kernel/livepatch/transition.c

   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 #define SIGNALS_TIMEOUT 15
  33
  34 struct klp_patch *klp_transition_patch;
  35
  36 static int klp_target_state = KLP_UNDEFINED;
  37
  38 static unsigned int klp_signals_cnt;
  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_PATCHED ? "patching" : "unpatching");
  91
  92         if (klp_transition_patch->replace && klp_target_state == KLP_PATCHED) {
  93                 klp_discard_replaced_patches(klp_transition_patch);
  94                 klp_discard_nops(klp_transition_patch);
  95         }
  96
  97         if (klp_target_state == KLP_UNPATCHED) {
  98                 /*
  99                  * All tasks have transitioned to KLP_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_UNDEFINED state */
 118         if (klp_target_state == KLP_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_UNDEFINED;
 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_UNDEFINED;
 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_PATCHED)
 138                         klp_post_patch_callback(obj);
 139                 else if (klp_target_state == KLP_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_PATCHED ? "patching" : "unpatching");
 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_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 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, 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         for (i = 0; i < nr_entries; i++) {
 213                 address = 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_func);
 228
 229                         if (list_is_singular(&ops->func_stack)) {
 230                                 /* original function */
 231                                 func_addr = (unsigned long)func->old_func;
 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 klp_object *obj;
 258         struct klp_func *func;
 259         int ret, nr_entries;
 260
 261         ret = stack_trace_save_tsk_reliable(task, entries, ARRAY_SIZE(entries));
 262         WARN_ON_ONCE(ret == -ENOSYS);
 263         if (ret < 0) {
 264                 snprintf(err_buf, STACK_ERR_BUF_SIZE,
 265                          "%s: %s:%d has an unreliable stack\n",
 266                          __func__, task->comm, task->pid);
 267                 return ret;
 268         }
 269         nr_entries = ret;
 270
 271         klp_for_each_object(klp_transition_patch, obj) {
 272                 if (!obj->patched)
 273                         continue;
 274                 klp_for_each_func(obj, func) {
 275                         ret = klp_check_stack_func(func, entries, nr_entries);
 276                         if (ret) {
 277                                 snprintf(err_buf, STACK_ERR_BUF_SIZE,
 278                                          "%s: %s:%d is sleeping on function %s\n",
 279                                          __func__, task->comm, task->pid,
 280                                          func->old_name);
 281                                 return ret;
 282                         }
 283                 }
 284         }
 285
 286         return 0;
 287 }
 288
 289 /*
 290  * Try to safely switch a task to the target patch state.  If it's currently
 291  * running, or it's sleeping on a to-be-patched or to-be-unpatched function, or
 292  * if the stack is unreliable, return false.
 293  */
 294 static bool klp_try_switch_task(struct task_struct *task)
 295 {
 296         struct rq *rq;
 297         struct rq_flags flags;
 298         int ret;
 299         bool success = false;
 300         char err_buf[STACK_ERR_BUF_SIZE];
 301
 302         err_buf[0] = '\0';
 303
 304         /* check if this task has already switched over */
 305         if (task->patch_state == klp_target_state)
 306                 return true;
 307
 308         /*
 309          * Now try to check the stack for any to-be-patched or to-be-unpatched
 310          * functions.  If all goes well, switch the task to the target patch
 311          * state.
 312          */
 313         rq = task_rq_lock(task, &flags);
 314
 315         if (task_running(rq, task) && task != current) {
 316                 snprintf(err_buf, STACK_ERR_BUF_SIZE,
 317                          "%s: %s:%d is running\n", __func__, task->comm,
 318                          task->pid);
 319                 goto done;
 320         }
 321
 322         ret = klp_check_stack(task, err_buf);
 323         if (ret)
 324                 goto done;
 325
 326         success = true;
 327
 328         clear_tsk_thread_flag(task, TIF_PATCH_PENDING);
 329         task->patch_state = klp_target_state;
 330
 331 done:
 332         task_rq_unlock(rq, task, &flags);
 333
 334         /*
 335          * Due to console deadlock issues, pr_debug() can't be used while
 336          * holding the task rq lock.  Instead we have to use a temporary buffer
 337          * and print the debug message after releasing the lock.
 338          */
 339         if (err_buf[0] != '\0')
 340                 pr_debug("%s", err_buf);
 341
 342         return success;
 343
 344 }
 345
 346 /*
 347  * Sends a fake signal to all non-kthread tasks with TIF_PATCH_PENDING set.
 348  * Kthreads with TIF_PATCH_PENDING set are woken up.
 349  */
 350 static void klp_send_signals(void)
 351 {
 352         struct task_struct *g, *task;
 353
 354         if (klp_signals_cnt == SIGNALS_TIMEOUT)
 355                 pr_notice("signaling remaining tasks\n");
 356
 357         read_lock(&tasklist_lock);
 358         for_each_process_thread(g, task) {
 359                 if (!klp_patch_pending(task))
 360                         continue;
 361
 362                 /*
 363                  * There is a small race here. We could see TIF_PATCH_PENDING
 364                  * set and decide to wake up a kthread or send a fake signal.
 365                  * Meanwhile the task could migrate itself and the action
 366                  * would be meaningless. It is not serious though.
 367                  */
 368                 if (task->flags & PF_KTHREAD) {
 369                         /*
 370                          * Wake up a kthread which sleeps interruptedly and
 371                          * still has not been migrated.
 372                          */
 373                         wake_up_state(task, TASK_INTERRUPTIBLE);
 374                 } else {
 375                         /*
 376                          * Send fake signal to all non-kthread tasks which are
 377                          * still not migrated.
 378                          */
 379                         spin_lock_irq(&task->sighand->siglock);
 380                         signal_wake_up(task, 0);
 381                         spin_unlock_irq(&task->sighand->siglock);
 382                 }
 383         }
 384         read_unlock(&tasklist_lock);
 385 }
 386
 387 /*
 388  * Try to switch all remaining tasks to the target patch state by walking the
 389  * stacks of sleeping tasks and looking for any to-be-patched or
 390  * to-be-unpatched functions.  If such functions are found, the task can't be
 391  * switched yet.
 392  *
 393  * If any tasks are still stuck in the initial patch state, schedule a retry.
 394  */
 395 void klp_try_complete_transition(void)
 396 {
 397         unsigned int cpu;
 398         struct task_struct *g, *task;
 399         struct klp_patch *patch;
 400         bool complete = true;
 401
 402         WARN_ON_ONCE(klp_target_state == KLP_UNDEFINED);
 403
 404         /*
 405          * Try to switch the tasks to the target patch state by walking their
 406          * stacks and looking for any to-be-patched or to-be-unpatched
 407          * functions.  If such functions are found on a stack, or if the stack
 408          * is deemed unreliable, the task can't be switched yet.
 409          *
 410          * Usually this will transition most (or all) of the tasks on a system
 411          * unless the patch includes changes to a very common function.
 412          */
 413         read_lock(&tasklist_lock);
 414         for_each_process_thread(g, task)
 415                 if (!klp_try_switch_task(task))
 416                         complete = false;
 417         read_unlock(&tasklist_lock);
 418
 419         /*
 420          * Ditto for the idle "swapper" tasks.
 421          */
 422         get_online_cpus();
 423         for_each_possible_cpu(cpu) {
 424                 task = idle_task(cpu);
 425                 if (cpu_online(cpu)) {
 426                         if (!klp_try_switch_task(task))
 427                                 complete = false;
 428                 } else if (task->patch_state != klp_target_state) {
 429                         /* offline idle tasks can be switched immediately */
 430                         clear_tsk_thread_flag(task, TIF_PATCH_PENDING);
 431                         task->patch_state = klp_target_state;
 432                 }
 433         }
 434         put_online_cpus();
 435
 436         if (!complete) {
 437                 if (klp_signals_cnt && !(klp_signals_cnt % SIGNALS_TIMEOUT))
 438                         klp_send_signals();
 439                 klp_signals_cnt++;
 440
 441                 /*
 442                  * Some tasks weren't able to be switched over.  Try again
 443                  * later and/or wait for other methods like kernel exit
 444                  * switching.
 445                  */
 446                 schedule_delayed_work(&klp_transition_work,
 447                                       round_jiffies_relative(HZ));
 448                 return;
 449         }
 450
 451         /* we're done, now cleanup the data structures */
 452         patch = klp_transition_patch;
 453         klp_complete_transition();
 454
 455         /*
 456          * It would make more sense to free the patch in
 457          * klp_complete_transition() but it is called also
 458          * from klp_cancel_transition().
 459          */
 460         if (!patch->enabled) {
 461                 klp_free_patch_start(patch);
 462                 schedule_work(&patch->free_work);
 463         }
 464 }
 465
 466 /*
 467  * Start the transition to the specified target patch state so tasks can begin
 468  * switching to it.
 469  */
 470 void klp_start_transition(void)
 471 {
 472         struct task_struct *g, *task;
 473         unsigned int cpu;
 474
 475         WARN_ON_ONCE(klp_target_state == KLP_UNDEFINED);
 476
 477         pr_notice("'%s': starting %s transition\n",
 478                   klp_transition_patch->mod->name,
 479                   klp_target_state == KLP_PATCHED ? "patching" : "unpatching");
 480
 481         /*
 482          * Mark all normal tasks as needing a patch state update.  They'll
 483          * switch either in klp_try_complete_transition() or as they exit the
 484          * kernel.
 485          */
 486         read_lock(&tasklist_lock);
 487         for_each_process_thread(g, task)
 488                 if (task->patch_state != klp_target_state)
 489                         set_tsk_thread_flag(task, TIF_PATCH_PENDING);
 490         read_unlock(&tasklist_lock);
 491
 492         /*
 493          * Mark all idle tasks as needing a patch state update.  They'll switch
 494          * either in klp_try_complete_transition() or at the idle loop switch
 495          * point.
 496          */
 497         for_each_possible_cpu(cpu) {
 498                 task = idle_task(cpu);
 499                 if (task->patch_state != klp_target_state)
 500                         set_tsk_thread_flag(task, TIF_PATCH_PENDING);
 501         }
 502
 503         klp_signals_cnt = 0;
 504 }
 505
 506 /*
 507  * Initialize the global target patch state and all tasks to the initial patch
 508  * state, and initialize all function transition states to true in preparation
 509  * for patching or unpatching.
 510  */
 511 void klp_init_transition(struct klp_patch *patch, int state)
 512 {
 513         struct task_struct *g, *task;
 514         unsigned int cpu;
 515         struct klp_object *obj;
 516         struct klp_func *func;
 517         int initial_state = !state;
 518
 519         WARN_ON_ONCE(klp_target_state != KLP_UNDEFINED);
 520
 521         klp_transition_patch = patch;
 522
 523         /*
 524          * Set the global target patch state which tasks will switch to.  This
 525          * has no effect until the TIF_PATCH_PENDING flags get set later.
 526          */
 527         klp_target_state = state;
 528
 529         pr_debug("'%s': initializing %s transition\n", patch->mod->name,
 530                  klp_target_state == KLP_PATCHED ? "patching" : "unpatching");
 531
 532         /*
 533          * Initialize all tasks to the initial patch state to prepare them for
 534          * switching to the target state.
 535          */
 536         read_lock(&tasklist_lock);
 537         for_each_process_thread(g, task) {
 538                 WARN_ON_ONCE(task->patch_state != KLP_UNDEFINED);
 539                 task->patch_state = initial_state;
 540         }
 541         read_unlock(&tasklist_lock);
 542
 543         /*
 544          * Ditto for the idle "swapper" tasks.
 545          */
 546         for_each_possible_cpu(cpu) {
 547                 task = idle_task(cpu);
 548                 WARN_ON_ONCE(task->patch_state != KLP_UNDEFINED);
 549                 task->patch_state = initial_state;
 550         }
 551
 552         /*
 553          * Enforce the order of the task->patch_state initializations and the
 554          * func->transition updates to ensure that klp_ftrace_handler() doesn't
 555          * see a func in transition with a task->patch_state of KLP_UNDEFINED.
 556          *
 557          * Also enforce the order of the klp_target_state write and future
 558          * TIF_PATCH_PENDING writes to ensure klp_update_patch_state() doesn't
 559          * set a task->patch_state to KLP_UNDEFINED.
 560          */
 561         smp_wmb();
 562
 563         /*
 564          * Set the func transition states so klp_ftrace_handler() will know to
 565          * switch to the transition logic.
 566          *
 567          * When patching, the funcs aren't yet in the func_stack and will be
 568          * made visible to the ftrace handler shortly by the calls to
 569          * klp_patch_object().
 570          *
 571          * When unpatching, the funcs are already in the func_stack and so are
 572          * already visible to the ftrace handler.
 573          */
 574         klp_for_each_object(patch, obj)
 575                 klp_for_each_func(obj, func)
 576                         func->transition = true;
 577 }
 578
 579 /*
 580  * This function can be called in the middle of an existing transition to
 581  * reverse the direction of the target patch state.  This can be done to
 582  * effectively cancel an existing enable or disable operation if there are any
 583  * tasks which are stuck in the initial patch state.
 584  */
 585 void klp_reverse_transition(void)
 586 {
 587         unsigned int cpu;
 588         struct task_struct *g, *task;
 589
 590         pr_debug("'%s': reversing transition from %s\n",
 591                  klp_transition_patch->mod->name,
 592                  klp_target_state == KLP_PATCHED ? "patching to unpatching" :
 593                                                    "unpatching to patching");
 594
 595         klp_transition_patch->enabled = !klp_transition_patch->enabled;
 596
 597         klp_target_state = !klp_target_state;
 598
 599         /*
 600          * Clear all TIF_PATCH_PENDING flags to prevent races caused by
 601          * klp_update_patch_state() running in parallel with
 602          * klp_start_transition().
 603          */
 604         read_lock(&tasklist_lock);
 605         for_each_process_thread(g, task)
 606                 clear_tsk_thread_flag(task, TIF_PATCH_PENDING);
 607         read_unlock(&tasklist_lock);
 608
 609         for_each_possible_cpu(cpu)
 610                 clear_tsk_thread_flag(idle_task(cpu), TIF_PATCH_PENDING);
 611
 612         /* Let any remaining calls to klp_update_patch_state() complete */
 613         klp_synchronize_transition();
 614
 615         klp_start_transition();
 616 }
 617
 618 /* Called from copy_process() during fork */
 619 void klp_copy_process(struct task_struct *child)
 620 {
 621         child->patch_state = current->patch_state;
 622
 623         /* TIF_PATCH_PENDING gets copied in setup_thread_stack() */
 624 }
 625
 626 /*
 627  * Drop TIF_PATCH_PENDING of all tasks on admin's request. This forces an
 628  * existing transition to finish.
 629  *
 630  * NOTE: klp_update_patch_state(task) requires the task to be inactive or
 631  * 'current'. This is not the case here and the consistency model could be
 632  * broken. Administrator, who is the only one to execute the
 633  * klp_force_transitions(), has to be aware of this.
 634  */
 635 void klp_force_transition(void)
 636 {
 637         struct klp_patch *patch;
 638         struct task_struct *g, *task;
 639         unsigned int cpu;
 640
 641         pr_warn("forcing remaining tasks to the patched state\n");
 642
 643         read_lock(&tasklist_lock);
 644         for_each_process_thread(g, task)
 645                 klp_update_patch_state(task);
 646         read_unlock(&tasklist_lock);
 647
 648         for_each_possible_cpu(cpu)
 649                 klp_update_patch_state(idle_task(cpu));
 650
 651         klp_for_each_patch(patch)
 652                 patch->forced = true;
 653 }