path: root/kernel/umh.c

// SPDX-License-Identifier: GPL-2.0-only
/*
 * umh - the kernel usermode helper
 */
#include <linux/module.h>
#include <linux/sched.h>
#include <linux/sched/task.h>
#include <linux/binfmts.h>
#include <linux/syscalls.h>
#include <linux/unistd.h>
#include <linux/kmod.h>
#include <linux/slab.h>
#include <linux/completion.h>
#include <linux/cred.h>
#include <linux/file.h>
#include <linux/fdtable.h>
#include <linux/workqueue.h>
#include <linux/security.h>
#include <linux/mount.h>
#include <linux/kernel.h>
#include <linux/init.h>
#include <linux/resource.h>
#include <linux/notifier.h>
#include <linux/suspend.h>
#include <linux/rwsem.h>
#include <linux/ptrace.h>
#include <linux/async.h>
#include <linux/uaccess.h>
#include <linux/shmem_fs.h>
#include <linux/pipe_fs_i.h>

#include <trace/events/module.h>

#define CAP_BSET	(void *)1
#define CAP_PI		(void *)2

static kernel_cap_t usermodehelper_bset = CAP_FULL_SET;
static kernel_cap_t usermodehelper_inheritable = CAP_FULL_SET;
static DEFINE_SPINLOCK(umh_sysctl_lock);
static DECLARE_RWSEM(umhelper_sem);
static LIST_HEAD(umh_list);
static DEFINE_MUTEX(umh_list_lock);

static void call_usermodehelper_freeinfo(struct subprocess_info *info)
{
	if (info->cleanup)
		(*info->cleanup)(info);
	kfree(info);
}

static void umh_complete(struct subprocess_info *sub_info)
{
	struct completion *comp = xchg(&sub_info->complete, NULL);
	/*
	 * See call_usermodehelper_exec(). If xchg() returns NULL
	 * we own sub_info, the UMH_KILLABLE caller has gone away
	 * or the caller used UMH_NO_WAIT.
	 */
	if (comp)
		complete(comp);
	else
		call_usermodehelper_freeinfo(sub_info);
}

/*
 * This is the task which runs the usermode application
 */
static int call_usermodehelper_exec_async(void *data)
{
	struct subprocess_info *sub_info = data;
	struct cred *new;
	int retval;

	spin_lock_irq(&current->sighand->siglock);
	flush_signal_handlers(current, 1);
	spin_unlock_irq(&current->sighand->siglock);

	/*
	 * Our parent (unbound workqueue) runs with elevated scheduling
	 * priority. Avoid propagating that into the userspace child.
	 */
	set_user_nice(current, 0);

	retval = -ENOMEM;
	new = prepare_kernel_cred(current);
	if (!new)
		goto out;

	spin_lock(&umh_sysctl_lock);
	new->cap_bset = cap_intersect(usermodehelper_bset, new->cap_bset);
	new->cap_inheritable = cap_intersect(usermodehelper_inheritable,
					     new->cap_inheritable);
	spin_unlock(&umh_sysctl_lock);

	if (sub_info->init) {
		retval = sub_info->init(sub_info, new);
		if (retval) {
			abort_creds(new);
			goto out;
		}
	}

	commit_creds(new);

	sub_info->pid = task_pid_nr(current);
	if (sub_info->file) {
		retval = do_execve_file(sub_info->file,
					sub_info->argv, sub_info->envp);
		if (!retval)
			current->flags |= PF_UMH;
	} else
		retval = do_execve(getname_kernel(sub_info->path),
				   (const char __user *const __user *)sub_info->argv,
				   (const char __user *const __user *)sub_info->envp);
out:
	sub_info->retval = retval;
	/*
	 * call_usermodehelper_exec_sync() will call umh_complete
	 * if UHM_WAIT_PROC.
	 */
	if (!(sub_info->wait & UMH_WAIT_PROC))
		umh_complete(sub_info);
	if (!retval)
		return 0;
	do_exit(0);
}

/* Handles UMH_WAIT_PROC.  */
static void call_usermodehelper_exec_sync(struct subprocess_info *sub_info)
{
	pid_t pid;

	/* If SIGCLD is ignored kernel_wait4 won't populate the status. */
	kernel_sigaction(SIGCHLD, SIG_DFL);
	pid = kernel_thread(call_usermodehelper_exec_async, sub_info, SIGCHLD);
	if (pid < 0) {
		sub_info->retval = pid;
	} else {
		int ret = -ECHILD;
		/*
		 * Normally it is bogus to call wait4() from in-kernel because
		 * wait4() wants to write the exit code to a userspace address.
		 * But call_usermodehelper_exec_sync() always runs as kernel
		 * thread (workqueue) and put_user() to a kernel address works
		 * OK for kernel threads, due to their having an mm_segment_t
		 * which spans the entire address space.
		 *
		 * Thus the __user pointer cast is valid here.
		 */
		kernel_wait4(pid, (int __user *)&ret, 0, NULL);

		/*
		 * If ret is 0, either call_usermodehelper_exec_async failed and
		 * the real error code is already in sub_info->retval or
		 * sub_info->retval is 0 anyway, so don't mess with it then.
		 */
		if (ret)
			sub_info->retval = ret;
	}

	/* Restore default kernel sig handler */
	kernel_sigaction(SIGCHLD, SIG_IGN);

	umh_complete(sub_info);
}

/*
 * We need to create the usermodehelper kernel thread from a task that is affine
 * to an optimized set of CPUs (or nohz housekeeping ones) such that they
 * inherit a widest affinity irrespective of call_usermodehelper() callers with
 * possibly reduced affinity (eg: per-cpu workqueues). We don't want
 * usermodehelper targets to contend a busy CPU.
 *
 * Unbound workqueues provide such wide affinity and allow to block on
 * UMH_WAIT_PROC requests without blocking pending request (up to some limit).
 *
 * Besides, workqueues provide the privilege level that caller might not have
 * to perform the usermodehelper request.
 *
 */
static void call_usermodehelper_exec_work(struct work_struct *work)
{
	struct subprocess_info *sub_info =
		container_of(work, struct subprocess_info, work);

	if (sub_info->wait & UMH_WAIT_PROC) {
		call_usermodehelper_exec_sync(sub_info);
	} else {
		pid_t pid;
		/*
		 * Use CLONE_PARENT to reparent it to kthreadd; we do not
		 * want to pollute current->children, and we need a parent
		 * that always ignores SIGCHLD to ensure auto-reaping.
		 */
		pid = kernel_thread(call_usermodehelper_exec_async, sub_info,
				    CLONE_PARENT | SIGCHLD);
		if (pid < 0) {
			sub_info->retval = pid;
			umh_complete(sub_info);
		}
	}
}

/*
 * If set, call_usermodehelper_exec() will exit immediately returning -EBUSY
 * (used for preventing user land processes from being created after the user
 * land has been frozen during a system-wide hibernation or suspend operation).
 * Should always be manipulated under umhelper_sem acquired for write.
 */
static enum umh_disable_depth usermodehelper_disabled = UMH_DISABLED;

/* Number of helpers running */
static atomic_t running_helpers = ATOMIC_INIT(0);

/*
 * Wait queue head used by usermodehelper_disable() to wait for all running
 * helpers to finish.
 */
static DECLARE_WAIT_QUEUE_HEAD(running_helpers_waitq);

/*
 * Used by usermodehelper_read_lock_wait() to wait for usermodehelper_disabled
 * to become 'false'.
 */
static DECLARE_WAIT_QUEUE_HEAD(usermodehelper_disabled_waitq);

/*
 * Time to wait for running_helpers to become zero before the setting of
 * usermodehelper_disabled in usermodehelper_disable() fails
 */
#define RUNNING_HELPERS_TIMEOUT	(5 * HZ)

int usermodehelper_read_trylock(void)
{
	DEFINE_WAIT(wait);
	int ret = 0;

	down_read(&umhelper_sem);
	for (;;) {
		prepare_to_wait(&usermodehelper_disabled_waitq, &wait,
				TASK_INTERRUPTIBLE);
		if (!usermodehelper_disabled)
			break;

		if (usermodehelper_disabled == UMH_DISABLED)
			ret = -EAGAIN;

		up_read(&umhelper_sem);

		if (ret)
			break;

		schedule();
		try_to_freeze();

		down_read(&umhelper_sem);
	}
	finish_wait(&usermodehelper_disabled_waitq, &wait);
	return ret;
}
EXPORT_SYMBOL_GPL(