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 struct klp_patch *klp_transition_patch;
  33
  34 static int klp_target_state = KLP_UNDEFINED;
  35
  36 /*
  37  * This work can be performed periodically to finish patching or unpatching any
  38  * "straggler" tasks which failed to transition in the first attempt.
  39  */
  40 static void klp_transition_work_fn(struct work_struct *work)
  41 {
  42         mutex_lock(&klp_mutex);
  43
  44         if (klp_transition_patch)
  45                 klp_try_complete_transition();
  46
  47         mutex_unlock(&klp_mutex);
  48 }
  49 static DECLARE_DELAYED_WORK(klp_transition_work, klp_transition_work_fn);
  50
  51 /*
  52  * This function is just a stub to implement a hard force
  53  * of synchronize_sched(). This requires synchronizing
  54  * tasks even in userspace and idle.
  55  */
  56 static void klp_sync(struct work_struct *work)
  57 {
  58 }
  59
  60 /*
  61  * We allow to patch also functions where RCU is not watching,
  62  * e.g. before user_exit(). We can not rely on the RCU infrastructure
  63  * to do the synchronization. Instead hard force the sched synchronization.
  64  *
  65  * This approach allows to use RCU functions for manipulating func_stack
  66  * safely.
  67  */
  68 static void klp_synchronize_transition(void)
  69 {
  70         schedule_on_each_cpu(klp_sync);
  71 }
  72
  73 /*
  74  * The transition to the target patch state is complete.  Clean up the data
  75  * structures.
  76  */
  77 static void klp_complete_transition(void)
  78 {
  79         struct klp_object *obj;
  80         struct klp_func *func;
  81         struct task_struct *g, *task;
  82         unsigned int cpu;
  83         bool immediate_func = false;
  84
  85         if (klp_target_state == KLP_UNPATCHED) {
  86                 /*
  87                  * All tasks have transitioned to KLP_UNPATCHED so we can now
  88                  * remove the new functions from the func_stack.
  89                  */
  90                 klp_unpatch_objects(klp_transition_patch);
  91
  92                 /*
  93                  * Make sure klp_ftrace_handler() can no longer see functions
  94                  * from this patch on the ops->func_stack.  Otherwise, after
  95                  * func->transition gets cleared, the handler may choose a
  96                  * removed function.
  97                  */
  98                 klp_synchronize_transition();
  99         }
 100
 101         if (klp_transition_patch->immediate)
 102                 goto done;
 103
 104         klp_for_each_object(klp_transition_patch, obj) {
 105                 klp_for_each_func(obj, func) {
 106                         func->transition = false;
 107                         if (func->immediate)
 108                                 immediate_func = true;
 109                 }
 110         }
 111
 112         if (klp_target_state == KLP_UNPATCHED && !immediate_func)
 113                 module_put(klp_transition_patch->mod);
 114
 115         /* Prevent klp_ftrace_handler() from seeing KLP_UNDEFINED state */
 116         if (klp_target_state == KLP_PATCHED)
 117                 klp_synchronize_transition();
 118
 119         read_lock(&tasklist_lock);
 120         for_each_process_thread(g, task) {
 121                 WARN_ON_ONCE(test_tsk_thread_flag(task, TIF_PATCH_PENDING));
 122                 task->patch_state = KLP_UNDEFINED;
 123         }
 124         read_unlock(&tasklist_lock);
 125
 126         for_each_possible_cpu(cpu) {
 127                 task = idle_task(cpu);
 128                 WARN_ON_ONCE(test_tsk_thread_flag(task, TIF_PATCH_PENDING));
 129                 task->patch_state = KLP_UNDEFINED;
 130         }
 131
 132 done:
 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         klp_target_state = KLP_UNPATCHED;
 149         klp_complete_transition();
 150 }
 151
 152 /*
 153  * Switch the patched state of the task to the set of functions in the target
 154  * patch state.
 155  *
 156  * NOTE: If task is not 'current', the caller must ensure the task is inactive.
 157  * Otherwise klp_ftrace_handler() might read the wrong 'patch_state' value.
 158  */
 159 void klp_update_patch_state(struct task_struct *task)
 160 {
 161         /*
 162          * A variant of synchronize_sched() is used to allow patching functions
 163          * where RCU is not watching, see klp_synchronize_transition().
 164          */
 165         preempt_disable_notrace();
 166
 167         /*
 168          * This test_and_clear_tsk_thread_flag() call also serves as a read
 169          * barrier (smp_rmb) for two cases:
 170          *
 171          * 1) Enforce the order of the TIF_PATCH_PENDING read and the
 172          *    klp_target_state read.  The corresponding write barrier is in
 173          *    klp_init_transition().
 174          *
 175          * 2) Enforce the order of the TIF_PATCH_PENDING read and a future read
 176          *    of func->transition, if klp_ftrace_handler() is called later on
 177          *    the same CPU.  See __klp_disable_patch().
 178          */
 179         if (test_and_clear_tsk_thread_flag(task, TIF_PATCH_PENDING))
 180                 task->patch_state = READ_ONCE(klp_target_state);
 181
 182         preempt_enable_notrace();
 183 }
 184
 185 /*
 186  * Determine whether the given stack trace includes any references to a
 187  * to-be-patched or to-be-unpatched function.
 188  */
 189 static int klp_check_stack_func(struct klp_func *func,
 190                                 struct stack_trace *trace)
 191 {
 192         unsigned long func_addr, func_size, address;
 193         struct klp_ops *ops;
 194         int i;
 195
 196         if (func->immediate)
 197                 return 0;
 198
 199         for (i = 0; i < trace->nr_entries; i++) {
 200                 address = trace->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_addr);
 215
 216                         if (list_is_singular(&ops->func_stack)) {
 217                                 /* original function */
 218                                 func_addr = func->old_addr;
 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, char *err_buf)
 242 {
 243         static unsigned long entries[MAX_STACK_ENTRIES];
 244         struct stack_trace trace;
 245         struct klp_object *obj;
 246         struct klp_func *func;
 247         int ret;
 248
 249         trace.skip = 0;
 250         trace.nr_entries = 0;
 251         trace.max_entries = MAX_STACK_ENTRIES;
 252         trace.entries = entries;
 253         ret = save_stack_trace_tsk_reliable(task, &trace);
 254         WARN_ON_ONCE(ret == -ENOSYS);
 255         if (ret) {
 256                 snprintf(err_buf, STACK_ERR_BUF_SIZE,
 257                          "%s: %s:%d has an unreliable stack\n",
 258                          __func__, task->comm, task->pid);
 259                 return ret;
 260         }
 261
 262         klp_for_each_object(klp_transition_patch, obj) {
 263                 if (!obj->patched)
 264                         continue;
 265                 klp_for_each_func(obj, func) {
 266                         ret = klp_check_stack_func(func, &trace);
 267                         if (ret) {
 268                                 snprintf(err_buf, STACK_ERR_BUF_SIZE,
 269                                          "%s: %s:%d is sleeping on function %s\n",
 270                                          __func__, task->comm, task->pid,
 271                                          func->old_name);
 272                                 return ret;
 273                         }
 274                 }
 275         }
 276
 277         return 0;
 278 }
 279
 280 /*
 281  * Try to safely switch a task to the target patch state.  If it's currently
 282  * running, or it's sleeping on a to-be-patched or to-be-unpatched function, or
 283  * if the stack is unreliable, return false.
 284  */
 285 static bool klp_try_switch_task(struct task_struct *task)
 286 {
 287         struct rq *rq;
 288         struct rq_flags flags;
 289         int ret;
 290         bool success = false;
 291         char err_buf[STACK_ERR_BUF_SIZE];
 292
 293         err_buf[0] = '\0';
 294
 295         /* check if this task has already switched over */
 296         if (task->patch_state == klp_target_state)
 297                 return true;
 298
 299         /*
 300          * For arches which don't have reliable stack traces, we have to rely
 301          * on other methods (e.g., switching tasks at kernel exit).
 302          */
 303         if (!klp_have_reliable_stack())
 304                 return false;
 305
 306         /*
 307          * Now try to check the stack for any to-be-patched or to-be-unpatched
 308          * functions.  If all goes well, switch the task to the target patch
 309          * state.
 310          */
 311         rq = task_rq_lock(task, &flags);
 312
 313         if (task_running(rq, task) && task != current) {
 314                 snprintf(err_buf, STACK_ERR_BUF_SIZE,
 315                          "%s: %s:%d is running\n", __func__, task->comm,
 316                          task->pid);
 317                 goto done;
 318         }
 319
 320         ret = klp_check_stack(task, err_buf);
 321         if (ret)
 322                 goto done;
 323
 324         success = true;
 325
 326         clear_tsk_thread_flag(task, TIF_PATCH_PENDING);
 327         task->patch_state = klp_target_state;
 328
 329 done:
 330         task_rq_unlock(rq, task, &flags);
 331
 332         /*
 333          * Due to console deadlock issues, pr_debug() can't be used while
 334          * holding the task rq lock.  Instead we have to use a temporary buffer
 335          * and print the debug message after releasing the lock.
 336          */
 337         if (err_buf[0] != '\0')
 338                 pr_debug("%s", err_buf);
 339
 340         return success;
 341
 342 }
 343
 344 /*
 345  * Try to switch all remaining tasks to the target patch state by walking the
 346  * stacks of sleeping tasks and looking for any to-be-patched or
 347  * to-be-unpatched functions.  If such functions are found, the task can't be
 348  * switched yet.
 349  *
 350  * If any tasks are still stuck in the initial patch state, schedule a retry.
 351  */
 352 void klp_try_complete_transition(void)
 353 {
 354         unsigned int cpu;
 355         struct task_struct *g, *task;
 356         bool complete = true;
 357
 358         WARN_ON_ONCE(klp_target_state == KLP_UNDEFINED);
 359
 360         /*
 361          * If the patch can be applied or reverted immediately, skip the
 362          * per-task transitions.
 363          */
 364         if (klp_transition_patch->immediate)
 365                 goto success;
 366
 367         /*
 368          * Try to switch the tasks to the target patch state by walking their
 369          * stacks and looking for any to-be-patched or to-be-unpatched
 370          * functions.  If such functions are found on a stack, or if the stack
 371          * is deemed unreliable, the task can't be switched yet.
 372          *
 373          * Usually this will transition most (or all) of the tasks on a system
 374          * unless the patch includes changes to a very common function.
 375          */
 376         read_lock(&tasklist_lock);
 377         for_each_process_thread(g, task)
 378                 if (!klp_try_switch_task(task))
 379                         complete = false;
 380         read_unlock(&tasklist_lock);
 381
 382         /*
 383          * Ditto for the idle "swapper" tasks.
 384          */
 385         get_online_cpus();
 386         for_each_possible_cpu(cpu) {
 387                 task = idle_task(cpu);
 388                 if (cpu_online(cpu)) {
 389                         if (!klp_try_switch_task(task))
 390                                 complete = false;
 391                 } else if (task->patch_state != klp_target_state) {
 392                         /* offline idle tasks can be switched immediately */
 393                         clear_tsk_thread_flag(task, TIF_PATCH_PENDING);
 394                         task->patch_state = klp_target_state;
 395                 }
 396         }
 397         put_online_cpus();
 398
 399         if (!complete) {
 400                 /*
 401                  * Some tasks weren't able to be switched over.  Try again
 402                  * later and/or wait for other methods like kernel exit
 403                  * switching.
 404                  */
 405                 schedule_delayed_work(&klp_transition_work,
 406                                       round_jiffies_relative(HZ));
 407                 return;
 408         }
 409
 410 success:
 411         pr_notice("'%s': %s complete\n", klp_transition_patch->mod->name,
 412                   klp_target_state == KLP_PATCHED ? "patching" : "unpatching");
 413
 414         /* we're done, now cleanup the data structures */
 415         klp_complete_transition();
 416 }
 417
 418 /*
 419  * Start the transition to the specified target patch state so tasks can begin
 420  * switching to it.
 421  */
 422 void klp_start_transition(void)
 423 {
 424         struct task_struct *g, *task;
 425         unsigned int cpu;
 426
 427         WARN_ON_ONCE(klp_target_state == KLP_UNDEFINED);
 428
 429         pr_notice("'%s': %s...\n", klp_transition_patch->mod->name,
 430                   klp_target_state == KLP_PATCHED ? "patching" : "unpatching");
 431
 432         /*
 433          * If the patch can be applied or reverted immediately, skip the
 434          * per-task transitions.
 435          */
 436         if (klp_transition_patch->immediate)
 437                 return;
 438
 439         /*
 440          * Mark all normal tasks as needing a patch state update.  They'll
 441          * switch either in klp_try_complete_transition() or as they exit the
 442          * kernel.
 443          */
 444         read_lock(&tasklist_lock);
 445         for_each_process_thread(g, task)
 446                 if (task->patch_state != klp_target_state)
 447                         set_tsk_thread_flag(task, TIF_PATCH_PENDING);
 448         read_unlock(&tasklist_lock);
 449
 450         /*
 451          * Mark all idle tasks as needing a patch state update.  They'll switch
 452          * either in klp_try_complete_transition() or at the idle loop switch
 453          * point.
 454          */
 455         for_each_possible_cpu(cpu) {
 456                 task = idle_task(cpu);
 457                 if (task->patch_state != klp_target_state)
 458                         set_tsk_thread_flag(task, TIF_PATCH_PENDING);
 459         }
 460 }
 461
 462 /*
 463  * Initialize the global target patch state and all tasks to the initial patch
 464  * state, and initialize all function transition states to true in preparation
 465  * for patching or unpatching.
 466  */
 467 void klp_init_transition(struct klp_patch *patch, int state)
 468 {
 469         struct task_struct *g, *task;
 470         unsigned int cpu;
 471         struct klp_object *obj;
 472         struct klp_func *func;
 473         int initial_state = !state;
 474
 475         WARN_ON_ONCE(klp_target_state != KLP_UNDEFINED);
 476
 477         klp_transition_patch = patch;
 478
 479         /*
 480          * Set the global target patch state which tasks will switch to.  This
 481          * has no effect until the TIF_PATCH_PENDING flags get set later.
 482          */
 483         klp_target_state = state;
 484
 485         /*
 486          * If the patch can be applied or reverted immediately, skip the
 487          * per-task transitions.
 488          */
 489         if (patch->immediate)
 490                 return;
 491
 492         /*
 493          * Initialize all tasks to the initial patch state to prepare them for
 494          * switching to the target state.
 495          */
 496         read_lock(&tasklist_lock);
 497         for_each_process_thread(g, task) {
 498                 WARN_ON_ONCE(task->patch_state != KLP_UNDEFINED);
 499                 task->patch_state = initial_state;
 500         }
 501         read_unlock(&tasklist_lock);
 502
 503         /*
 504          * Ditto for the idle "swapper" tasks.
 505          */
 506         for_each_possible_cpu(cpu) {
 507                 task = idle_task(cpu);
 508                 WARN_ON_ONCE(task->patch_state != KLP_UNDEFINED);
 509                 task->patch_state = initial_state;
 510         }
 511
 512         /*
 513          * Enforce the order of the task->patch_state initializations and the
 514          * func->transition updates to ensure that klp_ftrace_handler() doesn't
 515          * see a func in transition with a task->patch_state of KLP_UNDEFINED.
 516          *
 517          * Also enforce the order of the klp_target_state write and future
 518          * TIF_PATCH_PENDING writes to ensure klp_update_patch_state() doesn't
 519          * set a task->patch_state to KLP_UNDEFINED.
 520          */
 521         smp_wmb();
 522
 523         /*
 524          * Set the func transition states so klp_ftrace_handler() will know to
 525          * switch to the transition logic.
 526          *
 527          * When patching, the funcs aren't yet in the func_stack and will be
 528          * made visible to the ftrace handler shortly by the calls to
 529          * klp_patch_object().
 530          *
 531          * When unpatching, the funcs are already in the func_stack and so are
 532          * already visible to the ftrace handler.
 533          */
 534         klp_for_each_object(patch, obj)
 535                 klp_for_each_func(obj, func)
 536                         func->transition = true;
 537 }
 538
 539 /*
 540  * This function can be called in the middle of an existing transition to
 541  * reverse the direction of the target patch state.  This can be done to
 542  * effectively cancel an existing enable or disable operation if there are any
 543  * tasks which are stuck in the initial patch state.
 544  */
 545 void klp_reverse_transition(void)
 546 {
 547         unsigned int cpu;
 548         struct task_struct *g, *task;
 549
 550         klp_transition_patch->enabled = !klp_transition_patch->enabled;
 551
 552         klp_target_state = !klp_target_state;
 553
 554         /*
 555          * Clear all TIF_PATCH_PENDING flags to prevent races caused by
 556          * klp_update_patch_state() running in parallel with
 557          * klp_start_transition().
 558          */
 559         read_lock(&tasklist_lock);
 560         for_each_process_thread(g, task)
 561                 clear_tsk_thread_flag(task, TIF_PATCH_PENDING);
 562         read_unlock(&tasklist_lock);
 563
 564         for_each_possible_cpu(cpu)
 565                 clear_tsk_thread_flag(idle_task(cpu), TIF_PATCH_PENDING);
 566
 567         /* Let any remaining calls to klp_update_patch_state() complete */
 568         klp_synchronize_transition();
 569
 570         klp_start_transition();
 571 }
 572
 573 /* Called from copy_process() during fork */
 574 void klp_copy_process(struct task_struct *child)
 575 {
 576         child->patch_state = current->patch_state;
 577
 578         /* TIF_PATCH_PENDING gets copied in setup_thread_stack() */
 579 }