drivers/char/hw_random/core.c

/*
 * hw_random/core.c: HWRNG core API
 *
 * Copyright 2006 Michael Buesch <m@bues.ch>
 * Copyright 2005 (c) MontaVista Software, Inc.
 *
 * Please read Documentation/admin-guide/hw_random.rst for details on use.
 *
 * This software may be used and distributed according to the terms
 * of the GNU General Public License, incorporated herein by reference.
 */

#include <linux/delay.h>
#include <linux/device.h>
#include <linux/err.h>
#include <linux/fs.h>
#include <linux/hw_random.h>
#include <linux/kernel.h>
#include <linux/kthread.h>
#include <linux/lockdep.h>
#include <linux/miscdevice.h>
#include <linux/module.h>
#include <linux/notifier.h>
#include <linux/random.h>
#include <linux/rcupdate.h>
#include <linux/sched.h>
#include <linux/sched/signal.h>
#include <linux/slab.h>
#include <linux/string.h>
#include <linux/suspend.h>
#include <linux/sysfs.h>
#include <linux/uaccess.h>
#include <linux/workqueue.h>

#define RNG_MODULE_NAME		"hw_random"

#define RNG_BUFFER_SIZE		MAX(32, SMP_CACHE_BYTES)

static struct hwrng __rcu *current_rng;
/* the current rng has been explicitly chosen by user via sysfs */
static int cur_rng_set_by_user;
static struct task_struct *hwrng_fill;
/* list of registered rngs */
static LIST_HEAD(rng_list);
/* Protects rng_list, hwrng_fill and updating on current_rng */
static DEFINE_MUTEX(rng_mutex);
/* Protects rng read functions, data_avail, rng_buffer and rng_fillbuf */
static DEFINE_MUTEX(reading_mutex);
static int data_avail;
static u8 *rng_buffer, *rng_fillbuf;
/* Set to true during suspend-resume by PM notifier, protected by rng_mutex */
static bool hwrng_fillfn_stopped;
static unsigned short current_quality;
static unsigned short default_quality = 1024; /* default to maximum */

module_param(current_quality, ushort, 0644);
MODULE_PARM_DESC(current_quality,
		 "current hwrng entropy estimation per 1024 bits of input -- obsolete, use rng_quality instead");
module_param(default_quality, ushort, 0644);
MODULE_PARM_DESC(default_quality,
		 "default maximum entropy content of hwrng per 1024 bits of input");

static int hwrng_init(struct hwrng *rng);
static int hwrng_fillfn(void *unused);
static void hwrng_start_hwrng_fillfn(void);
static void hwrng_stop_hwrng_fillfn(void);

static size_t rng_buffer_size(void)
{
	return RNG_BUFFER_SIZE;
}

static void cleanup_rng_work(struct work_struct *work)
{
	struct hwrng *rng = container_of(work, struct hwrng, cleanup_work);

	/*
	 * Hold rng_mutex here so we serialize in case they set_current_rng
	 * on rng again immediately.
	 */
	mutex_lock(&rng_mutex);

	/* Skip if rng has been reinitialized. */
	if (kref_read(&rng->ref)) {
		mutex_unlock(&rng_mutex);
		return;
	}

	if (rng->cleanup)
		rng->cleanup(rng);

	complete(&rng->cleanup_done);
	mutex_unlock(&rng_mutex);
}

static inline void cleanup_rng(struct kref *kref)
{
	struct hwrng *rng = container_of(kref, struct hwrng, ref);

	schedule_work(&rng->cleanup_work);
}

static int set_current_rng(struct hwrng *rng)
{
	struct hwrng *old_rng;
	int err;

	BUG_ON(!mutex_is_locked(&rng_mutex));

	err = hwrng_init(rng);
	if (err)
		return err;

	old_rng = rcu_dereference_protected(current_rng,
					    lockdep_is_held(&rng_mutex));
	rcu_assign_pointer(current_rng, rng);

	if (old_rng) {
		synchronize_rcu();
		kref_put(&old_rng->ref, cleanup_rng);
	}

	/* if necessary, start hwrng thread */
	hwrng_start_hwrng_fillfn();

	return 0;
}

static void drop_current_rng(void)
{
	struct hwrng *rng;

	rng = rcu_dereference_protected(current_rng,
					lockdep_is_held(&rng_mutex));
	if (!rng)
		return;

	RCU_INIT_POINTER(current_rng, NULL);
	synchronize_rcu();

	hwrng_stop_hwrng_fillfn();

	/* decrease last reference for triggering the cleanup */
	kref_put(&rng->ref, cleanup_rng);
}

/* Returns NULL or refcounted hwrng */
static struct hwrng *get_current_rng_nolock(void)
{
	struct hwrng *rng;

	rng = rcu_dereference_protected(current_rng,
					lockdep_is_held(&rng_mutex));
	if (rng)
		kref_get(&rng->ref);

	return rng;
}

static struct hwrng *get_current_rng(void)
{
	struct hwrng *rng;

	rcu_read_lock();
	rng = rcu_dereference(current_rng);
	if (rng)
		kref_get(&rng->ref);

	rcu_read_unlock();

	return rng;
}

static void put_rng(struct hwrng *rng)
{
	if (rng)
		kref_put(&rng->ref, cleanup_rng);
}

static int hwrng_init(struct hwrng *rng)
{
	if (kref_get_unless_zero(&rng->ref))
		goto skip_init;

	if (rng->init) {
		int ret;

		ret =  rng->init(rng);
		if (ret)
			return ret;
	}

	kref_init(&rng->ref);
	reinit_completion(&rng->cleanup_done);

skip_init:
	current_quality = rng->quality; /* obsolete */

	return 0;
}

static int rng_dev_open(struct inode *inode, struct file *filp)
{
	/* enforce read-only access to this chrdev */
	if ((filp->f_mode & FMODE_READ) == 0)
		return -EINVAL;
	if (filp->f_mode & FMODE_WRITE)
		return -EINVAL;
	return 0;
}

static inline int rng_get_data(struct hwrng *rng, u8 *buffer, size_t size, bool wait)
{
	int present;

	BUG_ON(!mutex_is_locked(&reading_mutex));
	if (rng->read) {
		int err;

		err = rng->read(rng, buffer, size, wait);
		if (WARN_ON_ONCE(err > 0 && err > size))
			err = size;

		return err;
	}

	if (rng->data_present)
		present = rng->data_present(rng, wait);
	else
		present = 1;

	if (present)
		return rng->data_read(rng, (u32 *)buffer);

	return 0;
}

static ssize_t rng_dev_read(struct file *filp, char __user *buf,
			    size_t size, loff_t *offp)
{
	u8 buffer[RNG_BUFFER_SIZE];
	ssize_t ret = 0;
	int err = 0;
	int bytes_read, len;
	struct hwrng *rng;

	while (size) {
		rng = get_current_rng();
		if (!rng) {
			err = -ENODEV;
			goto out;
		}

		if (mutex_lock_interruptible(&reading_mutex)) {
			err = -ERESTARTSYS;
			goto out_put;
		}
		if (!data_avail) {
			bytes_read = rng_get_data(rng, rng_buffer,
				rng_buffer_size(),
				!(filp->f_flags & O_NONBLOCK));
			if (bytes_read < 0) {
				err = bytes_read;
				goto out_unlock_reading;
			} else if (bytes_read == 0 &&
				   (filp->f_flags & O_NONBLOCK)) {
				err = -EAGAIN;
				goto out_unlock_reading;
			}

			data_avail = bytes_read;
		}

		len = data_avail;
		if (len) {
			if (len > size)
				len = size;

			data_avail -= len;

			memcpy(buffer, rng_buffer + data_avail, len);
		}
		mutex_unlock(&reading_mutex);
		put_rng(rng);

		if (len) {
			if (copy_to_user(buf + ret, buffer, len)) {
				err = -EFAULT;
				goto out;
			}

			size -= len;
			ret += len;
		}


		if (need_resched())
			schedule_timeout_interruptible(1);

		if (signal_pending(current)) {
			err = -ERESTARTSYS;
			goto out;
		}
	}
out:
	memzero_explicit(buffer, sizeof(buffer));
	return ret ? : err;

out_unlock_reading:
	mutex_unlock(&reading_mutex);
out_put:
	put_rng(rng);
	goto out;
}

static const struct file_operations rng_chrdev_ops = {
	.owner		= THIS_MODULE,
	.open		= rng_dev_open,
	.read		= rng_dev_read,
	.llseek		= noop_llseek,
};

static const struct attribute_group *rng_dev_groups[];

static struct miscdevice rng_miscdev = {
	.minor		= HWRNG_MINOR,
	.name		= RNG_MODULE_NAME,
	.nodename	= "hwrng",
	.fops		= &rng_chrdev_ops,
	.groups		= rng_dev_groups,
};

static int enable_best_rng(void)
{
	struct hwrng *rng, *cur_rng, *new_rng = NULL;
	int ret = -ENODEV;

	BUG_ON(!mutex_is_locked(&rng_mutex));

	/* no rng to use? */
	if (list_empty(&rng_list)) {
		drop_current_rng();
		cur_rng_set_by_user = 0;
		return 0;
	}

	/* use the rng which offers the best quality */
	list_for_each_entry(rng, &rng_list, list) {
		if (!new_rng || rng->quality > new_rng->quality)
			new_rng = rng;
	}

	cur_rng = rcu_dereference_protected(current_rng,
					    lockdep_is_held(&rng_mutex));
	ret = ((new_rng == cur_rng) ? 0 : set_current_rng(new_rng));
	if (!ret)
		cur_rng_set_by_user = 0;

	return ret;
}

static ssize_t rng_current_store(struct device *dev,
				 struct device_attribute *attr,
				 const char *buf, size_t len)
{
	int err;
	struct hwrng *rng, *new_rng;

	err = mutex_lock_interruptible(&rng_mutex);
	if (err)
		return -ERESTARTSYS;

	if (sysfs_streq(buf, "")) {
		err = enable_best_rng();
	} else if (sysfs_streq(buf, "none")) {
		cur_rng_set_by_user = 1;
		drop_current_rng();
	} else {
		list_for_each_entry(rng, &rng_list, list) {
			if (sysfs_streq(rng->name, buf)) {
				err = set_current_rng(rng);
				if (!err)
					cur_rng_set_by_user = 1;
				break;
			}
		}
	}
	new_rng = get_current_rng_nolock();
	mutex_unlock(&rng_mutex);

	if (new_rng)
		put_rng(new_rng);

	return err ? : len;
}

static ssize_t rng_current_show(struct device *dev,
				struct device_attribute *attr,
				char *buf)
{
	ssize_t ret;
	struct hwrng *rng;

	rng = get_current_rng();

	ret = sysfs_emit(buf, "%s\n", rng ? rng->name : "none");
	put_rng(rng);

	return ret;
}

static ssize_t rng_available_show(struct device *dev,
				  struct device_attribute *attr,
				  char *buf)
{
	struct hwrng *rng;
	int len = 0;

	if (mutex_lock_interruptible(&rng_mutex))
		return -ERESTARTSYS;
	list_for_each_entry(rng, &rng_list, list)
		len += sysfs_emit_at(buf, len, "%s ", rng->name);
	len += sysfs_emit_at(buf, len, "none\n");
	mutex_unlock(&rng_mutex);

	return len;
}

static ssize_t rng_selected_show(struct device *dev,
				 struct device_attribute *attr,
				 char *buf)
{
	return sysfs_emit(buf, "%d\n", cur_rng_set_by_user);
}

static ssize_t rng_quality_show(struct device *dev,
				struct device_attribute *attr,
				char *buf)
{
	ssize_t ret;
	struct hwrng *rng;

	rng = get_current_rng();

	if (!rng) /* no need to put_rng */
		return -ENODEV;

	ret = sysfs_emit(buf, "%hu\n", rng->quality);
	put_rng(rng);

	return ret;
}

static ssize_t rng_quality_store(struct device *dev,
				 struct device_attribute *attr,
				 const char *buf, size_t len)
{
	struct hwrng *rng;
	u16 quality;
	int ret = -EINVAL;

	if (len < 2)
		return -EINVAL;

	ret = mutex_lock_interruptible(&rng_mutex);
	if (ret)
		return -ERESTARTSYS;

	ret = kstrtou16(buf, 0, &quality);
	if (ret || quality > 1024) {
		ret = -EINVAL;
		goto out;
	}

	rng = rcu_dereference_protected(current_rng, lockdep_is_held(&rng_mutex));
	if (!rng) {
		ret = -ENODEV;
		goto out;
	}

	rng->quality = quality;
	current_quality = quality; /* obsolete */

	/* the best available RNG may have changed */
	ret = enable_best_rng();

out:
	mutex_unlock(&rng_mutex);
	return ret ? ret : len;
}

static DEVICE_ATTR_RW(rng_current);
static DEVICE_ATTR_RO(rng_available);
static DEVICE_ATTR_RO(rng_selected);
static DEVICE_ATTR_RW(rng_quality);

static struct attribute *rng_dev_attrs[] = {
	&dev_attr_rng_current.attr,
	&dev_attr_rng_available.attr,
	&dev_attr_rng_selected.attr,
	&dev_attr_rng_quality.attr,
	NULL
};

ATTRIBUTE_GROUPS(rng_dev);

static void hwrng_start_hwrng_fillfn(void)
{
	lockdep_assert_held(&rng_mutex);

	/*
	 * PM notifier stopped the hwrng_fillfn kthread, prevent userspace to
	 * restart it until kernel freezes threads.
	 */
	if (hwrng_fillfn_stopped)
		return;

	if (!hwrng_fill) {
		hwrng_fill = kthread_run(hwrng_fillfn, NULL, "hwrng");
		if (IS_ERR(hwrng_fill)) {
			pr_err("hwrng_fill thread creation failed\n");
			hwrng_fill = NULL;
		}
	}
}

static void hwrng_stop_hwrng_fillfn(void)
{
	lockdep_assert_held(&rng_mutex);

	if (hwrng_fill) {
		kthread_stop(hwrng_fill);
		hwrng_fill = NULL;
	}
}

static int hwrng_fillfn(void *unused)
{
	size_t entropy, entropy_credit = 0; /* in 1/1024 of a bit */
	long rc;

	while (!kthread_should_stop()) {
		unsigned short quality;
		struct hwrng *rng;

		rng = get_current_rng();
		if (!rng) {
			/*
			 * Keep the task_struct alive until kthread_stop()
			 * is called to avoid UAF in drop_current_rng().
			 */
			while (!kthread_should_stop()) {
				set_current_state(TASK_INTERRUPTIBLE);
				if (!kthread_should_stop())
					schedule();
			}
			set_current_state(TASK_RUNNING);
			break;
		}

		mutex_lock(&reading_mutex);
		rc = rng_get_data(rng, rng_fillbuf, rng_buffer_size(), true);
		if (current_quality != rng->quality)
			rng->quality = current_quality; /* obsolete */
		quality = rng->quality;
		mutex_unlock(&reading_mutex);

		if (rc <= 0)
			hwrng_msleep(rng, 10000);

		put_rng(rng);

		if (rc <= 0)
			continue;

		/* If we cannot credit at least one bit of entropy,
		 * keep track of the remainder for the next iteration
		 */
		entropy = rc * quality * 8 + entropy_credit;
		if ((entropy >> 10) == 0)
			entropy_credit = entropy;

		/* Outside lock, sure, but y'know: randomness. */
		add_hwgenerator_randomness((void *)rng_fillbuf, rc,
					   entropy >> 10, true);
	}
	return 0;
}

static int hwrng_pm_notifier(struct notifier_block *nb, unsigned long action,
			     void *data)
{
	switch (action) {
	case PM_SUSPEND_PREPARE:
	case PM_HIBERNATION_PREPARE:
	case PM_RESTORE_PREPARE:
		mutex_lock(&rng_mutex);
		hwrng_stop_hwrng_fillfn();
		hwrng_fillfn_stopped = true;
		mutex_unlock(&rng_mutex);
		break;

	case PM_POST_SUSPEND:
	case PM_POST_HIBERNATION:
	case PM_POST_RESTORE:
		mutex_lock(&rng_mutex);
		hwrng_fillfn_stopped = false;
		if (rcu_access_pointer(current_rng))
			hwrng_start_hwrng_fillfn();
		mutex_unlock(&rng_mutex);
		break;
	}

	return NOTIFY_DONE;
}

static struct notifier_block hwrng_pm_nb = {
	.notifier_call = hwrng_pm_notifier,
};

int hwrng_register(struct hwrng *rng)
{
	int err = -EINVAL;
	struct hwrng *cur_rng, *tmp;

	if (!rng->name || (!rng->data_read && !rng->read))
		goto out;

	mutex_lock(&rng_mutex);

	/* Must not register two RNGs with the same name. */
	err = -EEXIST;
	list_for_each_entry(tmp, &rng_list, list) {
		if (strcmp(tmp->name, rng->name) == 0)
			goto out_unlock;
	}
	list_add_tail(&rng->list, &rng_list);

	INIT_WORK(&rng->cleanup_work, cleanup_rng_work);
	init_completion(&rng->cleanup_done);
	complete(&rng->cleanup_done);
	init_completion(&rng->dying);

	/* Adjust quality field to always have a proper value */
	rng->quality = min3(default_quality, 1024, rng->quality ?: 1024);

	if (!cur_rng_set_by_user) {
		cur_rng = rcu_dereference_protected(current_rng,
						    lockdep_is_held(&rng_mutex));
		if (!cur_rng || rng->quality > cur_rng->quality) {
			/*
			 * Set new rng as current as the new rng source
			 * provides better entropy quality and was not
			 * chosen by userspace.
			 */
			err = set_current_rng(rng);
			if (err)
				goto out_list_del;
		}
	}
	mutex_unlock(&rng_mutex);
	return 0;
out_list_del:
	list_del_init(&rng->list);
out_unlock:
	mutex_unlock(&rng_mutex);
out:
	return err;
}
EXPORT_SYMBOL_GPL(hwrng_register);

void hwrng_unregister(struct hwrng *rng)
{
	struct hwrng *cur_rng;
	int err;

	mutex_lock(&rng_mutex);

	list_del(&rng->list);
	complete_all(&rng->dying);

	cur_rng = rcu_dereference_protected(current_rng,
					    lockdep_is_held(&rng_mutex));
	if (cur_rng == rng) {
		err = enable_best_rng();
		if (err) {
			drop_current_rng();
			cur_rng_set_by_user = 0;
		}
	}

	mutex_unlock(&rng_mutex);
	wait_for_completion(&rng->cleanup_done);
}
EXPORT_SYMBOL_GPL(hwrng_unregister);

static void devm_hwrng_release(struct device *dev, void *res)
{
	hwrng_unregister(*(struct hwrng **)res);
}

static int devm_hwrng_match(struct device *dev, void *res, void *data)
{
	struct hwrng **r = res;

	if (WARN_ON(!r || !*r))
		return 0;

	return *r == data;
}

int devm_hwrng_register(struct device *dev, struct hwrng *rng)
{
	struct hwrng **ptr;
	int error;

	ptr = devres_alloc(devm_hwrng_release, sizeof(*ptr), GFP_KERNEL);
	if (!ptr)
		return -ENOMEM;

	error = hwrng_register(rng);
	if (error) {
		devres_free(ptr);
		return error;
	}

	*ptr = rng;
	devres_add(dev, ptr);
	return 0;
}
EXPORT_SYMBOL_GPL(devm_hwrng_register);

void devm_hwrng_unregister(struct device *dev, struct hwrng *rng)
{
	devres_release(dev, devm_hwrng_release, devm_hwrng_match, rng);
}
EXPORT_SYMBOL_GPL(devm_hwrng_unregister);

long hwrng_msleep(struct hwrng *rng, unsigned int msecs)
{
	unsigned long timeout = msecs_to_jiffies(msecs) + 1;

	return wait_for_completion_interruptible_timeout(&rng->dying, timeout);
}
EXPORT_SYMBOL_GPL(hwrng_msleep);

long hwrng_yield(struct hwrng *rng)
{
	return wait_for_completion_interruptible_timeout(&rng->dying, 1);
}
EXPORT_SYMBOL_GPL(hwrng_yield);

static int __init hwrng_modinit(void)
{
	int ret;

	/* kmalloc makes this safe for virt_to_page() in virtio_rng.c */
	rng_buffer = kmalloc(rng_buffer_size(), GFP_KERNEL);
	if (!rng_buffer)
		return -ENOMEM;

	rng_fillbuf = kmalloc(rng_buffer_size(), GFP_KERNEL);
	if (!rng_fillbuf) {
		kfree(rng_buffer);
		return -ENOMEM;
	}

	ret = misc_register(&rng_miscdev);
	if (ret)
		goto misc_err;

	ret = register_pm_notifier(&hwrng_pm_nb);
	if (ret)
		goto pm_err;

	return 0;

pm_err:
	misc_deregister(&rng_miscdev);
misc_err:
	kfree(rng_fillbuf);
	kfree(rng_buffer);

	return ret;
}

static void __exit hwrng_modexit(void)
{
	mutex_lock(&rng_mutex);
	WARN_ON(rcu_access_pointer(current_rng));
	kfree(rng_buffer);
	kfree(rng_fillbuf);
	mutex_unlock(&rng_mutex);

	unregister_pm_notifier(&hwrng_pm_nb);

	misc_deregister(&rng_miscdev);
}

fs_initcall(hwrng_modinit); /* depends on misc_register() */
module_exit(hwrng_modexit);

MODULE_DESCRIPTION("H/W Random Number Generator (RNG) driver");
MODULE_LICENSE("GPL");