media: dw2102: Fix null-ptr-deref in dw2102_i2c_transfer()

In dw2102_i2c_transfer, msg is controlled by user. When msg[i].buf
is null and msg[i].len is zero, former checks on msg[i].buf would be
passed. Malicious data finally reach dw2102_i2c_transfer. If accessing
msg[i].buf[0] without sanity check, null ptr deref would happen.
We add check on msg[i].len to prevent crash.

Similar commit:
commit 950e252c
("[media] dw2102: limit messages to buffer size")
Signed-off-by: Zhang Shurong <zhang_shurong@foxmail.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>

drivers/media/usb/dvb-usb/dw2102.c

@@ -128,6 +128,10 @@ static int dw2102_i2c_transfer(struct i2c_adapter *adap, struct i2c_msg msg[],
 
 	switch (num) {
 	case 2:
+		if (msg[0].len < 1) {
+			num = -EOPNOTSUPP;
+			break;
+		}
 		/* read stv0299 register */
 		value = msg[0].buf[0];/* register */
 		for (i = 0; i < msg[1].len; i++) {
@@ -139,6 +143,10 @@ static int dw2102_i2c_transfer(struct i2c_adapter *adap, struct i2c_msg msg[],
 	case 1:
 		switch (msg[0].addr) {
 		case 0x68:
+			if (msg[0].len < 2) {
+				num = -EOPNOTSUPP;
+				break;
+			}
 			/* write to stv0299 register */
 			buf6[0] = 0x2a;
 			buf6[1] = msg[0].buf[0];
@@ -148,6 +156,10 @@ static int dw2102_i2c_transfer(struct i2c_adapter *adap, struct i2c_msg msg[],
 			break;
 		case 0x60:
 			if (msg[0].flags == 0) {
+				if (msg[0].len < 4) {
+					num = -EOPNOTSUPP;
+					break;
+				}
 			/* write to tuner pll */
 				buf6[0] = 0x2c;
 				buf6[1] = 5;
@@ -159,6 +171,10 @@ static int dw2102_i2c_transfer(struct i2c_adapter *adap, struct i2c_msg msg[],
 				dw210x_op_rw(d->udev, 0xb2, 0, 0,
 						buf6, 7, DW210X_WRITE_MSG);
 			} else {
+				if (msg[0].len < 1) {
+					num = -EOPNOTSUPP;
+					break;
+				}
 			/* read from tuner */
 				dw210x_op_rw(d->udev, 0xb5, 0, 0,
 						buf6, 1, DW210X_READ_MSG);
@@ -166,12 +182,20 @@ static int dw2102_i2c_transfer(struct i2c_adapter *adap, struct i2c_msg msg[],
 			}
 			break;
 		case (DW2102_RC_QUERY):
+			if (msg[0].len < 2) {
+				num = -EOPNOTSUPP;
+				break;
+			}
 			dw210x_op_rw(d->udev, 0xb8, 0, 0,
 					buf6, 2, DW210X_READ_MSG);
 			msg[0].buf[0] = buf6[0];
 			msg[0].buf[1] = buf6[1];
 			break;
 		case (DW2102_VOLTAGE_CTRL):
+			if (msg[0].len < 1) {
+				num = -EOPNOTSUPP;
+				break;
+			}
 			buf6[0] = 0x30;
 			buf6[1] = msg[0].buf[0];
 			dw210x_op_rw(d->udev, 0xb2, 0, 0,