Re: [PATCH] staging: speakup: fix wraparound in uaccess length check

[Jann Horn <jannh@google.com>]

On Sat, Jul 7, 2018 at 1:29 AM Samuel Thibault
<samuel.thibault@ens-lyon.org> wrote:
>
> Re,
>
> Could you review, test, and resubmit the patch below instead?

Er... you mean, you want me to take your patch, add my Signed-off-by
below yours, and then send that?

> Samuel
>
>
> 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.

This looks sane to me. I've also tested it, and it seems to work.

Some random thing I noticed, but I don't think it has anything to do
with this issue: In some runs, when the console is repeatedly printing
"Debian GNU/Linux 9 debian tty1\n\ndebian login: " in response to me
pressing enter repeatedly, /dev/softsynthu (read in 1-byte steps)
seems to return things like "Debian GNU slash Linux 9 debian tty1 \n
debi login: ". I don't understand why it sometimes says "debi login"
instead of "debian login".
> Fixes: 425e586cf95b ("speakup: add unicode variant of /dev/softsynth")
> Cc: stable@vger.kernel.org
> Signed-off-by: Samuel Thibault <samuel.thibault@ens-lyon.org>
>
> --- a/drivers/staging/speakup/speakup_soft.c
> +++ b/drivers/staging/speakup/speakup_soft.c
> @@ -198,11 +198,15 @@ static ssize_t softsynthx_read(struct fi
>         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 fi
>         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';