linux-kernel - Re: [PATCH] Input: gtco - fix potential out-of-bound access

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Date:   Tue, 24 Oct 2017 13:04:03 +0200
From:   Andrey Konovalov <andreyknvl@...gle.com>
To:     Dmitry Torokhov <dmitry.torokhov@...il.com>
Cc:     linux-input@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] Input: gtco - fix potential out-of-bound access

On Tue, Oct 24, 2017 at 7:28 AM, Dmitry Torokhov
<dmitry.torokhov@...il.com> wrote:
> parse_hid_report_descriptor() has a while (i < length) loop, which
> only guarantees that there's at least 1 byte in the buffer, but the
> loop body can read multiple bytes which causes out-of-bounds access.
>
> Reported-by: Andrey Konovalov <andreyknvl@...gle.com>
> Signed-off-by: Dmitry Torokhov <dmitry.torokhov@...il.com>
> ---
>  drivers/input/tablet/gtco.c | 24 +++++++++++++++++-------
>  1 file changed, 17 insertions(+), 7 deletions(-)
>
> diff --git a/drivers/input/tablet/gtco.c b/drivers/input/tablet/gtco.c
> index b796e891e2ee..0351203b8c24 100644
> --- a/drivers/input/tablet/gtco.c
> +++ b/drivers/input/tablet/gtco.c
> @@ -230,13 +230,24 @@ static void parse_hid_report_descriptor(struct gtco *device, char * report,
>
>         /* Walk  this report and pull out the info we need */
>         while (i < length) {
> -               prefix = report[i];
> -
> -               /* Skip over prefix */
> -               i++;
> +               prefix = report[i++];
>
>                 /* Determine data size and save the data in the proper variable */
> -               size = PREF_SIZE(prefix);
> +               if (PREF_SIZE(prefix) < 1) {

AFAIU PREF_SIZE(prefix) == 0 is a perfectly valid item data size.

> +                       dev_err(ddev,
> +                               "Invalid size %d in element at offset %d\n",
> +                               PREF_SIZE(prefix), i);
> +                       break;
> +               }
> +
> +               size = 1U << (PREF_SIZE(prefix) - 1);

This formula doesn't work with PREF_SIZE(prefix) == 0.

> +               if (i + size >= length) {
> +                       dev_err(ddev,
> +                               "Not enough data (need %d, have %d)\n",
> +                               i + size, length);
> +                       break;
> +               }
> +
>                 switch (size) {
>                 case 1:
>                         data = report[i];
> @@ -244,8 +255,7 @@ static void parse_hid_report_descriptor(struct gtco *device, char * report,
>                 case 2:
>                         data16 = get_unaligned_le16(&report[i]);
>                         break;
> -               case 3:
> -                       size = 4;
> +               case 4:
>                         data32 = get_unaligned_le32(&report[i]);
>                         break;
>                 }
> --
> 2.15.0.rc0.271.g36b669edcc-goog
>
>
> --
> Dmitry