Input: elan_i2c_smbus - fix more potential stack buffer overflows

Commit 40f7090bb1b4 ("Input: elan_i2c_smbus - fix corrupted stack")
fixed most of the functions using i2c_smbus_read_block_data() to
allocate a buffer with the maximum block size. However three
functions were left unchanged:

* In elan_smbus_initialize(), increase the buffer size in the same
way.
* In elan_smbus_calibrate_result(), the buffer is provided by the
caller (calibrate_store()), so introduce a bounce buffer. Also
name the result buffer size.
* In elan_smbus_get_report(), the buffer is provided by the caller
but happens to be the right length. Add a compile-time assertion
to ensure this remains the case.

Cc: <[e-mail]> # 3.19+
Signed-off-by: Ben Hutchings <[e-mail]>
Reviewed-by: Benjamin Tissoires <[e-mail]>
Signed-off-by: Dmitry Torokhov <[e-mail]>

drivers/input/mouse/elan_i2c.h

···
 #define ETP_DISABLE_POWER	0x0001
 #define ETP_PRESSURE_OFFSET	25
 
+#define ETP_CALIBRATE_MAX_LEN	3
+
 /* IAP Firmware handling */
 #define ETP_PRODUCT_ID_FORMAT_STRING	"%d.0"
 #define ETP_FW_NAME		"elan_i2c_" ETP_PRODUCT_ID_FORMAT_STRING ".bin"

drivers/input/mouse/elan_i2c_core.c

···
 	int tries = 20;
 	int retval;
 	int error;
-	u8 val[3];
+	u8 val[ETP_CALIBRATE_MAX_LEN];
 
 	retval = mutex_lock_interruptible(&data->sysfs_mutex);
 	if (retval)

drivers/input/mouse/elan_i2c_smbus.c

···
 static int elan_smbus_initialize(struct i2c_client *client)
 {
 	u8 check[ETP_SMBUS_HELLOPACKET_LEN] = { 0x55, 0x55, 0x55, 0x55, 0x55 };
-	u8 values[ETP_SMBUS_HELLOPACKET_LEN] = { 0, 0, 0, 0, 0 };
+	u8 values[I2C_SMBUS_BLOCK_MAX] = {0};
 	int len, error;
 
 	/* Get hello packet */
···
 static int elan_smbus_calibrate_result(struct i2c_client *client, u8 *val)
 {
 	int error;
+	u8 buf[I2C_SMBUS_BLOCK_MAX] = {0};
+
+	BUILD_BUG_ON(ETP_CALIBRATE_MAX_LEN > sizeof(buf));
 
 	error = i2c_smbus_read_block_data(client,
-					  ETP_SMBUS_CALIBRATE_QUERY, val);
+					  ETP_SMBUS_CALIBRATE_QUERY, buf);
 	if (error < 0)
 		return error;
 
+	memcpy(val, buf, ETP_CALIBRATE_MAX_LEN);
 	return 0;
 }
 
···
 static int elan_smbus_get_report(struct i2c_client *client, u8 *report)
 {
 	int len;
+
+	BUILD_BUG_ON(I2C_SMBUS_BLOCK_MAX > ETP_SMBUS_REPORT_LEN);
 
 	len = i2c_smbus_read_block_data(client,
 					ETP_SMBUS_PACKET_QUERY,