 		task->patch_state = initial_state;
612 	}
613 
614 	/*
615 	 * Enforce the order of the task->patch_state initializations and the
616 	 * func->transition updates to ensure that klp_ftrace_handler() doesn't
617 	 * see a func in transition with a task->patch_state of KLP_UNDEFINED.
618 	 *
619 	 * Also enforce the order of the klp_target_state write and future
620 	 * TIF_PATCH_PENDING writes to ensure klp_update_patch_state() and
621 	 * __klp_sched_try_switch() don't set a task->patch_state to
622 	 * KLP_UNDEFINED.
623 	 */
624 	smp_wmb();
625 
626 	/*
627 	 * Set the func transition states so klp_ftrace_handler() will know to
628 	 * switch to the transition logic.
629 	 *
630 	 * When patching, the funcs aren't yet in the func_stack and will be
631 	 * made visible to the ftrace handler shortly by the calls to
632 	 * klp_patch_object().
633 	 *
634 	 * When unpatching, the funcs are already in the func_stack and so are
635 	 * already visible to the ftrace handler.
636 	 */
637 	klp_for_each_object(patch, obj)
638 		klp_for_each_func(obj, func)
639 			func->transition = true;
640 }
641 
642 /*
643  * This function can be called in the middle of an existing transition to
644  * reverse the direction of the target patch state.  This can be done to
645  * effectively cancel an existing enable or disable operation if there are any
646  * tasks which are stuck in the initial patch state.
647  */
klp_reverse_transition(void)648 void klp_reverse_transition(void)
649 {
650 	unsigned int cpu;
651 	struct task_struct *g, *task;
652 
653 	pr_debug("'%s': reversing transition from %s\n",
654 		 klp_transition_patch->mod->name,
655 		 klp_target_state == KLP_PATCHED ? "patching to unpatching" :
656 						   "unpatching to patching");
657 
658 	/*
659 	 * Clear all TIF_PATCH_PENDING flags to prevent races caused by
660 	 * klp_update_patch_state() or __klp_sched_try_switch() running in
661 	 * parallel with the reverse transition.
662 	 */
663 	read_lock(&tasklist_lock);
664 	for_each_process_thread(g, task)
665 		clear_tsk_thread_flag(task, TIF_PATCH_PENDING);
666 	read_unlock(&tasklist_lock);
667 
668 	for_each_possible_cpu(cpu)
669 		clear_tsk_thread_flag(idle_task(cpu), TIF_PATCH_PENDING);
670 
671 	/*
672 	 * Make sure all existing invocations of klp_update_patch_state() and
673 	 * __klp_sched_try_switch() see the cleared TIF_PATCH_PENDING before
674 	 * starting the reverse transition.
675 	 */
676 	klp_synchronize_transition();
677 
678 	/*
679 	 * All patching has stopped, now re-initialize the global variables to
680 	 * prepare for the reverse transition.
681 	 */
682 	klp_transition_patch->enabled = !klp_transition_patch->enabled;
683 	klp_target_state = !klp_target_state;
684 
685 	/*
686 	 * Enforce the order of the klp_target_state write and the
687 	 * TIF_PATCH_PENDING writes in klp_start_transition() to ensure
688 	 * klp_update_patch_state() and __klp_sched_try_switch() don't set
689 	 * task->patch_state to the wrong value.
690 	 */
691 	smp_wmb();
692 
693 	klp_start_transition();
694 }
695 
696 /* Called from copy_process() during fork */
klp_copy_process(struct task_struct * child)697 void klp_copy_process(struct task_struct *child)
698 {
699 
700 	/*
701 	 * The parent process may have gone through a KLP transition since
702 	 * the thread flag was copied in setup_thread_stack earlier. Bring
703 	 * the task flag up to date with the parent here.
704 	 *
705 	 * The operation is serialized against all klp_*_transition()
706 	 * operations by the tasklist_lock. The only exceptions are
707 	 * klp_update_patch_state(current) and __klp_sched_try_switch(), but we
708 	 * cannot race with them because we are current.
709 	 */
710 	if (test_tsk_thread_flag(current, TIF_PATCH_PENDING))
711 		set_tsk_thread_flag(child, TIF_PATCH_PENDING);
712 	else
713 		clear_tsk_thread_flag(child, TIF_PATCH_PENDING);
714 
715 	child->patch_state = current->patch_state;
716 }
717 
718 /*
719  * Drop TIF_PATCH_PENDING of all tasks on admin's request. This forces an
720  * existing transition to finish.
721  *
722  * NOTE: klp_update_patch_state(task) requires the task to be inactive or
723  * 'current'. This is not the case here and the consistency model could be
724  * broken. Administrator, who is the only one to execute the
725  * klp_force_transitions(), has to be aware of this.
726  */
klp_force_transition(void)727 void klp_force_transition(void)
728 {
729 	struct klp_patch *patch;
730 	struct task_struct *g, *task;
731 	unsigned int cpu;
732 
733 	pr_warn("forcing remaining tasks to the patched state\n");
734 
735 	read_lock(&tasklist_lock);
736 	for_each_process_thread(g, task)
737 		klp_update_patch_state(task);
738 	read_unlock(&tasklist_lock);
739 
740 	for_each_possible_cpu(cpu)
741 		klp_update_patch_state(idle_task(cpu));
742 
743 	/* Set forced flag for patches being removed. */
744 	if (klp_target_state == KLP_UNPATCHED)
745 		klp_transition_patch->forced = true;
746 	else if (klp_transition_patch->replace) {
747 		klp_for_each_patch(patch) {
748 			if (patch != klp_transition_patch)
749 				patch->forced = true;
750 		}
751 	}
752 }
753