i2c: ismt: prevent memory corruption in ismt_access()

The "data->block[0]" variable comes from the user and is a number
between 0-255.  It needs to be capped to prevent writing beyond the end
of dma_buffer[].

Fixes: 5e9a97b1 ("i2c: ismt: Adding support for I2C_SMBUS_BLOCK_PROC_CALL")
Reported-and-tested-by: Zheyu Ma <zheyuma97@gmail.com>
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/i2c/busses/i2c-ismt.c

@@ -528,6 +528,9 @@ static int ismt_access(struct i2c_adapter *adap, u16 addr,
 
 	case I2C_SMBUS_BLOCK_PROC_CALL:
 		dev_dbg(dev, "I2C_SMBUS_BLOCK_PROC_CALL\n");
+		if (data->block[0] > I2C_SMBUS_BLOCK_MAX)
+			return -EINVAL;
+
 		dma_size = I2C_SMBUS_BLOCK_MAX;
 		desc->tgtaddr_rw = ISMT_DESC_ADDR_RW(addr, 1);
 		desc->wr_len_cmd = data->block[0] + 1;