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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          * Now try to check the stack for any to-be-patched or to-be-unpatched
314          * functions.  If all goes well, switch the task to the target patch
315          * state.
316          */
317         rq = task_rq_lock(task, &flags);
318 
319         if (task_running(rq, task) && task != current) {
320                 snprintf(err_buf, STACK_ERR_BUF_SIZE,
321                          "%s: %s:%d is running\n", __func__, task->comm,
322                          task->pid);
323                 goto done;
324         }
325 
326         ret = klp_check_stack(task, err_buf);
327         if (ret)
328                 goto done;
329 
330         success = true;
331 
332         clear_tsk_thread_flag(task, TIF_PATCH_PENDING);
333         task->patch_state = klp_target_state;
334 
335 done:
336         task_rq_unlock(rq, task, &flags);
337 
338         /*
339          * Due to console deadlock issues, pr_debug() can't be used while
340          * holding the task rq lock.  Instead we have to use a temporary buffer
341          * and print the debug message after releasing the lock.
342          */
343         if (err_buf[0] != '\0')
344                 pr_debug("%s", err_buf);
345 
346         return success;
347 
348 }
349 
350 /*
351  * Try to switch all remaining tasks to the target patch state by walking the
352  * stacks of sleeping tasks and looking for any to-be-patched or
353  * to-be-unpatched functions.  If such functions are found, the task can't be
354  * switched yet.
355  *
356  * If any tasks are still stuck in the initial patch state, schedule a retry.
357  */
358 void klp_try_complete_transition(void)
359 {
360         unsigned int cpu;
361         struct task_struct *g, *task;
362         bool complete = true;
363 
364         WARN_ON_ONCE(klp_target_state == KLP_UNDEFINED);
365 
366         /*
367          * Try to switch the tasks to the target patch state by walking their
368          * stacks and looking for any to-be-patched or to-be-unpatched
369          * functions.  If such functions are found on a stack, or if the stack
370          * is deemed unreliable, the task can't be switched yet.
371          *
372          * Usually this will transition most (or all) of the tasks on a system
373          * unless the patch includes changes to a very common function.
374          */
375         read_lock(&tasklist_lock);
376         for_each_process_thread(g, task)
377                 if (!klp_try_switch_task(task))
378                         complete = false;
379         read_unlock(&tasklist_lock);
380 
381         /*
382          * Ditto for the idle "swapper" tasks.
383          */
384         get_online_cpus();
385         for_each_possible_cpu(cpu) {
386                 task = idle_task(cpu);
387                 if (cpu_online(cpu)) {
388                         if (!klp_try_switch_task(task))
389                                 complete = false;
390                 } else if (task->patch_state != klp_target_state) {
391                         /* offline idle tasks can be switched immediately */
392                         clear_tsk_thread_flag(task, TIF_PATCH_PENDING);
393                         task->patch_state = klp_target_state;
394                 }
395         }
396         put_online_cpus();
397 
398         if (!complete) {
399                 /*
400                  * Some tasks weren't able to be switched over.  Try again
401                  * later and/or wait for other methods like kernel exit
402                  * switching.
403                  */
404                 schedule_delayed_work(&klp_transition_work,
405                                       round_jiffies_relative(HZ));
406                 return;
407         }
408 
409         /* we're done, now cleanup the data structures */
410         klp_complete_transition();
411 }
412 
413 /*
414  * Start the transition to the specified target patch state so tasks can begin
415  * switching to it.
416  */
417 void klp_start_transition(void)
418 {
419         struct task_struct *g, *task;
420         unsigned int cpu;
421 
422         WARN_ON_ONCE(klp_target_state == KLP_UNDEFINED);
423 
424         pr_notice("'%s': starting %s transition\n",
425                   klp_transition_patch->mod->name,
426                   klp_target_state == KLP_PATCHED ? "patching" : "unpatching");
427 
428         /*
429          * Mark all normal tasks as needing a patch state update.  They'll
430          * switch either in klp_try_complete_transition() or as they exit the
431          * kernel.
432          */
433         read_lock(&tasklist_lock);
434         for_each_process_thread(g, task)
435                 if (task->patch_state != klp_target_state)
436                         set_tsk_thread_flag(task, TIF_PATCH_PENDING);
437         read_unlock(&tasklist_lock);
438 
439         /*
440          * Mark all idle tasks as needing a patch state update.  They'll switch
441          * either in klp_try_complete_transition() or at the idle loop switch
442          * point.
443          */
444         for_each_possible_cpu(cpu) {
445                 task = idle_task(cpu);
446                 if (task->patch_state != klp_target_state)
447                         set_tsk_thread_flag(task, TIF_PATCH_PENDING);
448         }
449 }
450 
451 /*
452  * Initialize the global target patch state and all tasks to the initial patch
453  * state, and initialize all function transition states to true in preparation
454  * for patching or unpatching.
455  */
456 void klp_init_transition(struct klp_patch *patch, int state)
457 {
458         struct task_struct *g, *task;
459         unsigned int cpu;
460         struct klp_object *obj;
461         struct klp_func *func;
462         int initial_state = !state;
463 
464         WARN_ON_ONCE(klp_target_state != KLP_UNDEFINED);
465 
466         klp_transition_patch = patch;
467 
468         /*
469          * Set the global target patch state which tasks will switch to.  This
470          * has no effect until the TIF_PATCH_PENDING flags get set later.
471          */
472         klp_target_state = state;
473 
474         pr_debug("'%s': initializing %s transition\n", patch->mod->name,
475                  klp_target_state == KLP_PATCHED ? "patching" : "unpatching");
476 
477         /*
478          * Initialize all tasks to the initial patch state to prepare them for
479          * switching to the target state.
480          */
481         read_lock(&tasklist_lock);
482         for_each_process_thread(g, task) {
483                 WARN_ON_ONCE(task->patch_state != KLP_UNDEFINED);
484                 task->patch_state = initial_state;
485         }
486         read_unlock(&tasklist_lock);
487 
488         /*
489          * Ditto for the idle "swapper" tasks.
490          */
491         for_each_possible_cpu(cpu) {
492                 task = idle_task(cpu);
493                 WARN_ON_ONCE(task->patch_state != KLP_UNDEFINED);
494                 task->patch_state = initial_state;
495         }
496 
497         /*
498          * Enforce the order of the task->patch_state initializations and the
499          * func->transition updates to ensure that klp_ftrace_handler() doesn't
500          * see a func in transition with a task->patch_state of KLP_UNDEFINED.
501          *
502          * Also enforce the order of the klp_target_state write and future
503          * TIF_PATCH_PENDING writes to ensure klp_update_patch_state() doesn't
504          * set a task->patch_state to KLP_UNDEFINED.
505          */
506         smp_wmb();
507 
508         /*
509          * Set the func transition states so klp_ftrace_handler() will know to
510          * switch to the transition logic.
511          *
512          * When patching, the funcs aren't yet in the func_stack and will be
513          * made visible to the ftrace handler shortly by the calls to
514          * klp_patch_object().
515          *
516          * When unpatching, the funcs are already in the func_stack and so are
517          * already visible to the ftrace handler.
518          */
519         klp_for_each_object(patch, obj)
520                 klp_for_each_func(obj, func)
521                         func->transition = true;
522 }
523 
524 /*
525  * This function can be called in the middle of an existing transition to
526  * reverse the direction of the target patch state.  This can be done to
527  * effectively cancel an existing enable or disable operation if there are any
528  * tasks which are stuck in the initial patch state.
529  */
530 void klp_reverse_transition(void)
531 {
532         unsigned int cpu;
533         struct task_struct *g, *task;
534 
535         pr_debug("'%s': reversing transition from %s\n",
536                  klp_transition_patch->mod->name,
537                  klp_target_state == KLP_PATCHED ? "patching to unpatching" :
538                                                    "unpatching to patching");
539 
540         klp_transition_patch->enabled = !klp_transition_patch->enabled;
541 
542         klp_target_state = !klp_target_state;
543 
544         /*
545          * Clear all TIF_PATCH_PENDING flags to prevent races caused by
546          * klp_update_patch_state() running in parallel with
547          * klp_start_transition().
548          */
549         read_lock(&tasklist_lock);
550         for_each_process_thread(g, task)
551                 clear_tsk_thread_flag(task, TIF_PATCH_PENDING);
552         read_unlock(&tasklist_lock);
553 
554         for_each_possible_cpu(cpu)
555                 clear_tsk_thread_flag(idle_task(cpu), TIF_PATCH_PENDING);
556 
557         /* Let any remaining calls to klp_update_patch_state() complete */
558         klp_synchronize_transition();
559 
560         klp_start_transition();
561 }
562 
563 /* Called from copy_process() during fork */
564 void klp_copy_process(struct task_struct *child)
565 {
566         child->patch_state = current->patch_state;
567 
568         /* TIF_PATCH_PENDING gets copied in setup_thread_stack() */
569 }
570 
571 /*
572  * Sends a fake signal to all non-kthread tasks with TIF_PATCH_PENDING set.
573  * Kthreads with TIF_PATCH_PENDING set are woken up. Only admin can request this
574  * action currently.
575  */
576 void klp_send_signals(void)
577 {
578         struct task_struct *g, *task;
579 
580         pr_notice("signaling remaining tasks\n");
581 
582         read_lock(&tasklist_lock);
583         for_each_process_thread(g, task) {
584                 if (!klp_patch_pending(task))
585                         continue;
586 
587                 /*
588                  * There is a small race here. We could see TIF_PATCH_PENDING
589                  * set and decide to wake up a kthread or send a fake signal.
590                  * Meanwhile the task could migrate itself and the action
591                  * would be meaningless. It is not serious though.
592                  */
593                 if (task->flags & PF_KTHREAD) {
594                         /*
595                          * Wake up a kthread which sleeps interruptedly and
596                          * still has not been migrated.
597                          */
598                         wake_up_state(task, TASK_INTERRUPTIBLE);
599                 } else {
600                         /*
601                          * Send fake signal to all non-kthread tasks which are
602                          * still not migrated.
603                          */
604                         spin_lock_irq(&task->sighand->siglock);
605                         signal_wake_up(task, 0);
606                         spin_unlock_irq(&task->sighand->siglock);
607                 }
608         }
609         read_unlock(&tasklist_lock);
610 }
611 
612 /*
613  * Drop TIF_PATCH_PENDING of all tasks on admin's request. This forces an
614  * existing transition to finish.
615  *
616  * NOTE: klp_update_patch_state(task) requires the task to be inactive or
617  * 'current'. This is not the case here and the consistency model could be
618  * broken. Administrator, who is the only one to execute the
619  * klp_force_transitions(), has to be aware of this.
620  */
621 void klp_force_transition(void)
622 {
623         struct task_struct *g, *task;
624         unsigned int cpu;
625 
626         pr_warn("forcing remaining tasks to the patched state\n");
627 
628         read_lock(&tasklist_lock);
629         for_each_process_thread(g, task)
630                 klp_update_patch_state(task);
631         read_unlock(&tasklist_lock);
632 
633         for_each_possible_cpu(cpu)
634                 klp_update_patch_state(idle_task(cpu));
635 
636         klp_forced = true;
637 }
638 

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