[PATCH v2 3/3] HID: mcp2221: validate report size in mcp2221_raw_event()

mcp2221_raw_event() never validates the size of incoming HID reports.
In the MCP2221_I2C_GET_DATA path it trusts the device-supplied data[3]
as the copy length without checking that 4 + data[3] bytes actually
exist in the received report. A malicious or misbehaving USB device can
send a short report with a large data[3], causing the memcpy to read
past the valid report data in the HID transfer buffer and leak
uninitialized kernel memory back to userspace through the I2C/SMBus
read path.

Add a minimum size check at entry and validate that the source range
fits within the received report before the copy.

Fixes: 67a95c21463d ("HID: mcp2221: add usb to i2c-smbus host bridge")
Cc: [email protected]
Signed-off-by: Jiangshan Yi <[email protected]>
---
 drivers/hid/hid-mcp2221.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/drivers/hid/hid-mcp2221.c b/drivers/hid/hid-mcp2221.c
index 9e03d1f..d52ce35 100644
--- a/drivers/hid/hid-mcp2221.c
+++ b/drivers/hid/hid-mcp2221.c
@@ -865,6 +865,9 @@ static int mcp2221_raw_event(struct hid_device *hdev,
 	u8 *buf;
 	struct mcp2221 *mcp = hid_get_drvdata(hdev);

+	if (size < 4)
+		return 0;
+
 	switch (data[0]) {

 	case MCP2221_I2C_WR_DATA:
@@ -930,6 +933,10 @@ static int mcp2221_raw_event(struct hid_device *hdev,
 					mcp->status = -EINVAL;
 					break;
 				}
+				if (4 + data[3] > size) {
+					mcp->status = -EINVAL;
+					break;
+				}
 				buf = mcp->rxbuf;
 				memcpy(&buf[mcp->rxbuf_idx], &data[4], data[3]);
 				mcp->rxbuf_idx = mcp->rxbuf_idx + data[3];
--
2.25.1