staging: speakup: fix wraparound in uaccess length check

If softsynthx_read() is called with `count < 3`, `count - 3` wraps, causing
the loop to copy as much data as available to the provided buffer. If
softsynthx_read() is invoked through sys_splice(), this causes an
unbounded kernel write; but even when userspace just reads from it
normally, a small size could cause userspace crashes.

Fixes: 425e586cf95b ("speakup: add unicode variant of /dev/softsynth")
Cc: stable@vger.kernel.org
Signed-off-by: Samuel Thibault <samuel.thibault@ens-lyon.org>
Signed-off-by: Jann Horn <jannh@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/speakup/speakup_soft.c

@@ -198,11 +198,15 @@ static ssize_t softsynthx_read(struct file *fp, char __user *buf, size_t count,
 	int chars_sent = 0;
 	char __user *cp;
 	char *init;
+	size_t bytes_per_ch = unicode ? 3 : 1;
 	u16 ch;
 	int empty;
 	unsigned long flags;
 	DEFINE_WAIT(wait);
 
+	if (count < bytes_per_ch)
+		return -EINVAL;
+
 	spin_lock_irqsave(&speakup_info.spinlock, flags);
 	while (1) {
 		prepare_to_wait(&speakup_event, &wait, TASK_INTERRUPTIBLE);
@@ -228,7 +232,7 @@ static ssize_t softsynthx_read(struct file *fp, char __user *buf, size_t count,
 	init = get_initstring();
 
 	/* Keep 3 bytes available for a 16bit UTF-8-encoded character */
-	while (chars_sent <= count - 3) {
+	while (chars_sent <= count - bytes_per_ch) {
 		if (speakup_info.flushing) {
 			speakup_info.flushing = 0;
 			ch = '\x18';