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