/*
 * 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/miscdevice.h>
#include <linux/module.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/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;
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 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 */
        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;
                }
        }

        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();

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

        /* 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 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;
}

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_unlock;
                }
        }
        mutex_unlock(&rng_mutex);
        return 0;
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) {
                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);

        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");