kernel/livepatch/transition.c

   1 // SPDX-License-Identifier: GPL-2.0-or-later
   2 /*
   3  * transition.c - Kernel Live Patching transition functions
   4  *
   5  * Copyright (C) 2015-2016 Josh Poimboeuf <jpoimboe@redhat.com>
   6  */
   7
   8 #define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
   9
  10 #include <linux/cpu.h>
  11 #include <linux/stacktrace.h>
  12 #include "core.h"
  13 #include "patch.h"
  14 #include "transition.h"
  15
  16 #define MAX_STACK_ENTRIES  100
  17 #define STACK_ERR_BUF_SIZE 128
  18
  19 #define SIGNALS_TIMEOUT 15
  20
  21 struct klp_patch *klp_transition_patch;
  22
  23 static int klp_target_state = KLP_UNDEFINED;
  24
  25 static unsigned int klp_signals_cnt;
  26
  27 /*
  28  * This work can be performed periodically to finish patching or unpatching any
  29  * "straggler" tasks which failed to transition in the first attempt.
  30  */
  31 static void klp_transition_work_fn(struct work_struct *work)
  32 {
  33         mutex_lock(&klp_mutex);
  34
  35         if (klp_transition_patch)
  36                 klp_try_complete_transition();
  37
  38         mutex_unlock(&klp_mutex);
  39 }
  40 static DECLARE_DELAYED_WORK(klp_transition_work, klp_transition_work_fn);
  41
  42 /*
  43  * This function is just a stub to implement a hard force
  44  * of synchronize_rcu(). This requires synchronizing
  45  * tasks even in userspace and idle.
  46  */
  47 static void klp_sync(struct work_struct *work)
  48 {
  49 }
  50
  51 /*
  52  * We allow to patch also functions where RCU is not watching,
  53  * e.g. before user_exit(). We can not rely on the RCU infrastructure
  54  * to do the synchronization. Instead hard force the sched synchronization.
  55  *
  56  * This approach allows to use RCU functions for manipulating func_stack
  57  * safely.
  58  */
  59 static void klp_synchronize_transition(void)
  60 {
  61         schedule_on_each_cpu(klp_sync);
  62 }
  63
  64 /*
  65  * The transition to the target patch state is complete.  Clean up the data
  66  * structures.
  67  */
  68 static void klp_complete_transition(void)
  69 {
  70         struct klp_object *obj;
  71         struct klp_func *func;
  72         struct task_struct *g, *task;
  73         unsigned int cpu;
  74
  75         pr_debug("'%s': completing %s transition\n",
  76                  klp_transition_patch->mod->name,
  77                  klp_target_state == KLP_PATCHED ? "patching" : "unpatching");
  78
  79         if (klp_transition_patch->replace && klp_target_state == KLP_PATCHED) {
  80                 klp_unpatch_replaced_patches(klp_transition_patch);
  81                 klp_discard_nops(klp_transition_patch);
  82         }
  83
  84         if (klp_target_state == KLP_UNPATCHED) {
  85                 /*
  86                  * All tasks have transitioned to KLP_UNPATCHED so we can now
  87                  * remove the new functions from the func_stack.
  88                  */
  89                 klp_unpatch_objects(klp_transition_patch);
  90
  91                 /*
  92                  * Make sure klp_ftrace_handler() can no longer see functions
  93                  * from this patch on the ops->func_stack.  Otherwise, after
  94                  * func->transition gets cleared, the handler may choose a
  95                  * removed function.
  96                  */
  97                 klp_synchronize_transition();
  98         }
  99
 100         klp_for_each_object(klp_transition_patch, obj)
 101                 klp_for_each_func(obj, func)
 102                         func->transition = false;
 103
 104         /* Prevent klp_ftrace_handler() from seeing KLP_UNDEFINED state */
 105         if (klp_target_state == KLP_PATCHED)
 106                 klp_synchronize_transition();
 107
 108         read_lock(&tasklist_lock);
 109         for_each_process_thread(g, task) {
 110                 WARN_ON_ONCE(test_tsk_thread_flag(task, TIF_PATCH_PENDING));
 111                 task->patch_state = KLP_UNDEFINED;
 112         }
 113         read_unlock(&tasklist_lock);
 114
 115         for_each_possible_cpu(cpu) {
 116                 task = idle_task(cpu);
 117                 WARN_ON_ONCE(test_tsk_thread_flag(task, TIF_PATCH_PENDING));
 118                 task->patch_state = KLP_UNDEFINED;
 119         }
 120
 121         klp_for_each_object(klp_transition_patch, obj) {
 122                 if (!klp_is_object_loaded(obj))
 123                         continue;
 124                 if (klp_target_state == KLP_PATCHED)
 125                         klp_post_patch_callback(obj);
 126                 else if (klp_target_state == KLP_UNPATCHED)
 127                         klp_post_unpatch_callback(obj);
 128         }
 129
 130         pr_notice("'%s': %s complete\n", klp_transition_patch->mod->name,
 131                   klp_target_state == KLP_PATCHED ? "patching" : "unpatching");
 132
 133         klp_target_state = KLP_UNDEFINED;
 134         klp_transition_patch = NULL;
 135 }
 136
 137 /*
 138  * This is called in the error path, to cancel a transition before it has
 139  * started, i.e. klp_init_transition() has been called but
 140  * klp_start_transition() hasn't.  If the transition *has* been started,
 141  * klp_reverse_transition() should be used instead.
 142  */
 143 void klp_cancel_transition(void)
 144 {
 145         if (WARN_ON_ONCE(klp_target_state != KLP_PATCHED))
 146                 return;
 147
 148         pr_debug("'%s': canceling patching transition, going to unpatch\n",
 149                  klp_transition_patch->mod->name);
 150
 151         klp_target_state = KLP_UNPATCHED;
 152         klp_complete_transition();
 153 }
 154
 155 /*
 156  * Switch the patched state of the task to the set of functions in the target
 157  * patch state.
 158  *
 159  * NOTE: If task is not 'current', the caller must ensure the task is inactive.
 160  * Otherwise klp_ftrace_handler() might read the wrong 'patch_state' value.
 161  */
 162 void klp_update_patch_state(struct task_struct *task)
 163 {
 164         /*
 165          * A variant of synchronize_rcu() is used to allow patching functions
 166          * where RCU is not watching, see klp_synchronize_transition().
 167          */
 168         preempt_disable_notrace();
 169
 170         /*
 171          * This test_and_clear_tsk_thread_flag() call also serves as a read
 172          * barrier (smp_rmb) for two cases:
 173          *
 174          * 1) Enforce the order of the TIF_PATCH_PENDING read and the
 175          *    klp_target_state read.  The corresponding write barrier is in
 176          *    klp_init_transition().
 177          *
 178          * 2) Enforce the order of the TIF_PATCH_PENDING read and a future read
 179          *    of func->transition, if klp_ftrace_handler() is called later on
 180          *    the same CPU.  See __klp_disable_patch().
 181          */
 182         if (test_and_clear_tsk_thread_flag(task, TIF_PATCH_PENDING))
 183                 task->patch_state = READ_ONCE(klp_target_state);
 184
 185         preempt_enable_notrace();
 186 }
 187
 188 /*
 189  * Determine whether the given stack trace includes any references to a
 190  * to-be-patched or to-be-unpatched function.
 191  */
 192 static int klp_check_stack_func(struct klp_func *func, unsigned long *entries,
 193                                 unsigned int nr_entries)
 194 {
 195         unsigned long func_addr, func_size, address;
 196         struct klp_ops *ops;
 197         int i;
 198
 199         for (i = 0; i < nr_entries; i++) {
 200                 address = entries[i];
 201
 202                 if (klp_target_state == KLP_UNPATCHED) {
 203                          /*
 204                           * Check for the to-be-unpatched function
 205                           * (the func itself).
 206                           */
 207                         func_addr = (unsigned long)func->new_func;
 208                         func_size = func->new_size;
 209                 } else {
 210                         /*
 211                          * Check for the to-be-patched function
 212                          * (the previous func).
 213                          */
 214                         ops = klp_find_ops(func->old_func);
 215
 216                         if (list_is_singular(&ops->func_stack)) {
 217                                 /* original function */
 218                                 func_addr = (unsigned long)func->old_func;
 219                                 func_size = func->old_size;
 220                         } else {
 221                                 /* previously patched function */
 222                                 struct klp_func *prev;
 223
 224                                 prev = list_next_entry(func, stack_node);
 225                                 func_addr = (unsigned long)prev->new_func;
 226                                 func_size = prev->new_size;
 227                         }
 228                 }
 229
 230                 if (address >= func_addr && address < func_addr + func_size)
 231                         return -EAGAIN;
 232         }
 233
 234         return 0;
 235 }
 236
 237 /*
 238  * Determine whether it's safe to transition the task to the target patch state
 239  * by looking for any to-be-patched or to-be-unpatched functions on its stack.
 240  */
 241 static int klp_check_stack(struct task_struct *task, const char **oldname)
 242 {
 243         static unsigned long entries[MAX_STACK_ENTRIES];
 244         struct klp_object *obj;
 245         struct klp_func *func;
 246         int ret, nr_entries;
 247
 248         ret = stack_trace_save_tsk_reliable(task, entries, ARRAY_SIZE(entries));
 249         if (ret < 0)
 250                 return -EINVAL;
 251         nr_entries = ret;
 252
 253         klp_for_each_object(klp_transition_patch, obj) {
 254                 if (!obj->patched)
 255                         continue;
 256                 klp_for_each_func(obj, func) {
 257                         ret = klp_check_stack_func(func, entries, nr_entries);
 258                         if (ret) {
 259                                 *oldname = func->old_name;
 260                                 return -EADDRINUSE;
 261                         }
 262                 }
 263         }
 264
 265         return 0;
 266 }
 267
 268 static int klp_check_and_switch_task(struct task_struct *task, void *arg)
 269 {
 270         int ret;
 271
 272         if (task_curr(task) && task != current)
 273                 return -EBUSY;
 274
 275         ret = klp_check_stack(task, arg);
 276         if (ret)
 277                 return ret;
 278
 279         clear_tsk_thread_flag(task, TIF_PATCH_PENDING);
 280         task->patch_state = klp_target_state;
 281         return 0;
 282 }
 283
 284 /*
 285  * Try to safely switch a task to the target patch state.  If it's currently
 286  * running, or it's sleeping on a to-be-patched or to-be-unpatched function, or
 287  * if the stack is unreliable, return false.
 288  */
 289 static bool klp_try_switch_task(struct task_struct *task)
 290 {
 291         const char *old_name;
 292         int ret;
 293
 294         /* check if this task has already switched over */
 295         if (task->patch_state == klp_target_state)
 296                 return true;
 297
 298         /*
 299          * For arches which don't have reliable stack traces, we have to rely
 300          * on other methods (e.g., switching tasks at kernel exit).
 301          */
 302         if (!klp_have_reliable_stack())
 303                 return false;
 304
 305         /*
 306          * Now try to check the stack for any to-be-patched or to-be-unpatched
 307          * functions.  If all goes well, switch the task to the target patch
 308          * state.
 309          */
 310         ret = task_call_func(task, klp_check_and_switch_task, &old_name);
 311         switch (ret) {
 312         case 0:         /* success */
 313                 break;
 314
 315         case -EBUSY:    /* klp_check_and_switch_task() */
 316                 pr_debug("%s: %s:%d is running\n",
 317                          __func__, task->comm, task->pid);
 318                 break;
 319         case -EINVAL:   /* klp_check_and_switch_task() */
 320                 pr_debug("%s: %s:%d has an unreliable stack\n",
 321                          __func__, task->comm, task->pid);
 322                 break;
 323         case -EADDRINUSE: /* klp_check_and_switch_task() */
 324                 pr_debug("%s: %s:%d is sleeping on function %s\n",
 325                          __func__, task->comm, task->pid, old_name);
 326                 break;
 327
 328         default:
 329                 pr_debug("%s: Unknown error code (%d) when trying to switch %s:%d\n",
 330                          __func__, ret, task->comm, task->pid);
 331                 break;
 332         }
 333
 334         return !ret;
 335 }
 336
 337 /*
 338  * Sends a fake signal to all non-kthread tasks with TIF_PATCH_PENDING set.
 339  * Kthreads with TIF_PATCH_PENDING set are woken up.
 340  */
 341 static void klp_send_signals(void)
 342 {
 343         struct task_struct *g, *task;
 344
 345         if (klp_signals_cnt == SIGNALS_TIMEOUT)
 346                 pr_notice("signaling remaining tasks\n");
 347
 348         read_lock(&tasklist_lock);
 349         for_each_process_thread(g, task) {
 350                 if (!klp_patch_pending(task))
 351                         continue;
 352
 353                 /*
 354                  * There is a small race here. We could see TIF_PATCH_PENDING
 355                  * set and decide to wake up a kthread or send a fake signal.
 356                  * Meanwhile the task could migrate itself and the action
 357                  * would be meaningless. It is not serious though.
 358                  */
 359                 if (task->flags & PF_KTHREAD) {
 360                         /*
 361                          * Wake up a kthread which sleeps interruptedly and
 362                          * still has not been migrated.
 363                          */
 364                         wake_up_state(task, TASK_INTERRUPTIBLE);
 365                 } else {
 366                         /*
 367                          * Send fake signal to all non-kthread tasks which are
 368                          * still not migrated.
 369                          */
 370                         set_notify_signal(task);
 371                 }
 372         }
 373         read_unlock(&tasklist_lock);
 374 }
 375
 376 /*
 377  * Try to switch all remaining tasks to the target patch state by walking the
 378  * stacks of sleeping tasks and looking for any to-be-patched or
 379  * to-be-unpatched functions.  If such functions are found, the task can't be
 380  * switched yet.
 381  *
 382  * If any tasks are still stuck in the initial patch state, schedule a retry.
 383  */
 384 void klp_try_complete_transition(void)
 385 {
 386         unsigned int cpu;
 387         struct task_struct *g, *task;
 388         struct klp_patch *patch;
 389         bool complete = true;
 390
 391         WARN_ON_ONCE(klp_target_state == KLP_UNDEFINED);
 392
 393         /*
 394          * Try to switch the tasks to the target patch state by walking their
 395          * stacks and looking for any to-be-patched or to-be-unpatched
 396          * functions.  If such functions are found on a stack, or if the stack
 397          * is deemed unreliable, the task can't be switched yet.
 398          *
 399          * Usually this will transition most (or all) of the tasks on a system
 400          * unless the patch includes changes to a very common function.
 401          */
 402         read_lock(&tasklist_lock);
 403         for_each_process_thread(g, task)
 404                 if (!klp_try_switch_task(task))
 405                         complete = false;
 406         read_unlock(&tasklist_lock);
 407
 408         /*
 409          * Ditto for the idle "swapper" tasks.
 410          */
 411         cpus_read_lock();
 412         for_each_possible_cpu(cpu) {
 413                 task = idle_task(cpu);
 414                 if (cpu_online(cpu)) {
 415                         if (!klp_try_switch_task(task)) {
 416                                 complete = false;
 417                                 /* Make idle task go through the main loop. */
 418                                 wake_up_if_idle(cpu);
 419                         }
 420                 } else if (task->patch_state != klp_target_state) {
 421                         /* offline idle tasks can be switched immediately */
 422                         clear_tsk_thread_flag(task, TIF_PATCH_PENDING);
 423                         task->patch_state = klp_target_state;
 424                 }
 425         }
 426         cpus_read_unlock();
 427
 428         if (!complete) {
 429                 if (klp_signals_cnt && !(klp_signals_cnt % SIGNALS_TIMEOUT))
 430                         klp_send_signals();
 431                 klp_signals_cnt++;
 432
 433                 /*
 434                  * Some tasks weren't able to be switched over.  Try again
 435                  * later and/or wait for other methods like kernel exit
 436                  * switching.
 437                  */
 438                 schedule_delayed_work(&klp_transition_work,
 439                                       round_jiffies_relative(HZ));
 440                 return;
 441         }
 442
 443         /* we're done, now cleanup the data structures */
 444         patch = klp_transition_patch;
 445         klp_complete_transition();
 446
 447         /*
 448          * It would make more sense to free the unused patches in
 449          * klp_complete_transition() but it is called also
 450          * from klp_cancel_transition().
 451          */
 452         if (!patch->enabled)
 453                 klp_free_patch_async(patch);
 454         else if (patch->replace)
 455                 klp_free_replaced_patches_async(patch);
 456 }
 457
 458 /*
 459  * Start the transition to the specified target patch state so tasks can begin
 460  * switching to it.
 461  */
 462 void klp_start_transition(void)
 463 {
 464         struct task_struct *g, *task;
 465         unsigned int cpu;
 466
 467         WARN_ON_ONCE(klp_target_state == KLP_UNDEFINED);
 468
 469         pr_notice("'%s': starting %s transition\n",
 470                   klp_transition_patch->mod->name,
 471                   klp_target_state == KLP_PATCHED ? "patching" : "unpatching");
 472
 473         /*
 474          * Mark all normal tasks as needing a patch state update.  They'll
 475          * switch either in klp_try_complete_transition() or as they exit the
 476          * kernel.
 477          */
 478         read_lock(&tasklist_lock);
 479         for_each_process_thread(g, task)
 480                 if (task->patch_state != klp_target_state)
 481                         set_tsk_thread_flag(task, TIF_PATCH_PENDING);
 482         read_unlock(&tasklist_lock);
 483
 484         /*
 485          * Mark all idle tasks as needing a patch state update.  They'll switch
 486          * either in klp_try_complete_transition() or at the idle loop switch
 487          * point.
 488          */
 489         for_each_possible_cpu(cpu) {
 490                 task = idle_task(cpu);
 491                 if (task->patch_state != klp_target_state)
 492                         set_tsk_thread_flag(task, TIF_PATCH_PENDING);
 493         }
 494
 495         klp_signals_cnt = 0;
 496 }
 497
 498 /*
 499  * Initialize the global target patch state and all tasks to the initial patch
 500  * state, and initialize all function transition states to true in preparation
 501  * for patching or unpatching.
 502  */
 503 void klp_init_transition(struct klp_patch *patch, int state)
 504 {
 505         struct task_struct *g, *task;
 506         unsigned int cpu;
 507         struct klp_object *obj;
 508         struct klp_func *func;
 509         int initial_state = !state;
 510
 511         WARN_ON_ONCE(klp_target_state != KLP_UNDEFINED);
 512
 513         klp_transition_patch = patch;
 514
 515         /*
 516          * Set the global target patch state which tasks will switch to.  This
 517          * has no effect until the TIF_PATCH_PENDING flags get set later.
 518          */
 519         klp_target_state = state;
 520
 521         pr_debug("'%s': initializing %s transition\n", patch->mod->name,
 522                  klp_target_state == KLP_PATCHED ? "patching" : "unpatching");
 523
 524         /*
 525          * Initialize all tasks to the initial patch state to prepare them for
 526          * switching to the target state.
 527          */
 528         read_lock(&tasklist_lock);
 529         for_each_process_thread(g, task) {
 530                 WARN_ON_ONCE(task->patch_state != KLP_UNDEFINED);
 531                 task->patch_state = initial_state;
 532         }
 533         read_unlock(&tasklist_lock);
 534
 535         /*
 536          * Ditto for the idle "swapper" tasks.
 537          */
 538         for_each_possible_cpu(cpu) {
 539                 task = idle_task(cpu);
 540                 WARN_ON_ONCE(task->patch_state != KLP_UNDEFINED);
 541                 task->patch_state = initial_state;
 542         }
 543
 544         /*
 545          * Enforce the order of the task->patch_state initializations and the
 546          * func->transition updates to ensure that klp_ftrace_handler() doesn't
 547          * see a func in transition with a task->patch_state of KLP_UNDEFINED.
 548          *
 549          * Also enforce the order of the klp_target_state write and future
 550          * TIF_PATCH_PENDING writes to ensure klp_update_patch_state() doesn't
 551          * set a task->patch_state to KLP_UNDEFINED.
 552          */
 553         smp_wmb();
 554
 555         /*
 556          * Set the func transition states so klp_ftrace_handler() will know to
 557          * switch to the transition logic.
 558          *
 559          * When patching, the funcs aren't yet in the func_stack and will be
 560          * made visible to the ftrace handler shortly by the calls to
 561          * klp_patch_object().
 562          *
 563          * When unpatching, the funcs are already in the func_stack and so are
 564          * already visible to the ftrace handler.
 565          */
 566         klp_for_each_object(patch, obj)
 567                 klp_for_each_func(obj, func)
 568                         func->transition = true;
 569 }
 570
 571 /*
 572  * This function can be called in the middle of an existing transition to
 573  * reverse the direction of the target patch state.  This can be done to
 574  * effectively cancel an existing enable or disable operation if there are any
 575  * tasks which are stuck in the initial patch state.
 576  */
 577 void klp_reverse_transition(void)
 578 {
 579         unsigned int cpu;
 580         struct task_struct *g, *task;
 581
 582         pr_debug("'%s': reversing transition from %s\n",
 583                  klp_transition_patch->mod->name,
 584                  klp_target_state == KLP_PATCHED ? "patching to unpatching" :
 585                                                    "unpatching to patching");
 586
 587         klp_transition_patch->enabled = !klp_transition_patch->enabled;
 588
 589         klp_target_state = !klp_target_state;
 590
 591         /*
 592          * Clear all TIF_PATCH_PENDING flags to prevent races caused by
 593          * klp_update_patch_state() running in parallel with
 594          * klp_start_transition().
 595          */
 596         read_lock(&tasklist_lock);
 597         for_each_process_thread(g, task)
 598                 clear_tsk_thread_flag(task, TIF_PATCH_PENDING);
 599         read_unlock(&tasklist_lock);
 600
 601         for_each_possible_cpu(cpu)
 602                 clear_tsk_thread_flag(idle_task(cpu), TIF_PATCH_PENDING);
 603
 604         /* Let any remaining calls to klp_update_patch_state() complete */
 605         klp_synchronize_transition();
 606
 607         klp_start_transition();
 608 }
 609
 610 /* Called from copy_process() during fork */
 611 void klp_copy_process(struct task_struct *child)
 612 {
 613         child->patch_state = current->patch_state;
 614
 615         /* TIF_PATCH_PENDING gets copied in setup_thread_stack() */
 616 }
 617
 618 /*
 619  * Drop TIF_PATCH_PENDING of all tasks on admin's request. This forces an
 620  * existing transition to finish.
 621  *
 622  * NOTE: klp_update_patch_state(task) requires the task to be inactive or
 623  * 'current'. This is not the case here and the consistency model could be
 624  * broken. Administrator, who is the only one to execute the
 625  * klp_force_transitions(), has to be aware of this.
 626  */
 627 void klp_force_transition(void)
 628 {
 629         struct klp_patch *patch;
 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         /* Set forced flag for patches being removed. */
 644         if (klp_target_state == KLP_UNPATCHED)
 645                 klp_transition_patch->forced = true;
 646         else if (klp_transition_patch->replace) {
 647                 klp_for_each_patch(patch) {
 648                         if (patch != klp_transition_patch)
 649                                 patch->forced = true;
 650                 }
 651         }
 652 }