random: convert to using fops->read_iter()

This is a pre-requisite to wiring up splice() again for the random
and urandom drivers. It also allows us to remove the INT_MAX check in
getrandom(), because import_single_range() applies capping internally.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
[Jason: rewrote get_random_bytes_user() to simplify and also incorporate
 additional suggestions from Al.]
Cc: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

Author: axboe

drivers/char/random.c

@@ -397,13 +397,13 @@ void get_random_bytes(void *buf, size_t len)
 }
 EXPORT_SYMBOL(get_random_bytes);
 
-static ssize_t get_random_bytes_user(void __user *ubuf, size_t len)
+static ssize_t get_random_bytes_user(struct iov_iter *iter)
 {
-	size_t block_len, left, ret = 0;
 	u32 chacha_state[CHACHA_STATE_WORDS];
-	u8 output[CHACHA_BLOCK_SIZE];
+	u8 block[CHACHA_BLOCK_SIZE];
+	size_t ret = 0, copied;
 
-	if (!len)
+	if (unlikely(!iov_iter_count(iter)))
 		return 0;
 
 	/*
@@ -417,38 +417,30 @@ static ssize_t get_random_bytes_user(void __user *ubuf, size_t len)
 	 * use chacha_state after, so we can simply return those bytes to
 	 * the user directly.
 	 */
-	if (len <= CHACHA_KEY_SIZE) {
-		ret = len - copy_to_user(ubuf, &chacha_state[4], len);
+	if (iov_iter_count(iter) <= CHACHA_KEY_SIZE) {
+		ret = copy_to_iter(&chacha_state[4], CHACHA_KEY_SIZE, iter);
 		goto out_zero_chacha;
 	}
 
 	for (;;) {
-		chacha20_block(chacha_state, output);
+		chacha20_block(chacha_state, block);
 		if (unlikely(chacha_state[12] == 0))
 			++chacha_state[13];
 
-		block_len = min_t(size_t, len, CHACHA_BLOCK_SIZE);
-		left = copy_to_user(ubuf, output, block_len);
-		if (left) {
-			ret += block_len - left;
-			break;
-		}
-
-		ubuf += block_len;
-		ret += block_len;
-		len -= block_len;
-		if (!len)
+		copied = copy_to_iter(block, sizeof(block), iter);
+		ret += copied;
+		if (!iov_iter_count(iter) || copied != sizeof(block))
 			break;
 
-		BUILD_BUG_ON(PAGE_SIZE % CHACHA_BLOCK_SIZE != 0);
+		BUILD_BUG_ON(PAGE_SIZE % sizeof(block) != 0);
 		if (ret % PAGE_SIZE == 0) {
 			if (signal_pending(current))
 				break;
 			cond_resched();
 		}
 	}
 
-	memzero_explicit(output, sizeof(output));
+	memzero_explicit(block, sizeof(block));
 out_zero_chacha:
 	memzero_explicit(chacha_state, sizeof(chacha_state));
 	return ret ? ret : -EFAULT;
@@ -1248,6 +1240,10 @@ static void __cold try_to_generate_entropy(void)
 
 SYSCALL_DEFINE3(getrandom, char __user *, ubuf, size_t, len, unsigned int, flags)
 {
+	struct iov_iter iter;
+	struct iovec iov;
+	int ret;
+
 	if (flags & ~(GRND_NONBLOCK | GRND_RANDOM | GRND_INSECURE))
 		return -EINVAL;
 
@@ -1258,19 +1254,18 @@ SYSCALL_DEFINE3(getrandom, char __user *, ubuf, size_t, len, unsigned int, flags
 	if ((flags & (GRND_INSECURE | GRND_RANDOM)) == (GRND_INSECURE | GRND_RANDOM))
 		return -EINVAL;
 
-	if (len > INT_MAX)
-		len = INT_MAX;
-
 	if (!crng_ready() && !(flags & GRND_INSECURE)) {
-		int ret;
-
 		if (flags & GRND_NONBLOCK)
 			return -EAGAIN;
 		ret = wait_for_random_bytes();
 		if (unlikely(ret))
 			return ret;
 	}
-	return get_random_bytes_user(ubuf, len);
+
+	ret = import_single_range(READ, ubuf, len, &iov, &iter);
+	if (unlikely(ret))
+		return ret;
+	return get_random_bytes_user(&iter);
 }
 
 static __poll_t random_poll(struct file *file, poll_table *wait)
@@ -1314,8 +1309,7 @@ static ssize_t random_write(struct file *file, const char __user *ubuf,
 	return (ssize_t)len;
 }
 
-static ssize_t urandom_read(struct file *file, char __user *ubuf,
-			    size_t len, loff_t *ppos)
+static ssize_t urandom_read_iter(struct kiocb *kiocb, struct iov_iter *iter)
 {
 	static int maxwarn = 10;
 
@@ -1331,23 +1325,22 @@ static ssize_t urandom_read(struct file *file, char __user *ubuf,
 			++urandom_warning.missed;
 		else if (ratelimit_disable || __ratelimit(&urandom_warning)) {
 			--maxwarn;
-			pr_notice("%s: uninitialized urandom read (%zd bytes read)\n",
-				  current->comm, len);
+			pr_notice("%s: uninitialized urandom read (%zu bytes read)\n",
+				  current->comm, iov_iter_count(iter));
 		}
 	}
 
-	return get_random_bytes_user(ubuf, len);
+	return get_random_bytes_user(iter);
 }
 
-static ssize_t random_read(struct file *file, char __user *ubuf,
-			   size_t len, loff_t *ppos)
+static ssize_t random_read_iter(struct kiocb *kiocb, struct iov_iter *iter)
 {
 	int ret;
 
 	ret = wait_for_random_bytes();
 	if (ret != 0)
 		return ret;
-	return get_random_bytes_user(ubuf, len);
+	return get_random_bytes_user(iter);
 }
 
 static long random_ioctl(struct file *f, unsigned int cmd, unsigned long arg)
@@ -1409,7 +1402,7 @@ static int random_fasync(int fd, struct file *filp, int on)
 }
 
 const struct file_operations random_fops = {
-	.read = random_read,
+	.read_iter = random_read_iter,
 	.write = random_write,
 	.poll = random_poll,
 	.unlocked_ioctl = random_ioctl,
@@ -1419,7 +1412,7 @@ const struct file_operations random_fops = {
 };
 
 const struct file_operations urandom_fops = {
-	.read = urandom_read,
+	.read_iter = urandom_read_iter,
 	.write = random_write,
 	.unlocked_ioctl = random_ioctl,
 	.compat_ioctl = compat_ptr_ioctl,