staging: speakup: fix wraparound in uaccess length check

commit b96fba8d5855c3617adbfb43ca4723a808cac954 upstream.

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>

diff --git a/drivers/staging/speakup/speakup_soft.c b/drivers/staging/speakup/speakup_soft.c
index d99daf6..fe229d6 100644 (file)
--- a/drivers/staging/speakup/speakup_soft.c
+++ b/drivers/staging/speakup/speakup_soft.c
@@ -207,11 +207,15 @@ 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);
@@ -237,7 +241,7 @@ 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';