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_TRANSITION_IDLE;
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_TRANSITION_PATCHED ? "patching" : "unpatching");
100  
101  	if (klp_transition_patch->replace && klp_target_state == KLP_TRANSITION_PATCHED) {
102  		klp_unpatch_replaced_patches(klp_transition_patch);
103  		klp_discard_nops(klp_transition_patch);
104  	}
105  
106  	if (klp_target_state == KLP_TRANSITION_UNPATCHED) {
107  		/*
108  		 * All tasks have transitioned to KLP_TRANSITION_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_TRANSITION_IDLE state */
127  	if (klp_target_state == KLP_TRANSITION_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_TRANSITION_IDLE;
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_TRANSITION_IDLE;
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_TRANSITION_PATCHED)
147  			klp_post_patch_callback(obj);
148  		else if (klp_target_state == KLP_TRANSITION_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_TRANSITION_PATCHED ? "patching" : "unpatching");
154  
155  	klp_target_state = KLP_TRANSITION_IDLE;
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_TRANSITION_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_TRANSITION_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_TRANSITION_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_TRANSITION_IDLE);
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_TRANSITION_IDLE);
536  
537  	pr_notice("'%s': starting %s transition\n",
538  		  klp_transition_patch->mod->name,
539  		  klp_target_state == KLP_TRANSITION_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_TRANSITION_IDLE);
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_TRANSITION_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_TRANSITION_IDLE);
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_TRANSITION_IDLE);
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_TRANSITION_IDLE.
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_TRANSITION_IDLE.
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_TRANSITION_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_TRANSITION_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