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

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

[Qasim Ijaz]

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>
---
 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",
+					rdesc[607]);
+			}
 		} 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",
-				rdesc[607]);
+				"GT7868Q fixup: report descriptor only %u bytes, skipping\n",
+				*size);
 		}
 	}
 
-- 
2.39.5