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

From: Dmitry Torokhov
Date: Mon Jun 21 2021 - 01:25:16 EST


On Sun, Jun 20, 2021 at 09:37:47AM -0700, Linus Torvalds wrote:
> On Sun, Jun 20, 2021 at 5:01 AM Alexander Larkin <avlarkin82@xxxxxxxxx> 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").

Yes, we can discuss whether "short" ioctl should clear out the part of
map that is not supplied by the call, but given that I consider joydev
legacy my preference would be to leave this as it was.

>
> 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?

Yes, this makes sense to me and it is safe as joydev->keypam is
guaranteed to be the right size.

Are you going to reformat this and resend or should I?


Thanks.

--
Dmitry