linux-kernel - Re: [PATCH RESEND] HID: multitouch: fix slab out-of-bounds access in mt_report

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Message-ID: <c90e88a4-7fff-49fa-8a6f-24f3671d9390@kernel.org>
Date: Tue, 22 Jul 2025 11:16:15 +0200
From: Jiri Slaby <jirislaby@...nel.org>
To: Qasim Ijaz <qasdev00@...il.com>, jikos@...nel.org, bentiss@...nel.org
Cc: linux-input@...r.kernel.org, linux-kernel@...r.kernel.org,
 stable@...r.kernel.org, Dmitry Savin <envelsavinds@...il.com>
Subject: Re: [PATCH RESEND] HID: multitouch: fix slab out-of-bounds access in
 mt_report_fixup()

On 22. 07. 25, 10:00, Qasim Ijaz wrote:
> A malicious HID device can trigger a slab out-of-bounds during
> mt_report_fixup() by passing in report descriptor smaller than
> 607 bytes. mt_report_fixup() attempts to patch byte offset 607
> of the descriptor with 0x25 by first checking if byte offset
> 607 is 0x15 however it lacks bounds checks to verify if the
> descriptor is big enough before conducting this check. Fix
> this vulnerability by ensuring the descriptor size is
> greater than or equal to 608 before accessing it.
> 
> Below is the KASAN splat after the out of bounds access happens:
> 
> [   13.671954] ==================================================================
> [   13.672667] BUG: KASAN: slab-out-of-bounds in mt_report_fixup+0x103/0x110
> [   13.673297] Read of size 1 at addr ffff888103df39df by task kworker/0:1/10
> [   13.673297]
> [   13.673297] CPU: 0 UID: 0 PID: 10 Comm: kworker/0:1 Not tainted 6.15.0-00005-gec5d573d83f4-dirty #3
> [   13.673297] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.16.2-debian-1.16.2-1 04/04
> [   13.673297] Call Trace:
> [   13.673297]  <TASK>
> [   13.673297]  dump_stack_lvl+0x5f/0x80
> [   13.673297]  print_report+0xd1/0x660
> [   13.673297]  kasan_report+0xe5/0x120
> [   13.673297]  __asan_report_load1_noabort+0x18/0x20
> [   13.673297]  mt_report_fixup+0x103/0x110
> [   13.673297]  hid_open_report+0x1ef/0x810
> [   13.673297]  mt_probe+0x422/0x960
> [   13.673297]  hid_device_probe+0x2e2/0x6f0
> [   13.673297]  really_probe+0x1c6/0x6b0
> [   13.673297]  __driver_probe_device+0x24f/0x310
> [   13.673297]  driver_probe_device+0x4e/0x220
> [   13.673297]  __device_attach_driver+0x169/0x320
> [   13.673297]  bus_for_each_drv+0x11d/0x1b0
> [   13.673297]  __device_attach+0x1b8/0x3e0
> [   13.673297]  device_initial_probe+0x12/0x20
> [   13.673297]  bus_probe_device+0x13d/0x180
> [   13.673297]  device_add+0xe3a/0x1670
> [   13.673297]  hid_add_device+0x31d/0xa40
> [...]
> 
> Fixes: c8000deb6836 ("HID: multitouch: Add support for GT7868Q")
> Cc: stable@...r.kernel.org
> Signed-off-by: Qasim Ijaz <qasdev00@...il.com>
> Reviewed-by: Dmitry Savin <envelsavinds@...il.com>
> ---
>   drivers/hid/hid-multitouch.c | 25 ++++++++++++++++---------
>   1 file changed, 16 insertions(+), 9 deletions(-)
> 
> diff --git a/drivers/hid/hid-multitouch.c b/drivers/hid/hid-multitouch.c
> index 7ac8e16e6158..af4abe3ba410 100644
> --- a/drivers/hid/hid-multitouch.c
> +++ b/drivers/hid/hid-multitouch.c
> @@ -1461,18 +1461,25 @@ static const __u8 *mt_report_fixup(struct hid_device *hdev, __u8 *rdesc,
>   	if (hdev->vendor == I2C_VENDOR_ID_GOODIX &&
>   	    (hdev->product == I2C_DEVICE_ID_GOODIX_01E8 ||
>   	     hdev->product == I2C_DEVICE_ID_GOODIX_01E9)) {
> -		if (rdesc[607] == 0x15) {
> -			rdesc[607] = 0x25;
> -			dev_info(
> -				&hdev->dev,
> -				"GT7868Q report descriptor fixup is applied.\n");
> +		if (*size >= 608) {
> +			if (rdesc[607] == 0x15) {
> +				rdesc[607] = 0x25;
> +				dev_info(
> +					&hdev->dev,
> +					"GT7868Q report descriptor fixup is applied.\n");
> +			} else {
> +				dev_info(
> +					&hdev->dev,
> +					"The byte is not expected for fixing the report descriptor. \
> +					It's possible that the touchpad firmware is not suitable for applying the fix. \
> +					got: %x\n",

This is wrong. You have all the spaces/tabs in the string now. Drop all 
the backslashes, and open and close the string on every line.

> +					rdesc[607]);
> +			}

As this is superlong and superindented, perhaps introduce a new function 
for these devices?

>   		} else {
>   			dev_info(
>   				&hdev->dev,
> -				"The byte is not expected for fixing the report descriptor. \
> -It's possible that the touchpad firmware is not suitable for applying the fix. \
> -got: %x\n",

This was horrid too, yeah.

> -				rdesc[607]);
> +				"GT7868Q fixup: report descriptor only %u bytes, skipping\n",

A predicate missing. Eg. "has only", or "is only".

> +				*size);
>   		}
>   	}
>   

thanks,
-- 
js
suse labs