kernel/umh.c

   1 // SPDX-License-Identifier: GPL-2.0-only
   2 /*
   3  * umh - the kernel usermode helper
   4  */
   5 #include <linux/module.h>
   6 #include <linux/sched.h>
   7 #include <linux/sched/task.h>
   8 #include <linux/binfmts.h>
   9 #include <linux/syscalls.h>
  10 #include <linux/unistd.h>
  11 #include <linux/kmod.h>
  12 #include <linux/slab.h>
  13 #include <linux/completion.h>
  14 #include <linux/cred.h>
  15 #include <linux/file.h>
  16 #include <linux/fdtable.h>
  17 #include <linux/fs_struct.h>
  18 #include <linux/workqueue.h>
  19 #include <linux/security.h>
  20 #include <linux/mount.h>
  21 #include <linux/kernel.h>
  22 #include <linux/init.h>
  23 #include <linux/resource.h>
  24 #include <linux/notifier.h>
  25 #include <linux/suspend.h>
  26 #include <linux/rwsem.h>
  27 #include <linux/ptrace.h>
  28 #include <linux/async.h>
  29 #include <linux/uaccess.h>
  30 #include <linux/initrd.h>
  31 #include <linux/freezer.h>
  32
  33 #include <trace/events/module.h>
  34
  35 static kernel_cap_t usermodehelper_bset = CAP_FULL_SET;
  36 static kernel_cap_t usermodehelper_inheritable = CAP_FULL_SET;
  37 static DEFINE_SPINLOCK(umh_sysctl_lock);
  38 static DECLARE_RWSEM(umhelper_sem);
  39
  40 static void call_usermodehelper_freeinfo(struct subprocess_info *info)
  41 {
  42         if (info->cleanup)
  43                 (*info->cleanup)(info);
  44         kfree(info);
  45 }
  46
  47 static void umh_complete(struct subprocess_info *sub_info)
  48 {
  49         struct completion *comp = xchg(&sub_info->complete, NULL);
  50         /*
  51          * See call_usermodehelper_exec(). If xchg() returns NULL
  52          * we own sub_info, the UMH_KILLABLE caller has gone away
  53          * or the caller used UMH_NO_WAIT.
  54          */
  55         if (comp)
  56                 complete(comp);
  57         else
  58                 call_usermodehelper_freeinfo(sub_info);
  59 }
  60
  61 /*
  62  * This is the task which runs the usermode application
  63  */
  64 static int call_usermodehelper_exec_async(void *data)
  65 {
  66         struct subprocess_info *sub_info = data;
  67         struct cred *new;
  68         int retval;
  69
  70         spin_lock_irq(&current->sighand->siglock);
  71         flush_signal_handlers(current, 1);
  72         spin_unlock_irq(&current->sighand->siglock);
  73
  74         /*
  75          * Initial kernel threads share ther FS with init, in order to
  76          * get the init root directory. But we've now created a new
  77          * thread that is going to execve a user process and has its own
  78          * 'struct fs_struct'. Reset umask to the default.
  79          */
  80         current->fs->umask = 0022;
  81
  82         /*
  83          * Our parent (unbound workqueue) runs with elevated scheduling
  84          * priority. Avoid propagating that into the userspace child.
  85          */
  86         set_user_nice(current, 0);
  87
  88         retval = -ENOMEM;
  89         new = prepare_kernel_cred(current);
  90         if (!new)
  91                 goto out;
  92
  93         spin_lock(&umh_sysctl_lock);
  94         new->cap_bset = cap_intersect(usermodehelper_bset, new->cap_bset);
  95         new->cap_inheritable = cap_intersect(usermodehelper_inheritable,
  96                                              new->cap_inheritable);
  97         spin_unlock(&umh_sysctl_lock);
  98
  99         if (sub_info->init) {
 100                 retval = sub_info->init(sub_info, new);
 101                 if (retval) {
 102                         abort_creds(new);
 103                         goto out;
 104                 }
 105         }
 106
 107         commit_creds(new);
 108
 109         wait_for_initramfs();
 110         retval = kernel_execve(sub_info->path,
 111                                (const char *const *)sub_info->argv,
 112                                (const char *const *)sub_info->envp);
 113 out:
 114         sub_info->retval = retval;
 115         /*
 116          * call_usermodehelper_exec_sync() will call umh_complete
 117          * if UHM_WAIT_PROC.
 118          */
 119         if (!(sub_info->wait & UMH_WAIT_PROC))
 120                 umh_complete(sub_info);
 121         if (!retval)
 122                 return 0;
 123         do_exit(0);
 124 }
 125
 126 /* Handles UMH_WAIT_PROC.  */
 127 static void call_usermodehelper_exec_sync(struct subprocess_info *sub_info)
 128 {
 129         pid_t pid;
 130
 131         /* If SIGCLD is ignored do_wait won't populate the status. */
 132         kernel_sigaction(SIGCHLD, SIG_DFL);
 133         pid = user_mode_thread(call_usermodehelper_exec_async, sub_info, SIGCHLD);
 134         if (pid < 0)
 135                 sub_info->retval = pid;
 136         else
 137                 kernel_wait(pid, &sub_info->retval);
 138
 139         /* Restore default kernel sig handler */
 140         kernel_sigaction(SIGCHLD, SIG_IGN);
 141         umh_complete(sub_info);
 142 }
 143
 144 /*
 145  * We need to create the usermodehelper kernel thread from a task that is affine
 146  * to an optimized set of CPUs (or nohz housekeeping ones) such that they
 147  * inherit a widest affinity irrespective of call_usermodehelper() callers with
 148  * possibly reduced affinity (eg: per-cpu workqueues). We don't want
 149  * usermodehelper targets to contend a busy CPU.
 150  *
 151  * Unbound workqueues provide such wide affinity and allow to block on
 152  * UMH_WAIT_PROC requests without blocking pending request (up to some limit).
 153  *
 154  * Besides, workqueues provide the privilege level that caller might not have
 155  * to perform the usermodehelper request.
 156  *
 157  */
 158 static void call_usermodehelper_exec_work(struct work_struct *work)
 159 {
 160         struct subprocess_info *sub_info =
 161                 container_of(work, struct subprocess_info, work);
 162
 163         if (sub_info->wait & UMH_WAIT_PROC) {
 164                 call_usermodehelper_exec_sync(sub_info);
 165         } else {
 166                 pid_t pid;
 167                 /*
 168                  * Use CLONE_PARENT to reparent it to kthreadd; we do not
 169                  * want to pollute current->children, and we need a parent
 170                  * that always ignores SIGCHLD to ensure auto-reaping.
 171                  */
 172                 pid = user_mode_thread(call_usermodehelper_exec_async, sub_info,
 173                                        CLONE_PARENT | SIGCHLD);
 174                 if (pid < 0) {
 175                         sub_info->retval = pid;
 176                         umh_complete(sub_info);
 177                 }
 178         }
 179 }
 180
 181 /*
 182  * If set, call_usermodehelper_exec() will exit immediately returning -EBUSY
 183  * (used for preventing user land processes from being created after the user
 184  * land has been frozen during a system-wide hibernation or suspend operation).
 185  * Should always be manipulated under umhelper_sem acquired for write.
 186  */
 187 static enum umh_disable_depth usermodehelper_disabled = UMH_DISABLED;
 188
 189 /* Number of helpers running */
 190 static atomic_t running_helpers = ATOMIC_INIT(0);
 191
 192 /*
 193  * Wait queue head used by usermodehelper_disable() to wait for all running
 194  * helpers to finish.
 195  */
 196 static DECLARE_WAIT_QUEUE_HEAD(running_helpers_waitq);
 197
 198 /*
 199  * Used by usermodehelper_read_lock_wait() to wait for usermodehelper_disabled
 200  * to become 'false'.
 201  */
 202 static DECLARE_WAIT_QUEUE_HEAD(usermodehelper_disabled_waitq);
 203
 204 /*
 205  * Time to wait for running_helpers to become zero before the setting of
 206  * usermodehelper_disabled in usermodehelper_disable() fails
 207  */
 208 #define RUNNING_HELPERS_TIMEOUT (5 * HZ)
 209
 210 int usermodehelper_read_trylock(void)
 211 {
 212         DEFINE_WAIT(wait);
 213         int ret = 0;
 214
 215         down_read(&umhelper_sem);
 216         for (;;) {
 217                 prepare_to_wait(&usermodehelper_disabled_waitq, &wait,
 218                                 TASK_INTERRUPTIBLE);
 219                 if (!usermodehelper_disabled)
 220                         break;
 221
 222                 if (usermodehelper_disabled == UMH_DISABLED)
 223                         ret = -EAGAIN;
 224
 225                 up_read(&umhelper_sem);
 226
 227                 if (ret)
 228                         break;
 229
 230                 schedule();
 231                 try_to_freeze();
 232
 233                 down_read(&umhelper_sem);
 234         }
 235         finish_wait(&usermodehelper_disabled_waitq, &wait);
 236         return ret;
 237 }
 238 EXPORT_SYMBOL_GPL(usermodehelper_read_trylock);
 239
 240 long usermodehelper_read_lock_wait(long timeout)
 241 {
 242         DEFINE_WAIT(wait);
 243
 244         if (timeout < 0)
 245                 return -EINVAL;
 246
 247         down_read(&umhelper_sem);
 248         for (;;) {
 249                 prepare_to_wait(&usermodehelper_disabled_waitq, &wait,
 250                                 TASK_UNINTERRUPTIBLE);
 251                 if (!usermodehelper_disabled)
 252                         break;
 253
 254                 up_read(&umhelper_sem);
 255
 256                 timeout = schedule_timeout(timeout);
 257                 if (!timeout)
 258                         break;
 259
 260                 down_read(&umhelper_sem);
 261         }
 262         finish_wait(&usermodehelper_disabled_waitq, &wait);
 263         return timeout;
 264 }
 265 EXPORT_SYMBOL_GPL(usermodehelper_read_lock_wait);
 266
 267 void usermodehelper_read_unlock(void)
 268 {
 269         up_read(&umhelper_sem);
 270 }
 271 EXPORT_SYMBOL_GPL(usermodehelper_read_unlock);
 272
 273 /**
 274  * __usermodehelper_set_disable_depth - Modify usermodehelper_disabled.
 275  * @depth: New value to assign to usermodehelper_disabled.
 276  *
 277  * Change the value of usermodehelper_disabled (under umhelper_sem locked for
 278  * writing) and wakeup tasks waiting for it to change.
 279  */
 280 void __usermodehelper_set_disable_depth(enum umh_disable_depth depth)
 281 {
 282         down_write(&umhelper_sem);
 283         usermodehelper_disabled = depth;
 284         wake_up(&usermodehelper_disabled_waitq);
 285         up_write(&umhelper_sem);
 286 }
 287
 288 /**
 289  * __usermodehelper_disable - Prevent new helpers from being started.
 290  * @depth: New value to assign to usermodehelper_disabled.
 291  *
 292  * Set usermodehelper_disabled to @depth and wait for running helpers to exit.
 293  */
 294 int __usermodehelper_disable(enum umh_disable_depth depth)
 295 {
 296         long retval;
 297
 298         if (!depth)
 299                 return -EINVAL;
 300
 301         down_write(&umhelper_sem);
 302         usermodehelper_disabled = depth;
 303         up_write(&umhelper_sem);
 304
 305         /*
 306          * From now on call_usermodehelper_exec() won't start any new
 307          * helpers, so it is sufficient if running_helpers turns out to
 308          * be zero at one point (it may be increased later, but that
 309          * doesn't matter).
 310          */
 311         retval = wait_event_timeout(running_helpers_waitq,
 312                                         atomic_read(&running_helpers) == 0,
 313                                         RUNNING_HELPERS_TIMEOUT);
 314         if (retval)
 315                 return 0;
 316
 317         __usermodehelper_set_disable_depth(UMH_ENABLED);
 318         return -EAGAIN;
 319 }
 320
 321 static void helper_lock(void)
 322 {
 323         atomic_inc(&running_helpers);
 324         smp_mb__after_atomic();
 325 }
 326
 327 static void helper_unlock(void)
 328 {
 329         if (atomic_dec_and_test(&running_helpers))
 330                 wake_up(&running_helpers_waitq);
 331 }
 332
 333 /**
 334  * call_usermodehelper_setup - prepare to call a usermode helper
 335  * @path: path to usermode executable
 336  * @argv: arg vector for process
 337  * @envp: environment for process
 338  * @gfp_mask: gfp mask for memory allocation
 339  * @init: an init function
 340  * @cleanup: a cleanup function
 341  * @data: arbitrary context sensitive data
 342  *
 343  * Returns either %NULL on allocation failure, or a subprocess_info
 344  * structure.  This should be passed to call_usermodehelper_exec to
 345  * exec the process and free the structure.
 346  *
 347  * The init function is used to customize the helper process prior to
 348  * exec.  A non-zero return code causes the process to error out, exit,
 349  * and return the failure to the calling process
 350  *
 351  * The cleanup function is just before the subprocess_info is about to
 352  * be freed.  This can be used for freeing the argv and envp.  The
 353  * Function must be runnable in either a process context or the
 354  * context in which call_usermodehelper_exec is called.
 355  */
 356 struct subprocess_info *call_usermodehelper_setup(const char *path, char **argv,
 357                 char **envp, gfp_t gfp_mask,
 358                 int (*init)(struct subprocess_info *info, struct cred *new),
 359                 void (*cleanup)(struct subprocess_info *info),
 360                 void *data)
 361 {
 362         struct subprocess_info *sub_info;
 363         sub_info = kzalloc(sizeof(struct subprocess_info), gfp_mask);
 364         if (!sub_info)
 365                 goto out;
 366
 367         INIT_WORK(&sub_info->work, call_usermodehelper_exec_work);
 368
 369 #ifdef CONFIG_STATIC_USERMODEHELPER
 370         sub_info->path = CONFIG_STATIC_USERMODEHELPER_PATH;
 371 #else
 372         sub_info->path = path;
 373 #endif
 374         sub_info->argv = argv;
 375         sub_info->envp = envp;
 376
 377         sub_info->cleanup = cleanup;
 378         sub_info->init = init;
 379         sub_info->data = data;
 380   out:
 381         return sub_info;
 382 }
 383 EXPORT_SYMBOL(call_usermodehelper_setup);
 384
 385 /**
 386  * call_usermodehelper_exec - start a usermode application
 387  * @sub_info: information about the subprocess
 388  * @wait: wait for the application to finish and return status.
 389  *        when UMH_NO_WAIT don't wait at all, but you get no useful error back
 390  *        when the program couldn't be exec'ed. This makes it safe to call
 391  *        from interrupt context.
 392  *
 393  * Runs a user-space application.  The application is started
 394  * asynchronously if wait is not set, and runs as a child of system workqueues.
 395  * (ie. it runs with full root capabilities and optimized affinity).
 396  *
 397  * Note: successful return value does not guarantee the helper was called at
 398  * all. You can't rely on sub_info->{init,cleanup} being called even for
 399  * UMH_WAIT_* wait modes as STATIC_USERMODEHELPER_PATH="" turns all helpers
 400  * into a successful no-op.
 401  */
 402 int call_usermodehelper_exec(struct subprocess_info *sub_info, int wait)
 403 {
 404         unsigned int state = TASK_UNINTERRUPTIBLE;
 405         DECLARE_COMPLETION_ONSTACK(done);
 406         int retval = 0;
 407
 408         if (!sub_info->path) {
 409                 call_usermodehelper_freeinfo(sub_info);
 410                 return -EINVAL;
 411         }
 412         helper_lock();
 413         if (usermodehelper_disabled) {
 414                 retval = -EBUSY;
 415                 goto out;
 416         }
 417
 418         /*
 419          * If there is no binary for us to call, then just return and get out of
 420          * here.  This allows us to set STATIC_USERMODEHELPER_PATH to "" and
 421          * disable all call_usermodehelper() calls.
 422          */
 423         if (strlen(sub_info->path) == 0)
 424                 goto out;
 425
 426         /*
 427          * Set the completion pointer only if there is a waiter.
 428          * This makes it possible to use umh_complete to free
 429          * the data structure in case of UMH_NO_WAIT.
 430          */
 431         sub_info->complete = (wait == UMH_NO_WAIT) ? NULL : &done;
 432         sub_info->wait = wait;
 433
 434         queue_work(system_unbound_wq, &sub_info->work);
 435         if (wait == UMH_NO_WAIT)        /* task has freed sub_info */
 436                 goto unlock;
 437
 438         if (wait & UMH_FREEZABLE)
 439                 state |= TASK_FREEZABLE;
 440
 441         if (wait & UMH_KILLABLE) {
 442                 retval = wait_for_completion_state(&done, state | TASK_KILLABLE);
 443                 if (!retval)
 444                         goto wait_done;
 445
 446                 /* umh_complete() will see NULL and free sub_info */
 447                 if (xchg(&sub_info->complete, NULL))
 448                         goto unlock;
 449
 450                 /*
 451                  * fallthrough; in case of -ERESTARTSYS now do uninterruptible
 452                  * wait_for_completion_state(). Since umh_complete() shall call
 453                  * complete() in a moment if xchg() above returned NULL, this
 454                  * uninterruptible wait_for_completion_state() will not block
 455                  * SIGKILL'ed processes for long.
 456                  */
 457         }
 458         wait_for_completion_state(&done, state);
 459
 460 wait_done:
 461         retval = sub_info->retval;
 462 out:
 463         call_usermodehelper_freeinfo(sub_info);
 464 unlock:
 465         helper_unlock();
 466         return retval;
 467 }
 468 EXPORT_SYMBOL(call_usermodehelper_exec);
 469
 470 /**
 471  * call_usermodehelper() - prepare and start a usermode application
 472  * @path: path to usermode executable
 473  * @argv: arg vector for process
 474  * @envp: environment for process
 475  * @wait: wait for the application to finish and return status.
 476  *        when UMH_NO_WAIT don't wait at all, but you get no useful error back
 477  *        when the program couldn't be exec'ed. This makes it safe to call
 478  *        from interrupt context.
 479  *
 480  * This function is the equivalent to use call_usermodehelper_setup() and
 481  * call_usermodehelper_exec().
 482  */
 483 int call_usermodehelper(const char *path, char **argv, char **envp, int wait)
 484 {
 485         struct subprocess_info *info;
 486         gfp_t gfp_mask = (wait == UMH_NO_WAIT) ? GFP_ATOMIC : GFP_KERNEL;
 487
 488         info = call_usermodehelper_setup(path, argv, envp, gfp_mask,
 489                                          NULL, NULL, NULL);
 490         if (info == NULL)
 491                 return -ENOMEM;
 492
 493         return call_usermodehelper_exec(info, wait);
 494 }
 495 EXPORT_SYMBOL(call_usermodehelper);
 496
 497 #if defined(CONFIG_SYSCTL)
 498 static int proc_cap_handler(struct ctl_table *table, int write,
 499                          void *buffer, size_t *lenp, loff_t *ppos)
 500 {
 501         struct ctl_table t;
 502         unsigned long cap_array[2];
 503         kernel_cap_t new_cap, *cap;
 504         int err;
 505
 506         if (write && (!capable(CAP_SETPCAP) ||
 507                       !capable(CAP_SYS_MODULE)))
 508                 return -EPERM;
 509
 510         /*
 511          * convert from the global kernel_cap_t to the ulong array to print to
 512          * userspace if this is a read.
 513          *
 514          * Legacy format: capabilities are exposed as two 32-bit values
 515          */
 516         cap = table->data;
 517         spin_lock(&umh_sysctl_lock);
 518         cap_array[0] = (u32) cap->val;
 519         cap_array[1] = cap->val >> 32;
 520         spin_unlock(&umh_sysctl_lock);
 521
 522         t = *table;
 523         t.data = &cap_array;
 524
 525         /*
 526          * actually read or write and array of ulongs from userspace.  Remember
 527          * these are least significant 32 bits first
 528          */
 529         err = proc_doulongvec_minmax(&t, write, buffer, lenp, ppos);
 530         if (err < 0)
 531                 return err;
 532
 533         new_cap.val = (u32)cap_array[0];
 534         new_cap.val += (u64)cap_array[1] << 32;
 535
 536         /*
 537          * Drop everything not in the new_cap (but don't add things)
 538          */
 539         if (write) {
 540                 spin_lock(&umh_sysctl_lock);
 541                 *cap = cap_intersect(*cap, new_cap);
 542                 spin_unlock(&umh_sysctl_lock);
 543         }
 544
 545         return 0;
 546 }
 547
 548 static struct ctl_table usermodehelper_table[] = {
 549         {
 550                 .procname       = "bset",
 551                 .data           = &usermodehelper_bset,
 552                 .maxlen         = 2 * sizeof(unsigned long),
 553                 .mode           = 0600,
 554                 .proc_handler   = proc_cap_handler,
 555         },
 556         {
 557                 .procname       = "inheritable",
 558                 .data           = &usermodehelper_inheritable,
 559                 .maxlen         = 2 * sizeof(unsigned long),
 560                 .mode           = 0600,
 561                 .proc_handler   = proc_cap_handler,
 562         },
 563         { }
 564 };
 565
 566 static int __init init_umh_sysctls(void)
 567 {
 568         register_sysctl_init("kernel/usermodehelper", usermodehelper_table);
 569         return 0;
 570 }
 571 early_initcall(init_umh_sysctls);
 572 #endif /* CONFIG_SYSCTL */