[PATCH v2 3/3] HID: mcp2221: avoid stale rxbuf pointer

[Enrik Berkhan <Enrik.Berkhan@xxxxxxx>]

In case the MCP2221 driver receives an unexpected read complete report
from the device, the data should not be copied to mcp->rxbuf. The
pointer might be NULL or even stale, having been set during an earlier
transaction.

Further, some bounds checking has been added.

Signed-off-by: Enrik Berkhan <Enrik.Berkhan@xxxxxxx>
---
 drivers/hid/hid-mcp2221.c | 36 +++++++++++++++++++++++++++++-------
 1 file changed, 29 insertions(+), 7 deletions(-)

diff --git a/drivers/hid/hid-mcp2221.c b/drivers/hid/hid-mcp2221.c
index 0705526231ec..471b29c3c501 100644
--- a/drivers/hid/hid-mcp2221.c
+++ b/drivers/hid/hid-mcp2221.c
@@ -98,6 +98,7 @@ struct mcp2221 {
 	u8 *rxbuf;
 	u8 txbuf[64];
 	int rxbuf_idx;
+	int rxbuf_len;
 	int status;
 	u8 cur_i2c_clk_div;
 	struct gpio_chip *gc;
@@ -294,11 +295,12 @@ static int mcp_i2c_smbus_read(struct mcp2221 *mcp,
 		mcp->rxbuf = smbus_buf;
 	}
 
+	mcp->rxbuf_len = total_len;
+	mcp->rxbuf_idx = 0;
+
 	ret = mcp_send_data_req_status(mcp, mcp->txbuf, 4);
 	if (ret)
-		return ret;
-
-	mcp->rxbuf_idx = 0;
+		goto out_invalidate_rxbuf;
 
 	do {
 		memset(mcp->txbuf, 0, 4);
@@ -306,15 +308,20 @@ static int mcp_i2c_smbus_read(struct mcp2221 *mcp,
 
 		ret = mcp_send_data_req_status(mcp, mcp->txbuf, 1);
 		if (ret)
-			return ret;
+			goto out_invalidate_rxbuf;
 
 		ret = mcp_chk_last_cmd_status(mcp);
 		if (ret)
-			return ret;
+			goto out_invalidate_rxbuf;
 
 		usleep_range(980, 1000);
 	} while (mcp->rxbuf_idx < total_len);
 
+out_invalidate_rxbuf:
+	mcp->rxbuf = NULL;
+	mcp->rxbuf_len = 0;
+	mcp->rxbuf_idx = 0;
+
 	return ret;
 }
 
@@ -499,8 +506,12 @@ static int mcp_smbus_xfer(struct i2c_adapter *adapter, u16 addr,
 
 			mcp->rxbuf_idx = 0;
 			mcp->rxbuf = data->block;
+			mcp->rxbuf_len = sizeof(data->block);
 			mcp->txbuf[0] = MCP2221_I2C_GET_DATA;
 			ret = mcp_send_data_req_status(mcp, mcp->txbuf, 1);
+			mcp->rxbuf_idx = 0;
+			mcp->rxbuf = NULL;
+			mcp->rxbuf_len = 0;
 			if (ret)
 				goto exit;
 		} else {
@@ -522,8 +533,12 @@ static int mcp_smbus_xfer(struct i2c_adapter *adapter, u16 addr,
 
 			mcp->rxbuf_idx = 0;
 			mcp->rxbuf = data->block;
+			mcp->rxbuf_len = sizeof(data->block);
 			mcp->txbuf[0] = MCP2221_I2C_GET_DATA;
 			ret = mcp_send_data_req_status(mcp, mcp->txbuf, 1);
+			mcp->rxbuf_idx = 0;
+			mcp->rxbuf = NULL;
+			mcp->rxbuf_len = 0;
 			if (ret)
 				goto exit;
 		} else {
@@ -734,6 +749,7 @@ static int mcp2221_raw_event(struct hid_device *hdev,
 				struct hid_report *report, u8 *data, int size)
 {
 	u8 *buf;
+	int len;
 	struct mcp2221 *mcp = hid_get_drvdata(hdev);
 
 	switch (data[0]) {
@@ -792,9 +808,15 @@ static int mcp2221_raw_event(struct hid_device *hdev,
 				break;
 			}
 			if (data[2] == MCP2221_I2C_READ_COMPL) {
+				if (mcp->rxbuf == NULL || mcp->rxbuf_idx >= mcp->rxbuf_len)
+					return 1; /* no complete() in this case */
+
 				buf = mcp->rxbuf;
-				memcpy(&buf[mcp->rxbuf_idx], &data[4], data[3]);
-				mcp->rxbuf_idx = mcp->rxbuf_idx + data[3];
+				len = data[3];
+				if (len > mcp->rxbuf_len - mcp->rxbuf_idx)
+					len = mcp->rxbuf_len - mcp->rxbuf_idx;
+				memcpy(&buf[mcp->rxbuf_idx], &data[4], len);
+				mcp->rxbuf_idx = mcp->rxbuf_idx + len;
 				mcp->status = 0;
 				break;
 			}
-- 
2.34.1