linux-kernel - Re: [PATCH] Input: joydev - prevent potential write out of bounds in ioctl

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Date:   Sun, 20 Jun 2021 09:37:47 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Alexander Larkin <avlarkin82@...il.com>
Cc:     Dmitry Torokhov <dmitry.torokhov@...il.com>,
        Dan Carpenter <dan.carpenter@...cle.com>,
        linux-input@...r.kernel.org, lkml <linux-kernel@...r.kernel.org>,
        Security Officers <security@...nel.org>,
        Murray McAllister <murray.mcallister@...il.com>
Subject: Re: [PATCH] Input: joydev - prevent potential write out of bounds in ioctl

On Sun, Jun 20, 2021 at 5:01 AM Alexander Larkin <avlarkin82@...il.com> wrote:
>
>     The problem is that the check of user input values that is just
>     before the fixed line of code is for the part of first values
>     (before len or before len/2), but then the usage of all the values
>     including i >= len (or i >= len/2) could be.

No, I think the problem is simpler than that.

> -       for (i = 0; i < joydev->nabs; i++)
> +       for (i = 0; i < len && i < joydev->nabs; i++)
>                 joydev->absmap[joydev->abspam[i]] = i;

This part is unnecessary - all values of "joydev->abspam[i]" have been
validated (either they are the old ones, or the new ones that we just
validated).

>         memcpy(joydev->keypam, keypam, len);
>
> -       for (i = 0; i < joydev->nkey; i++)
> +       for (i = 0; i < (len / 2) && i < joydev->nkey; i++)
>                 joydev->keymap[keypam[i] - BTN_MISC] = i;

The problem here is not that we walk past "len/2", but that the code
*should* have used

        joydev->keymap[joydev->keypam[i] - BTN_MISC] = i;

(note the "keypam[1]" vs "joydev->keypam[i]").

And the reason it *should* walk the whole "joydev->nkey" is that if
there are later cases with the same keypam value, the later ones
should override the previous ones (well, that "should" is more a
"traditionally have").

So I think the right patch is this one-liner

  diff --git a/drivers/input/joydev.c b/drivers/input/joydev.c
  index da8963a9f044..947d440a3be6 100644
  --- a/drivers/input/joydev.c
  +++ b/drivers/input/joydev.c
  @@ -499,7 +499,7 @@ static int joydev_handle_JSIOCSBTNMAP(struct
joydev *joydev,
        memcpy(joydev->keypam, keypam, len);

        for (i = 0; i < joydev->nkey; i++)
  -             joydev->keymap[keypam[i] - BTN_MISC] = i;
  +             joydev->keymap[joydev->keypam[i] - BTN_MISC] = i;

    out:
        kfree(keypam);

(whitespace-damaged, I would like Dmitry to think about it rather than
apply this mindlessly.

Dmitry?

              Linus