Re: [PATCH RESEND] HID: multitouch: fix slab out-of-bounds access in mt_report_fixup()

[Qasim Ijaz <qasdev00@gmail.com>]

On Tue, Jul 22, 2025 at 11:16:15AM +0200, Jiri Slaby wrote:
> 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@vger.kernel.org
> > Signed-off-by: Qasim Ijaz <qasdev00@gmail.com>
> > Reviewed-by: Dmitry Savin <envelsavinds@gmail.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".
> 

Thanks for the feedback Jiri, I took the advice on board, is something
like this better?

 static const __u8 *mt_report_fixup(struct hid_device *hdev, __u8 *rdesc,
				   unsigned int *size)
 {
          if (hdev->vendor == I2C_VENDOR_ID_GOODIX &&
              (hdev->product == I2C_DEVICE_ID_GOODIX_01E8 ||
               hdev->product == I2C_DEVICE_ID_GOODIX_01E9)) {
		 if (*size < 608) {
			 dev_info(
				 &hdev->dev,
				 "GT7868Q fixup: report descriptor is only %u bytes, skipping\n",
				 *size);
                          return rdesc;
                  }
 
		 if (rdesc[607] == 0x15) {
			 rdesc[607] = 0x25;
			 dev_info(
				 &hdev->dev,
				 "GT7868Q fixup: report descriptor fixup is applied.\n");
		 } else {
			 dev_info(&hdev->dev,
				 "GT7868Q fixup: offset 607 is %x (expected 0x15), "
				 "descriptor may be malformed, skipping\n",
				 rdesc[607]);
		 }
	  }
 
 	  return rdesc;
 }

the key changes I made are:

- Move size check to the top, this way the indentation level is decent
- get rid of message backslashes
- shorten the fixup failure message when rdesc[607] is not 0x15 and make
  it a bit clearer since this message was the longest - just a minor
  cleanup
- added "is only %u bytes" as you suggested

if this is all good I can send v2.

Thanks
qasim
> > +				*size);
> >   		}
> >   	}
> 
> thanks,
> -- 
> js
> suse labs
>