i2c: ismt: Return EMSGSIZE for block reads with bogus length

Compare the number of bytes actually seen on the wire to the byte
count field returned by the slave device.

Previously we just overwrote the byte count returned by the slave
with the real byte count and let the caller figure out if the
message was sane.

Signed-off-by: Stephen Douthit <stephend@adiengineering.com>
Tested-by: Dan Priamo <danp@adiengineering.com>
Acked-by: Neil Horman <nhorman@tuxdriver.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>
Cc: stable@kernel.org

drivers/i2c/busses/i2c-ismt.c

@@ -341,8 +341,10 @@ static int ismt_process_desc(const struct ismt_desc *desc,
 			break;
 		case I2C_SMBUS_BLOCK_DATA:
 		case I2C_SMBUS_I2C_BLOCK_DATA:
+			if (desc->rxbytes != dma_buffer[0] + 1)
+				return -EMSGSIZE;
+
 			memcpy(data->block, dma_buffer, desc->rxbytes);
-			data->block[0] = desc->rxbytes - 1;
 			break;
 		}
 		return 0;