iio: magnetometer: rm3100: add boundary check for the value read from RM3100_REG_TMRC

Recently, we encounter kernel crash in function rm3100_common_probe
caused by out of bound access of array rm3100_samp_rates (because of
underlying hardware failures). Add boundary check to prevent out of
bound access.

Fixes: 121354b ("iio: magnetometer: Add driver support for PNI RM3100")
Suggested-by: Zhouyi Zhou <zhouzhouyi@gmail.com>
Signed-off-by: zhili.liu <zhili.liu@ucas.com.cn>
Link: https://lore.kernel.org/r/1704157631-3814-1-git-send-email-zhouzhouyi@gmail.com
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

Author: zhili.liu

drivers/iio/magnetometer/rm3100-core.c

@@ -530,6 +530,7 @@ int rm3100_common_probe(struct device *dev, struct regmap *regmap, int irq)
 	struct rm3100_data *data;
 	unsigned int tmp;
 	int ret;
+	int samp_rate_index;
 
 	indio_dev = devm_iio_device_alloc(dev, sizeof(*data));
 	if (!indio_dev)
@@ -586,9 +587,14 @@ int rm3100_common_probe(struct device *dev, struct regmap *regmap, int irq)
 	ret = regmap_read(regmap, RM3100_REG_TMRC, &tmp);
 	if (ret < 0)
 		return ret;
+
+	samp_rate_index = tmp - RM3100_TMRC_OFFSET;
+	if (samp_rate_index < 0 || samp_rate_index >=  RM3100_SAMP_NUM) {
+		dev_err(dev, "The value read from RM3100_REG_TMRC is invalid!\n");
+		return -EINVAL;
+	}
 	/* Initializing max wait time, which is double conversion time. */
-	data->conversion_time = rm3100_samp_rates[tmp - RM3100_TMRC_OFFSET][2]
-				* 2;
+	data->conversion_time = rm3100_samp_rates[samp_rate_index][2] * 2;
 
 	/* Cycle count values may not be what we want. */
 	if ((tmp - RM3100_TMRC_OFFSET) == 0)