Merge branch 'for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/livepatchin...
[muen/linux.git] / kernel / livepatch / transition.c
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 "core.h"
13 #include "patch.h"
14 #include "transition.h"
15 #include "../sched/sched.h"
16
17 #define MAX_STACK_ENTRIES  100
18 #define STACK_ERR_BUF_SIZE 128
19
20 #define SIGNALS_TIMEOUT 15
21
22 struct klp_patch *klp_transition_patch;
23
24 static int klp_target_state = KLP_UNDEFINED;
25
26 static unsigned int klp_signals_cnt;
27
28 /*
29  * This work can be performed periodically to finish patching or unpatching any
30  * "straggler" tasks which failed to transition in the first attempt.
31  */
32 static void klp_transition_work_fn(struct work_struct *work)
33 {
34         mutex_lock(&klp_mutex);
35
36         if (klp_transition_patch)
37                 klp_try_complete_transition();
38
39         mutex_unlock(&klp_mutex);
40 }
41 static DECLARE_DELAYED_WORK(klp_transition_work, klp_transition_work_fn);
42
43 /*
44  * This function is just a stub to implement a hard force
45  * of synchronize_rcu(). This requires synchronizing
46  * tasks even in userspace and idle.
47  */
48 static void klp_sync(struct work_struct *work)
49 {
50 }
51
52 /*
53  * We allow to patch also functions where RCU is not watching,
54  * e.g. before user_exit(). We can not rely on the RCU infrastructure
55  * to do the synchronization. Instead hard force the sched synchronization.
56  *
57  * This approach allows to use RCU functions for manipulating func_stack
58  * safely.
59  */
60 static void klp_synchronize_transition(void)
61 {
62         schedule_on_each_cpu(klp_sync);
63 }
64
65 /*
66  * The transition to the target patch state is complete.  Clean up the data
67  * structures.
68  */
69 static void klp_complete_transition(void)
70 {
71         struct klp_object *obj;
72         struct klp_func *func;
73         struct task_struct *g, *task;
74         unsigned int cpu;
75
76         pr_debug("'%s': completing %s transition\n",
77                  klp_transition_patch->mod->name,
78                  klp_target_state == KLP_PATCHED ? "patching" : "unpatching");
79
80         if (klp_transition_patch->replace && klp_target_state == KLP_PATCHED) {
81                 klp_discard_replaced_patches(klp_transition_patch);
82                 klp_discard_nops(klp_transition_patch);
83         }
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         klp_for_each_object(klp_transition_patch, obj)
102                 klp_for_each_func(obj, func)
103                         func->transition = false;
104
105         /* Prevent klp_ftrace_handler() from seeing KLP_UNDEFINED state */
106         if (klp_target_state == KLP_PATCHED)
107                 klp_synchronize_transition();
108
109         read_lock(&tasklist_lock);
110         for_each_process_thread(g, task) {
111                 WARN_ON_ONCE(test_tsk_thread_flag(task, TIF_PATCH_PENDING));
112                 task->patch_state = KLP_UNDEFINED;
113         }
114         read_unlock(&tasklist_lock);
115
116         for_each_possible_cpu(cpu) {
117                 task = idle_task(cpu);
118                 WARN_ON_ONCE(test_tsk_thread_flag(task, TIF_PATCH_PENDING));
119                 task->patch_state = KLP_UNDEFINED;
120         }
121
122         klp_for_each_object(klp_transition_patch, obj) {
123                 if (!klp_is_object_loaded(obj))
124                         continue;
125                 if (klp_target_state == KLP_PATCHED)
126                         klp_post_patch_callback(obj);
127                 else if (klp_target_state == KLP_UNPATCHED)
128                         klp_post_unpatch_callback(obj);
129         }
130
131         pr_notice("'%s': %s complete\n", klp_transition_patch->mod->name,
132                   klp_target_state == KLP_PATCHED ? "patching" : "unpatching");
133
134         klp_target_state = KLP_UNDEFINED;
135         klp_transition_patch = NULL;
136 }
137
138 /*
139  * This is called in the error path, to cancel a transition before it has
140  * started, i.e. klp_init_transition() has been called but
141  * klp_start_transition() hasn't.  If the transition *has* been started,
142  * klp_reverse_transition() should be used instead.
143  */
144 void klp_cancel_transition(void)
145 {
146         if (WARN_ON_ONCE(klp_target_state != KLP_PATCHED))
147                 return;
148
149         pr_debug("'%s': canceling patching transition, going to unpatch\n",
150                  klp_transition_patch->mod->name);
151
152         klp_target_state = KLP_UNPATCHED;
153         klp_complete_transition();
154 }
155
156 /*
157  * Switch the patched state of the task to the set of functions in the target
158  * patch state.
159  *
160  * NOTE: If task is not 'current', the caller must ensure the task is inactive.
161  * Otherwise klp_ftrace_handler() might read the wrong 'patch_state' value.
162  */
163 void klp_update_patch_state(struct task_struct *task)
164 {
165         /*
166          * A variant of synchronize_rcu() is used to allow patching functions
167          * where RCU is not watching, see klp_synchronize_transition().
168          */
169         preempt_disable_notrace();
170
171         /*
172          * This test_and_clear_tsk_thread_flag() call also serves as a read
173          * barrier (smp_rmb) for two cases:
174          *
175          * 1) Enforce the order of the TIF_PATCH_PENDING read and the
176          *    klp_target_state read.  The corresponding write barrier is in
177          *    klp_init_transition().
178          *
179          * 2) Enforce the order of the TIF_PATCH_PENDING read and a future read
180          *    of func->transition, if klp_ftrace_handler() is called later on
181          *    the same CPU.  See __klp_disable_patch().
182          */
183         if (test_and_clear_tsk_thread_flag(task, TIF_PATCH_PENDING))
184                 task->patch_state = READ_ONCE(klp_target_state);
185
186         preempt_enable_notrace();
187 }
188
189 /*
190  * Determine whether the given stack trace includes any references to a
191  * to-be-patched or to-be-unpatched function.
192  */
193 static int klp_check_stack_func(struct klp_func *func, unsigned long *entries,
194                                 unsigned int nr_entries)
195 {
196         unsigned long func_addr, func_size, address;
197         struct klp_ops *ops;
198         int i;
199
200         for (i = 0; i < nr_entries; i++) {
201                 address = entries[i];
202
203                 if (klp_target_state == KLP_UNPATCHED) {
204                          /*
205                           * Check for the to-be-unpatched function
206                           * (the func itself).
207                           */
208                         func_addr = (unsigned long)func->new_func;
209                         func_size = func->new_size;
210                 } else {
211                         /*
212                          * Check for the to-be-patched function
213                          * (the previous func).
214                          */
215                         ops = klp_find_ops(func->old_func);
216
217                         if (list_is_singular(&ops->func_stack)) {
218                                 /* original function */
219                                 func_addr = (unsigned long)func->old_func;
220                                 func_size = func->old_size;
221                         } else {
222                                 /* previously patched function */
223                                 struct klp_func *prev;
224
225                                 prev = list_next_entry(func, stack_node);
226                                 func_addr = (unsigned long)prev->new_func;
227                                 func_size = prev->new_size;
228                         }
229                 }
230
231                 if (address >= func_addr && address < func_addr + func_size)
232                         return -EAGAIN;
233         }
234
235         return 0;
236 }
237
238 /*
239  * Determine whether it's safe to transition the task to the target patch state
240  * by looking for any to-be-patched or to-be-unpatched functions on its stack.
241  */
242 static int klp_check_stack(struct task_struct *task, char *err_buf)
243 {
244         static unsigned long entries[MAX_STACK_ENTRIES];
245         struct klp_object *obj;
246         struct klp_func *func;
247         int ret, nr_entries;
248
249         ret = stack_trace_save_tsk_reliable(task, entries, ARRAY_SIZE(entries));
250         if (ret < 0) {
251                 snprintf(err_buf, STACK_ERR_BUF_SIZE,
252                          "%s: %s:%d has an unreliable stack\n",
253                          __func__, task->comm, task->pid);
254                 return ret;
255         }
256         nr_entries = ret;
257
258         klp_for_each_object(klp_transition_patch, obj) {
259                 if (!obj->patched)
260                         continue;
261                 klp_for_each_func(obj, func) {
262                         ret = klp_check_stack_func(func, entries, nr_entries);
263                         if (ret) {
264                                 snprintf(err_buf, STACK_ERR_BUF_SIZE,
265                                          "%s: %s:%d is sleeping on function %s\n",
266                                          __func__, task->comm, task->pid,
267                                          func->old_name);
268                                 return ret;
269                         }
270                 }
271         }
272
273         return 0;
274 }
275
276 /*
277  * Try to safely switch a task to the target patch state.  If it's currently
278  * running, or it's sleeping on a to-be-patched or to-be-unpatched function, or
279  * if the stack is unreliable, return false.
280  */
281 static bool klp_try_switch_task(struct task_struct *task)
282 {
283         static char err_buf[STACK_ERR_BUF_SIZE];
284         struct rq *rq;
285         struct rq_flags flags;
286         int ret;
287         bool success = false;
288
289         err_buf[0] = '\0';
290
291         /* check if this task has already switched over */
292         if (task->patch_state == klp_target_state)
293                 return true;
294
295         /*
296          * For arches which don't have reliable stack traces, we have to rely
297          * on other methods (e.g., switching tasks at kernel exit).
298          */
299         if (!klp_have_reliable_stack())
300                 return false;
301
302         /*
303          * Now try to check the stack for any to-be-patched or to-be-unpatched
304          * functions.  If all goes well, switch the task to the target patch
305          * state.
306          */
307         rq = task_rq_lock(task, &flags);
308
309         if (task_running(rq, task) && task != current) {
310                 snprintf(err_buf, STACK_ERR_BUF_SIZE,
311                          "%s: %s:%d is running\n", __func__, task->comm,
312                          task->pid);
313                 goto done;
314         }
315
316         ret = klp_check_stack(task, err_buf);
317         if (ret)
318                 goto done;
319
320         success = true;
321
322         clear_tsk_thread_flag(task, TIF_PATCH_PENDING);
323         task->patch_state = klp_target_state;
324
325 done:
326         task_rq_unlock(rq, task, &flags);
327
328         /*
329          * Due to console deadlock issues, pr_debug() can't be used while
330          * holding the task rq lock.  Instead we have to use a temporary buffer
331          * and print the debug message after releasing the lock.
332          */
333         if (err_buf[0] != '\0')
334                 pr_debug("%s", err_buf);
335
336         return success;
337 }
338
339 /*
340  * Sends a fake signal to all non-kthread tasks with TIF_PATCH_PENDING set.
341  * Kthreads with TIF_PATCH_PENDING set are woken up.
342  */
343 static void klp_send_signals(void)
344 {
345         struct task_struct *g, *task;
346
347         if (klp_signals_cnt == SIGNALS_TIMEOUT)
348                 pr_notice("signaling remaining tasks\n");
349
350         read_lock(&tasklist_lock);
351         for_each_process_thread(g, task) {
352                 if (!klp_patch_pending(task))
353                         continue;
354
355                 /*
356                  * There is a small race here. We could see TIF_PATCH_PENDING
357                  * set and decide to wake up a kthread or send a fake signal.
358                  * Meanwhile the task could migrate itself and the action
359                  * would be meaningless. It is not serious though.
360                  */
361                 if (task->flags & PF_KTHREAD) {
362                         /*
363                          * Wake up a kthread which sleeps interruptedly and
364                          * still has not been migrated.
365                          */
366                         wake_up_state(task, TASK_INTERRUPTIBLE);
367                 } else {
368                         /*
369                          * Send fake signal to all non-kthread tasks which are
370                          * still not migrated.
371                          */
372                         spin_lock_irq(&task->sighand->siglock);
373                         signal_wake_up(task, 0);
374                         spin_unlock_irq(&task->sighand->siglock);
375                 }
376         }
377         read_unlock(&tasklist_lock);
378 }
379
380 /*
381  * Try to switch all remaining tasks to the target patch state by walking the
382  * stacks of sleeping tasks and looking for any to-be-patched or
383  * to-be-unpatched functions.  If such functions are found, the task can't be
384  * switched yet.
385  *
386  * If any tasks are still stuck in the initial patch state, schedule a retry.
387  */
388 void klp_try_complete_transition(void)
389 {
390         unsigned int cpu;
391         struct task_struct *g, *task;
392         struct klp_patch *patch;
393         bool complete = true;
394
395         WARN_ON_ONCE(klp_target_state == KLP_UNDEFINED);
396
397         /*
398          * Try to switch the tasks to the target patch state by walking their
399          * stacks and looking for any to-be-patched or to-be-unpatched
400          * functions.  If such functions are found on a stack, or if the stack
401          * is deemed unreliable, the task can't be switched yet.
402          *
403          * Usually this will transition most (or all) of the tasks on a system
404          * unless the patch includes changes to a very common function.
405          */
406         read_lock(&tasklist_lock);
407         for_each_process_thread(g, task)
408                 if (!klp_try_switch_task(task))
409                         complete = false;
410         read_unlock(&tasklist_lock);
411
412         /*
413          * Ditto for the idle "swapper" tasks.
414          */
415         get_online_cpus();
416         for_each_possible_cpu(cpu) {
417                 task = idle_task(cpu);
418                 if (cpu_online(cpu)) {
419                         if (!klp_try_switch_task(task))
420                                 complete = false;
421                 } else if (task->patch_state != klp_target_state) {
422                         /* offline idle tasks can be switched immediately */
423                         clear_tsk_thread_flag(task, TIF_PATCH_PENDING);
424                         task->patch_state = klp_target_state;
425                 }
426         }
427         put_online_cpus();
428
429         if (!complete) {
430                 if (klp_signals_cnt && !(klp_signals_cnt % SIGNALS_TIMEOUT))
431                         klp_send_signals();
432                 klp_signals_cnt++;
433
434                 /*
435                  * Some tasks weren't able to be switched over.  Try again
436                  * later and/or wait for other methods like kernel exit
437                  * switching.
438                  */
439                 schedule_delayed_work(&klp_transition_work,
440                                       round_jiffies_relative(HZ));
441                 return;
442         }
443
444         /* we're done, now cleanup the data structures */
445         patch = klp_transition_patch;
446         klp_complete_transition();
447
448         /*
449          * It would make more sense to free the patch in
450          * klp_complete_transition() but it is called also
451          * from klp_cancel_transition().
452          */
453         if (!patch->enabled) {
454                 klp_free_patch_start(patch);
455                 schedule_work(&patch->free_work);
456         }
457 }
458
459 /*
460  * Start the transition to the specified target patch state so tasks can begin
461  * switching to it.
462  */
463 void klp_start_transition(void)
464 {
465         struct task_struct *g, *task;
466         unsigned int cpu;
467
468         WARN_ON_ONCE(klp_target_state == KLP_UNDEFINED);
469
470         pr_notice("'%s': starting %s transition\n",
471                   klp_transition_patch->mod->name,
472                   klp_target_state == KLP_PATCHED ? "patching" : "unpatching");
473
474         /*
475          * Mark all normal tasks as needing a patch state update.  They'll
476          * switch either in klp_try_complete_transition() or as they exit the
477          * kernel.
478          */
479         read_lock(&tasklist_lock);
480         for_each_process_thread(g, task)
481                 if (task->patch_state != klp_target_state)
482                         set_tsk_thread_flag(task, TIF_PATCH_PENDING);
483         read_unlock(&tasklist_lock);
484
485         /*
486          * Mark all idle tasks as needing a patch state update.  They'll switch
487          * either in klp_try_complete_transition() or at the idle loop switch
488          * point.
489          */
490         for_each_possible_cpu(cpu) {
491                 task = idle_task(cpu);
492                 if (task->patch_state != klp_target_state)
493                         set_tsk_thread_flag(task, TIF_PATCH_PENDING);
494         }
495
496         klp_signals_cnt = 0;
497 }
498
499 /*
500  * Initialize the global target patch state and all tasks to the initial patch
501  * state, and initialize all function transition states to true in preparation
502  * for patching or unpatching.
503  */
504 void klp_init_transition(struct klp_patch *patch, int state)
505 {
506         struct task_struct *g, *task;
507         unsigned int cpu;
508         struct klp_object *obj;
509         struct klp_func *func;
510         int initial_state = !state;
511
512         WARN_ON_ONCE(klp_target_state != KLP_UNDEFINED);
513
514         klp_transition_patch = patch;
515
516         /*
517          * Set the global target patch state which tasks will switch to.  This
518          * has no effect until the TIF_PATCH_PENDING flags get set later.
519          */
520         klp_target_state = state;
521
522         pr_debug("'%s': initializing %s transition\n", patch->mod->name,
523                  klp_target_state == KLP_PATCHED ? "patching" : "unpatching");
524
525         /*
526          * Initialize all tasks to the initial patch state to prepare them for
527          * switching to the target state.
528          */
529         read_lock(&tasklist_lock);
530         for_each_process_thread(g, task) {
531                 WARN_ON_ONCE(task->patch_state != KLP_UNDEFINED);
532                 task->patch_state = initial_state;
533         }
534         read_unlock(&tasklist_lock);
535
536         /*
537          * Ditto for the idle "swapper" tasks.
538          */
539         for_each_possible_cpu(cpu) {
540                 task = idle_task(cpu);
541                 WARN_ON_ONCE(task->patch_state != KLP_UNDEFINED);
542                 task->patch_state = initial_state;
543         }
544
545         /*
546          * Enforce the order of the task->patch_state initializations and the
547          * func->transition updates to ensure that klp_ftrace_handler() doesn't
548          * see a func in transition with a task->patch_state of KLP_UNDEFINED.
549          *
550          * Also enforce the order of the klp_target_state write and future
551          * TIF_PATCH_PENDING writes to ensure klp_update_patch_state() doesn't
552          * set a task->patch_state to KLP_UNDEFINED.
553          */
554         smp_wmb();
555
556         /*
557          * Set the func transition states so klp_ftrace_handler() will know to
558          * switch to the transition logic.
559          *
560          * When patching, the funcs aren't yet in the func_stack and will be
561          * made visible to the ftrace handler shortly by the calls to
562          * klp_patch_object().
563          *
564          * When unpatching, the funcs are already in the func_stack and so are
565          * already visible to the ftrace handler.
566          */
567         klp_for_each_object(patch, obj)
568                 klp_for_each_func(obj, func)
569                         func->transition = true;
570 }
571
572 /*
573  * This function can be called in the middle of an existing transition to
574  * reverse the direction of the target patch state.  This can be done to
575  * effectively cancel an existing enable or disable operation if there are any
576  * tasks which are stuck in the initial patch state.
577  */
578 void klp_reverse_transition(void)
579 {
580         unsigned int cpu;
581         struct task_struct *g, *task;
582
583         pr_debug("'%s': reversing transition from %s\n",
584                  klp_transition_patch->mod->name,
585                  klp_target_state == KLP_PATCHED ? "patching to unpatching" :
586                                                    "unpatching to patching");
587
588         klp_transition_patch->enabled = !klp_transition_patch->enabled;
589
590         klp_target_state = !klp_target_state;
591
592         /*
593          * Clear all TIF_PATCH_PENDING flags to prevent races caused by
594          * klp_update_patch_state() running in parallel with
595          * klp_start_transition().
596          */
597         read_lock(&tasklist_lock);
598         for_each_process_thread(g, task)
599                 clear_tsk_thread_flag(task, TIF_PATCH_PENDING);
600         read_unlock(&tasklist_lock);
601
602         for_each_possible_cpu(cpu)
603                 clear_tsk_thread_flag(idle_task(cpu), TIF_PATCH_PENDING);
604
605         /* Let any remaining calls to klp_update_patch_state() complete */
606         klp_synchronize_transition();
607
608         klp_start_transition();
609 }
610
611 /* Called from copy_process() during fork */
612 void klp_copy_process(struct task_struct *child)
613 {
614         child->patch_state = current->patch_state;
615
616         /* TIF_PATCH_PENDING gets copied in setup_thread_stack() */
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  */
628 void klp_force_transition(void)
629 {
630         struct klp_patch *patch;
631         struct task_struct *g, *task;
632         unsigned int cpu;
633
634         pr_warn("forcing remaining tasks to the patched state\n");
635
636         read_lock(&tasklist_lock);
637         for_each_process_thread(g, task)
638                 klp_update_patch_state(task);
639         read_unlock(&tasklist_lock);
640
641         for_each_possible_cpu(cpu)
642                 klp_update_patch_state(idle_task(cpu));
643
644         klp_for_each_patch(patch)
645                 patch->forced = true;
646 }