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Message-ID: <aIC4X_pDf6SFd55c@gmail.com>
Date: Wed, 23 Jul 2025 11:24:31 +0100
From: Qasim Ijaz <qasdev00@...il.com>
To: Jiri Slaby <jirislaby@...nel.org>
Cc: jikos@...nel.org, bentiss@...nel.org, 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 Wed, Jul 23, 2025 at 06:40:52AM +0200, Jiri Slaby wrote:
> On 22. 07. 25, 13:19, Qasim Ijaz wrote:
> > 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@...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".
> > >
> >
> > Thanks for the feedback Jiri, I took the advice on board, is something
> > like this better?
>
> Definitely.
>
> > 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.
>
> I would invert the conditions. So the code would look like:
>
> static bool goodix_needs_fixup(hdev)
> {
> if (hdev->vendor != I2C_VENDOR_ID_GOODIX)
> return false;
>
> return hdev->product == I2C_DEVICE_ID_GOODIX_01E8 ||
> hdev->product == I2C_DEVICE_ID_GOODIX_01E9;
> }
>
> static const __u8 *mt_report_fixup(struct hid_device *hdev, __u8 *rdesc,
> unsigned int *size)
> {
> if (!goodix_needs_fixup(hdev))
> return rdesc;
>
> if (*size < 608) {
> dev_info(&hdev->dev,
> "GT7868Q fixup: report descriptor is only %u bytes,
> skipping\n",
> *size);
> return rdesc;
> }
>
> if (rdesc[607] != 0x15) {
> dev_info(&hdev->dev,
> "GT7868Q fixup: offset 607 is %x (expected 0x15), descriptor may be
> malformed, skipping\n",
> rdesc[607]);
> return rdesc;
> }
>
> rdesc[607] = 0x25;
> dev_info(&hdev->dev,
> "GT7868Q fixup: report descriptor fixup is applied.\n");
>
> return rdesc;
> }
Thanks Jiri, this looks good. I think Ill split this into 2 patches
one for fixing the OOB with a size check and the second for a general
function cleanup. Hope that sounds good.
Thanks
qasim
>
> thanks,
> --
> js
> suse labs
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