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TOMOYO Linux Cross Reference
Linux/kernel/livepatch/transition.c

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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 
 32 struct klp_patch *klp_transition_patch;
 33 
 34 static int klp_target_state = KLP_UNDEFINED;
 35 
 36 static 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  */
 42 static 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 }
 51 static 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  */
 58 static 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  */
 70 static 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  */
 79 static 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  */
156 void 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  */
175 void 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  */
205 static 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  */
254 static 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  */
298 static 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 
342 done:
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  */
365 void 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  */
424 void 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  */
463 void 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  */
537 void 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 */
571 void 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  */
583 void 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  */
628 void 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 }
645 

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