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 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_UNDEFINED;
  27
  28 static unsigned int klp_signals_cnt;
  29
  30 /*
  31  * When a livepatch is in progress, enable klp stack checking in
  32  * cond_resched().  This helps CPU-bound kthreads get patched.
  33  */
  34 #if defined(CONFIG_PREEMPT_DYNAMIC) && defined(CONFIG_HAVE_PREEMPT_DYNAMIC_CALL)
  35
  36 #define klp_cond_resched_enable() sched_dynamic_klp_enable()
  37 #define klp_cond_resched_disable() sched_dynamic_klp_disable()
  38
  39 #else /* !CONFIG_PREEMPT_DYNAMIC || !CONFIG_HAVE_PREEMPT_DYNAMIC_CALL */
  40
  41 DEFINE_STATIC_KEY_FALSE(klp_sched_try_switch_key);
  42 EXPORT_SYMBOL(klp_sched_try_switch_key);
  43
  44 #define klp_cond_resched_enable() static_branch_enable(&klp_sched_try_switch_key)
  45 #define klp_cond_resched_disable() static_branch_disable(&klp_sched_try_switch_key)
  46
  47 #endif /* CONFIG_PREEMPT_DYNAMIC && CONFIG_HAVE_PREEMPT_DYNAMIC_CALL */
  48
  49 /*
  50  * This work can be performed periodically to finish patching or unpatching any
  51  * "straggler" tasks which failed to transition in the first attempt.
  52  */
  53 static void klp_transition_work_fn(struct work_struct *work)
  54 {
  55         mutex_lock(&klp_mutex);
  56
  57         if (klp_transition_patch)
  58                 klp_try_complete_transition();
  59
  60         mutex_unlock(&klp_mutex);
  61 }
  62 static DECLARE_DELAYED_WORK(klp_transition_work, klp_transition_work_fn);
  63
  64 /*
  65  * This function is just a stub to implement a hard force
  66  * of synchronize_rcu(). This requires synchronizing
  67  * tasks even in userspace and idle.
  68  */
  69 static void klp_sync(struct work_struct *work)
  70 {
  71 }
  72
  73 /*
  74  * We allow to patch also functions where RCU is not watching,
  75  * e.g. before user_exit(). We can not rely on the RCU infrastructure
  76  * to do the synchronization. Instead hard force the sched synchronization.
  77  *
  78  * This approach allows to use RCU functions for manipulating func_stack
  79  * safely.
  80  */
  81 static void klp_synchronize_transition(void)
  82 {
  83         schedule_on_each_cpu(klp_sync);
  84 }
  85
  86 /*
  87  * The transition to the target patch state is complete.  Clean up the data
  88  * structures.
  89  */
  90 static void klp_complete_transition(void)
  91 {
  92         struct klp_object *obj;
  93         struct klp_func *func;
  94         struct task_struct *g, *task;
  95         unsigned int cpu;
  96
  97         pr_debug("'%s': completing %s transition\n",
  98                  klp_transition_patch->mod->name,
  99                  klp_target_state == KLP_PATCHED ? "patching" : "unpatching");
 100
 101         if (klp_transition_patch->replace && klp_target_state == KLP_PATCHED) {
 102                 klp_unpatch_replaced_patches(klp_transition_patch);
 103                 klp_discard_nops(klp_transition_patch);
 104         }
 105
 106         if (klp_target_state == KLP_UNPATCHED) {
 107                 /*
 108                  * All tasks have transitioned to KLP_UNPATCHED so we can now
 109                  * remove the new functions from the func_stack.
 110                  */
 111                 klp_unpatch_objects(klp_transition_patch);
 112
 113                 /*
 114                  * Make sure klp_ftrace_handler() can no longer see functions
 115                  * from this patch on the ops->func_stack.  Otherwise, after
 116                  * func->transition gets cleared, the handler may choose a
 117                  * removed function.
 118                  */
 119                 klp_synchronize_transition();
 120         }
 121
 122         klp_for_each_object(klp_transition_patch, obj)
 123                 klp_for_each_func(obj, func)
 124                         func->transition = false;
 125
 126         /* Prevent klp_ftrace_handler() from seeing KLP_UNDEFINED state */
 127         if (klp_target_state == KLP_PATCHED)
 128                 klp_synchronize_transition();
 129
 130         read_lock(&tasklist_lock);
 131         for_each_process_thread(g, task) {
 132                 WARN_ON_ONCE(test_tsk_thread_flag(task, TIF_PATCH_PENDING));
 133                 task->patch_state = KLP_UNDEFINED;
 134         }
 135         read_unlock(&tasklist_lock);
 136
 137         for_each_possible_cpu(cpu) {
 138                 task = idle_task(cpu);
 139                 WARN_ON_ONCE(test_tsk_thread_flag(task, TIF_PATCH_PENDING));
 140                 task->patch_state = KLP_UNDEFINED;
 141         }
 142
 143         klp_for_each_object(klp_transition_patch, obj) {
 144                 if (!klp_is_object_loaded(obj))
 145                         continue;
 146                 if (klp_target_state == KLP_PATCHED)
 147                         klp_post_patch_callback(obj);
 148                 else if (klp_target_state == KLP_UNPATCHED)
 149                         klp_post_unpatch_callback(obj);
 150         }
 151
 152         pr_notice("'%s': %s complete\n", klp_transition_patch->mod->name,
 153                   klp_target_state == KLP_PATCHED ? "patching" : "unpatching");
 154
 155         klp_target_state = KLP_UNDEFINED;
 156         klp_transition_patch = NULL;
 157 }
 158
 159 /*
 160  * This is called in the error path, to cancel a transition before it has
 161  * started, i.e. klp_init_transition() has been called but
 162  * klp_start_transition() hasn't.  If the transition *has* been started,
 163  * klp_reverse_transition() should be used instead.
 164  */
 165 void klp_cancel_transition(void)
 166 {
 167         if (WARN_ON_ONCE(klp_target_state != KLP_PATCHED))
 168                 return;
 169
 170         pr_debug("'%s': canceling patching transition, going to unpatch\n",
 171                  klp_transition_patch->mod->name);
 172
 173         klp_target_state = KLP_UNPATCHED;
 174         klp_complete_transition();
 175 }
 176
 177 /*
 178  * Switch the patched state of the task to the set of functions in the target
 179  * patch state.
 180  *
 181  * NOTE: If task is not 'current', the caller must ensure the task is inactive.
 182  * Otherwise klp_ftrace_handler() might read the wrong 'patch_state' value.
 183  */
 184 void klp_update_patch_state(struct task_struct *task)
 185 {
 186         /*
 187          * A variant of synchronize_rcu() is used to allow patching functions
 188          * where RCU is not watching, see klp_synchronize_transition().
 189          */
 190         preempt_disable_notrace();
 191
 192         /*
 193          * This test_and_clear_tsk_thread_flag() call also serves as a read
 194          * barrier (smp_rmb) for two cases:
 195          *
 196          * 1) Enforce the order of the TIF_PATCH_PENDING read and the
 197          *    klp_target_state read.  The corresponding write barriers are in
 198          *    klp_init_transition() and klp_reverse_transition().
 199          *
 200          * 2) Enforce the order of the TIF_PATCH_PENDING read and a future read
 201          *    of func->transition, if klp_ftrace_handler() is called later on
 202          *    the same CPU.  See __klp_disable_patch().
 203          */
 204         if (test_and_clear_tsk_thread_flag(task, TIF_PATCH_PENDING))
 205                 task->patch_state = READ_ONCE(klp_target_state);
 206
 207         preempt_enable_notrace();
 208 }
 209
 210 /*
 211  * Determine whether the given stack trace includes any references to a
 212  * to-be-patched or to-be-unpatched function.
 213  */
 214 static int klp_check_stack_func(struct klp_func *func, unsigned long *entries,
 215                                 unsigned int nr_entries)
 216 {
 217         unsigned long func_addr, func_size, address;
 218         struct klp_ops *ops;
 219         int i;
 220
 221         if (klp_target_state == KLP_UNPATCHED) {
 222                  /*
 223                   * Check for the to-be-unpatched function
 224                   * (the func itself).
 225                   */
 226                 func_addr = (unsigned long)func->new_func;
 227                 func_size = func->new_size;
 228         } else {
 229                 /*
 230                  * Check for the to-be-patched function
 231                  * (the previous func).
 232                  */
 233                 ops = klp_find_ops(func->old_func);
 234
 235                 if (list_is_singular(&ops->func_stack)) {
 236                         /* original function */
 237                         func_addr = (unsigned long)func->old_func;
 238                         func_size = func->old_size;
 239                 } else {
 240                         /* previously patched function */
 241                         struct klp_func *prev;
 242
 243                         prev = list_next_entry(func, stack_node);
 244                         func_addr = (unsigned long)prev->new_func;
 245                         func_size = prev->new_size;
 246                 }
 247         }
 248
 249         for (i = 0; i < nr_entries; i++) {
 250                 address = entries[i];
 251
 252                 if (address >= func_addr && address < func_addr + func_size)
 253                         return -EAGAIN;
 254         }
 255
 256         return 0;
 257 }
 258
 259 /*
 260  * Determine whether it's safe to transition the task to the target patch state
 261  * by looking for any to-be-patched or to-be-unpatched functions on its stack.
 262  */
 263 static int klp_check_stack(struct task_struct *task, const char **oldname)
 264 {
 265         unsigned long *entries = this_cpu_ptr(klp_stack_entries);
 266         struct klp_object *obj;
 267         struct klp_func *func;
 268         int ret, nr_entries;
 269
 270         /* Protect 'klp_stack_entries' */
 271         lockdep_assert_preemption_disabled();
 272
 273         ret = stack_trace_save_tsk_reliable(task, entries, MAX_STACK_ENTRIES);
 274         if (ret < 0)
 275                 return -EINVAL;
 276         nr_entries = ret;
 277
 278         klp_for_each_object(klp_transition_patch, obj) {
 279                 if (!obj->patched)
 280                         continue;
 281                 klp_for_each_func(obj, func) {
 282                         ret = klp_check_stack_func(func, entries, nr_entries);
 283                         if (ret) {
 284                                 *oldname = func->old_name;
 285                                 return -EADDRINUSE;
 286                         }
 287                 }
 288         }
 289
 290         return 0;
 291 }
 292
 293 static int klp_check_and_switch_task(struct task_struct *task, void *arg)
 294 {
 295         int ret;
 296
 297         if (task_curr(task) && task != current)
 298                 return -EBUSY;
 299
 300         ret = klp_check_stack(task, arg);
 301         if (ret)
 302                 return ret;
 303
 304         clear_tsk_thread_flag(task, TIF_PATCH_PENDING);
 305         task->patch_state = klp_target_state;
 306         return 0;
 307 }
 308
 309 /*
 310  * Try to safely switch a task to the target patch state.  If it's currently
 311  * running, or it's sleeping on a to-be-patched or to-be-unpatched function, or
 312  * if the stack is unreliable, return false.
 313  */
 314 static bool klp_try_switch_task(struct task_struct *task)
 315 {
 316         const char *old_name;
 317         int ret;
 318
 319         /* check if this task has already switched over */
 320         if (task->patch_state == klp_target_state)
 321                 return true;
 322
 323         /*
 324          * For arches which don't have reliable stack traces, we have to rely
 325          * on other methods (e.g., switching tasks at kernel exit).
 326          */
 327         if (!klp_have_reliable_stack())
 328                 return false;
 329
 330         /*
 331          * Now try to check the stack for any to-be-patched or to-be-unpatched
 332          * functions.  If all goes well, switch the task to the target patch
 333          * state.
 334          */
 335         if (task == current)
 336                 ret = klp_check_and_switch_task(current, &old_name);
 337         else
 338                 ret = task_call_func(task, klp_check_and_switch_task, &old_name);
 339
 340         switch (ret) {
 341         case 0:         /* success */
 342                 break;
 343
 344         case -EBUSY:    /* klp_check_and_switch_task() */
 345                 pr_debug("%s: %s:%d is running\n",
 346                          __func__, task->comm, task->pid);
 347                 break;
 348         case -EINVAL:   /* klp_check_and_switch_task() */
 349                 pr_debug("%s: %s:%d has an unreliable stack\n",
 350                          __func__, task->comm, task->pid);
 351                 break;
 352         case -EADDRINUSE: /* klp_check_and_switch_task() */
 353                 pr_debug("%s: %s:%d is sleeping on function %s\n",
 354                          __func__, task->comm, task->pid, old_name);
 355                 break;
 356
 357         default:
 358                 pr_debug("%s: Unknown error code (%d) when trying to switch %s:%d\n",
 359                          __func__, ret, task->comm, task->pid);
 360                 break;
 361         }
 362
 363         return !ret;
 364 }
 365
 366 void __klp_sched_try_switch(void)
 367 {
 368         if (likely(!klp_patch_pending(current)))
 369                 return;
 370
 371         /*
 372          * This function is called from cond_resched() which is called in many
 373          * places throughout the kernel.  Using the klp_mutex here might
 374          * deadlock.
 375          *
 376          * Instead, disable preemption to prevent racing with other callers of
 377          * klp_try_switch_task().  Thanks to task_call_func() they won't be
 378          * able to switch this task while it's running.
 379          */
 380         preempt_disable();
 381
 382         /*
 383          * Make sure current didn't get patched between the above check and
 384          * preempt_disable().
 385          */
 386         if (unlikely(!klp_patch_pending(current)))
 387                 goto out;
 388
 389         /*
 390          * Enforce the order of the TIF_PATCH_PENDING read above and the
 391          * klp_target_state read in klp_try_switch_task().  The corresponding
 392          * write barriers are in klp_init_transition() and
 393          * klp_reverse_transition().
 394          */
 395         smp_rmb();
 396
 397         klp_try_switch_task(current);
 398
 399 out:
 400         preempt_enable();
 401 }
 402 EXPORT_SYMBOL(__klp_sched_try_switch);
 403
 404 /*
 405  * Sends a fake signal to all non-kthread tasks with TIF_PATCH_PENDING set.
 406  * Kthreads with TIF_PATCH_PENDING set are woken up.
 407  */
 408 static void klp_send_signals(void)
 409 {
 410         struct task_struct *g, *task;
 411
 412         if (klp_signals_cnt == SIGNALS_TIMEOUT)
 413                 pr_notice("signaling remaining tasks\n");
 414
 415         read_lock(&tasklist_lock);
 416         for_each_process_thread(g, task) {
 417                 if (!klp_patch_pending(task))
 418                         continue;
 419
 420                 /*
 421                  * There is a small race here. We could see TIF_PATCH_PENDING
 422                  * set and decide to wake up a kthread or send a fake signal.
 423                  * Meanwhile the task could migrate itself and the action
 424                  * would be meaningless. It is not serious though.
 425                  */
 426                 if (task->flags & PF_KTHREAD) {
 427                         /*
 428                          * Wake up a kthread which sleeps interruptedly and
 429                          * still has not been migrated.
 430                          */
 431                         wake_up_state(task, TASK_INTERRUPTIBLE);
 432                 } else {
 433                         /*
 434                          * Send fake signal to all non-kthread tasks which are
 435                          * still not migrated.
 436                          */
 437                         set_notify_signal(task);
 438                 }
 439         }
 440         read_unlock(&tasklist_lock);
 441 }
 442
 443 /*
 444  * Try to switch all remaining tasks to the target patch state by walking the
 445  * stacks of sleeping tasks and looking for any to-be-patched or
 446  * to-be-unpatched functions.  If such functions are found, the task can't be
 447  * switched yet.
 448  *
 449  * If any tasks are still stuck in the initial patch state, schedule a retry.
 450  */
 451 void klp_try_complete_transition(void)
 452 {
 453         unsigned int cpu;
 454         struct task_struct *g, *task;
 455         struct klp_patch *patch;
 456         bool complete = true;
 457
 458         WARN_ON_ONCE(klp_target_state == KLP_UNDEFINED);
 459
 460         /*
 461          * Try to switch the tasks to the target patch state by walking their
 462          * stacks and looking for any to-be-patched or to-be-unpatched
 463          * functions.  If such functions are found on a stack, or if the stack
 464          * is deemed unreliable, the task can't be switched yet.
 465          *
 466          * Usually this will transition most (or all) of the tasks on a system
 467          * unless the patch includes changes to a very common function.
 468          */
 469         read_lock(&tasklist_lock);
 470         for_each_process_thread(g, task)
 471                 if (!klp_try_switch_task(task))
 472                         complete = false;
 473         read_unlock(&tasklist_lock);
 474
 475         /*
 476          * Ditto for the idle "swapper" tasks.
 477          */
 478         cpus_read_lock();
 479         for_each_possible_cpu(cpu) {
 480                 task = idle_task(cpu);
 481                 if (cpu_online(cpu)) {
 482                         if (!klp_try_switch_task(task)) {
 483                                 complete = false;
 484                                 /* Make idle task go through the main loop. */
 485                                 wake_up_if_idle(cpu);
 486                         }
 487                 } else if (task->patch_state != klp_target_state) {
 488                         /* offline idle tasks can be switched immediately */
 489                         clear_tsk_thread_flag(task, TIF_PATCH_PENDING);
 490                         task->patch_state = klp_target_state;
 491                 }
 492         }
 493         cpus_read_unlock();
 494
 495         if (!complete) {
 496                 if (klp_signals_cnt && !(klp_signals_cnt % SIGNALS_TIMEOUT))
 497                         klp_send_signals();
 498                 klp_signals_cnt++;
 499
 500                 /*
 501                  * Some tasks weren't able to be switched over.  Try again
 502                  * later and/or wait for other methods like kernel exit
 503                  * switching.
 504                  */
 505                 schedule_delayed_work(&klp_transition_work,
 506                                       round_jiffies_relative(HZ));
 507                 return;
 508         }
 509
 510         /* Done!  Now cleanup the data structures. */
 511         klp_cond_resched_disable();
 512         patch = klp_transition_patch;
 513         klp_complete_transition();
 514
 515         /*
 516          * It would make more sense to free the unused patches in
 517          * klp_complete_transition() but it is called also
 518          * from klp_cancel_transition().
 519          */
 520         if (!patch->enabled)
 521                 klp_free_patch_async(patch);
 522         else if (patch->replace)
 523                 klp_free_replaced_patches_async(patch);
 524 }
 525
 526 /*
 527  * Start the transition to the specified target patch state so tasks can begin
 528  * switching to it.
 529  */
 530 void klp_start_transition(void)
 531 {
 532         struct task_struct *g, *task;
 533         unsigned int cpu;
 534
 535         WARN_ON_ONCE(klp_target_state == KLP_UNDEFINED);
 536
 537         pr_notice("'%s': starting %s transition\n",
 538                   klp_transition_patch->mod->name,
 539                   klp_target_state == KLP_PATCHED ? "patching" : "unpatching");
 540
 541         /*
 542          * Mark all normal tasks as needing a patch state update.  They'll
 543          * switch either in klp_try_complete_transition() or as they exit the
 544          * kernel.
 545          */
 546         read_lock(&tasklist_lock);
 547         for_each_process_thread(g, task)
 548                 if (task->patch_state != klp_target_state)
 549                         set_tsk_thread_flag(task, TIF_PATCH_PENDING);
 550         read_unlock(&tasklist_lock);
 551
 552         /*
 553          * Mark all idle tasks as needing a patch state update.  They'll switch
 554          * either in klp_try_complete_transition() or at the idle loop switch
 555          * point.
 556          */
 557         for_each_possible_cpu(cpu) {
 558                 task = idle_task(cpu);
 559                 if (task->patch_state != klp_target_state)
 560                         set_tsk_thread_flag(task, TIF_PATCH_PENDING);
 561         }
 562
 563         klp_cond_resched_enable();
 564
 565         klp_signals_cnt = 0;
 566 }
 567
 568 /*
 569  * Initialize the global target patch state and all tasks to the initial patch
 570  * state, and initialize all function transition states to true in preparation
 571  * for patching or unpatching.
 572  */
 573 void klp_init_transition(struct klp_patch *patch, int state)
 574 {
 575         struct task_struct *g, *task;
 576         unsigned int cpu;
 577         struct klp_object *obj;
 578         struct klp_func *func;
 579         int initial_state = !state;
 580
 581         WARN_ON_ONCE(klp_target_state != KLP_UNDEFINED);
 582
 583         klp_transition_patch = patch;
 584
 585         /*
 586          * Set the global target patch state which tasks will switch to.  This
 587          * has no effect until the TIF_PATCH_PENDING flags get set later.
 588          */
 589         klp_target_state = state;
 590
 591         pr_debug("'%s': initializing %s transition\n", patch->mod->name,
 592                  klp_target_state == KLP_PATCHED ? "patching" : "unpatching");
 593
 594         /*
 595          * Initialize all tasks to the initial patch state to prepare them for
 596          * switching to the target state.
 597          */
 598         read_lock(&tasklist_lock);
 599         for_each_process_thread(g, task) {
 600                 WARN_ON_ONCE(task->patch_state != KLP_UNDEFINED);
 601                 task->patch_state = initial_state;
 602         }
 603         read_unlock(&tasklist_lock);
 604
 605         /*
 606          * Ditto for the idle "swapper" tasks.
 607          */
 608         for_each_possible_cpu(cpu) {
 609                 task = idle_task(cpu);
 610                 WARN_ON_ONCE(task->patch_state != KLP_UNDEFINED);
 611                 task->patch_state = initial_state;
 612         }
 613
 614         /*
 615          * Enforce the order of the task->patch_state initializations and the
 616          * func->transition updates to ensure that klp_ftrace_handler() doesn't
 617          * see a func in transition with a task->patch_state of KLP_UNDEFINED.
 618          *
 619          * Also enforce the order of the klp_target_state write and future
 620          * TIF_PATCH_PENDING writes to ensure klp_update_patch_state() and
 621          * __klp_sched_try_switch() don't set a task->patch_state to
 622          * KLP_UNDEFINED.
 623          */
 624         smp_wmb();
 625
 626         /*
 627          * Set the func transition states so klp_ftrace_handler() will know to
 628          * switch to the transition logic.
 629          *
 630          * When patching, the funcs aren't yet in the func_stack and will be
 631          * made visible to the ftrace handler shortly by the calls to
 632          * klp_patch_object().
 633          *
 634          * When unpatching, the funcs are already in the func_stack and so are
 635          * already visible to the ftrace handler.
 636          */
 637         klp_for_each_object(patch, obj)
 638                 klp_for_each_func(obj, func)
 639                         func->transition = true;
 640 }
 641
 642 /*
 643  * This function can be called in the middle of an existing transition to
 644  * reverse the direction of the target patch state.  This can be done to
 645  * effectively cancel an existing enable or disable operation if there are any
 646  * tasks which are stuck in the initial patch state.
 647  */
 648 void klp_reverse_transition(void)
 649 {
 650         unsigned int cpu;
 651         struct task_struct *g, *task;
 652
 653         pr_debug("'%s': reversing transition from %s\n",
 654                  klp_transition_patch->mod->name,
 655                  klp_target_state == KLP_PATCHED ? "patching to unpatching" :
 656                                                    "unpatching to patching");
 657
 658         /*
 659          * Clear all TIF_PATCH_PENDING flags to prevent races caused by
 660          * klp_update_patch_state() or __klp_sched_try_switch() running in
 661          * parallel with the reverse transition.
 662          */
 663         read_lock(&tasklist_lock);
 664         for_each_process_thread(g, task)
 665                 clear_tsk_thread_flag(task, TIF_PATCH_PENDING);
 666         read_unlock(&tasklist_lock);
 667
 668         for_each_possible_cpu(cpu)
 669                 clear_tsk_thread_flag(idle_task(cpu), TIF_PATCH_PENDING);
 670
 671         /*
 672          * Make sure all existing invocations of klp_update_patch_state() and
 673          * __klp_sched_try_switch() see the cleared TIF_PATCH_PENDING before
 674          * starting the reverse transition.
 675          */
 676         klp_synchronize_transition();
 677
 678         /*
 679          * All patching has stopped, now re-initialize the global variables to
 680          * prepare for the reverse transition.
 681          */
 682         klp_transition_patch->enabled = !klp_transition_patch->enabled;
 683         klp_target_state = !klp_target_state;
 684
 685         /*
 686          * Enforce the order of the klp_target_state write and the
 687          * TIF_PATCH_PENDING writes in klp_start_transition() to ensure
 688          * klp_update_patch_state() and __klp_sched_try_switch() don't set
 689          * task->patch_state to the wrong value.
 690          */
 691         smp_wmb();
 692
 693         klp_start_transition();
 694 }
 695
 696 /* Called from copy_process() during fork */
 697 void klp_copy_process(struct task_struct *child)
 698 {
 699
 700         /*
 701          * The parent process may have gone through a KLP transition since
 702          * the thread flag was copied in setup_thread_stack earlier. Bring
 703          * the task flag up to date with the parent here.
 704          *
 705          * The operation is serialized against all klp_*_transition()
 706          * operations by the tasklist_lock. The only exceptions are
 707          * klp_update_patch_state(current) and __klp_sched_try_switch(), but we
 708          * cannot race with them because we are current.
 709          */
 710         if (test_tsk_thread_flag(current, TIF_PATCH_PENDING))
 711                 set_tsk_thread_flag(child, TIF_PATCH_PENDING);
 712         else
 713                 clear_tsk_thread_flag(child, TIF_PATCH_PENDING);
 714
 715         child->patch_state = current->patch_state;
 716 }
 717
 718 /*
 719  * Drop TIF_PATCH_PENDING of all tasks on admin's request. This forces an
 720  * existing transition to finish.
 721  *
 722  * NOTE: klp_update_patch_state(task) requires the task to be inactive or
 723  * 'current'. This is not the case here and the consistency model could be
 724  * broken. Administrator, who is the only one to execute the
 725  * klp_force_transitions(), has to be aware of this.
 726  */
 727 void klp_force_transition(void)
 728 {
 729         struct klp_patch *patch;
 730         struct task_struct *g, *task;
 731         unsigned int cpu;
 732
 733         pr_warn("forcing remaining tasks to the patched state\n");
 734
 735         read_lock(&tasklist_lock);
 736         for_each_process_thread(g, task)
 737                 klp_update_patch_state(task);
 738         read_unlock(&tasklist_lock);
 739
 740         for_each_possible_cpu(cpu)
 741                 klp_update_patch_state(idle_task(cpu));
 742
 743         /* Set forced flag for patches being removed. */
 744         if (klp_target_state == KLP_UNPATCHED)
 745                 klp_transition_patch->forced = true;
 746         else if (klp_transition_patch->replace) {
 747                 klp_for_each_patch(patch) {
 748                         if (patch != klp_transition_patch)
 749                                 patch->forced = true;
 750                 }
 751         }
 752 }