[PATCH v5] drivers/iio: Remove all strcpy() uses

[Len Baker <len.baker@gmx.com>]

strcpy() performs no bounds checking on the destination buffer. This
could result in linear overflows beyond the end of the buffer, leading
to all kinds of misbehaviors. So, remove all the uses and add
devm_kstrdup() or devm_kasprintf() instead.

Also, modify the "for" loop conditions to clarify the access to the
st->orientation.rotation buffer.

This patch is an effort to clean up the proliferation of str*()
functions in the kernel and a previous step in the path to remove
the strcpy function from the kernel entirely [1].

[1] https://github.com/KSPP/linux/issues/88

Signed-off-by: Len Baker <len.baker@gmx.com>
---
The previous versions can be found in:

v1
https://lore.kernel.org/linux-hardening/20210801171157.17858-1-len.baker@gmx.com/

v2
https://lore.kernel.org/linux-hardening/20210807152225.9403-1-len.baker@gmx.com/

v3
https://lore.kernel.org/linux-hardening/20210814090618.8920-1-len.baker@gmx.com/

v4
https://lore.kernel.org/linux-hardening/20210814135509.4500-1-len.baker@gmx.com/

Changelog v1 -> v2
- Modify the commit changelog to inform that the motivation of this
  patch is to remove the strcpy() function from the kernel entirely
  (Jonathan Cameron).

Changelog v2 -> v3
- Rewrite the code using devm_kstrdup() and devm_kasprintf() functions
  (Andy Shevchenko).
- Rebase against v5.14-rc5.

Changelog v3 -> v4
- Reorder the variables (Andy Shevchenko).
- Get the device in the definition block (Andy Shevchenko).
- Reword the comment (Andy Shevchenko).
- Change the conditions in the "if" statement to clarify the "0" check
  (Andy Shevchenko).

Changelog v4 -> v5
- Use the strcmp() function to clarify the "0" check (Joe Perches).
- Modify the "for" loop conditions to clarify the access to the
  st->orientation.rotation buffer (Joe Perches).

 drivers/iio/imu/inv_mpu6050/inv_mpu_magn.c | 36 +++++++++++++---------
 1 file changed, 21 insertions(+), 15 deletions(-)

diff --git a/drivers/iio/imu/inv_mpu6050/inv_mpu_magn.c b/drivers/iio/imu/inv_mpu6050/inv_mpu_magn.c
index f282e9cc34c5..6aee6c989485 100644
--- a/drivers/iio/imu/inv_mpu6050/inv_mpu_magn.c
+++ b/drivers/iio/imu/inv_mpu6050/inv_mpu_magn.c
@@ -261,6 +261,7 @@ int inv_mpu_magn_set_rate(const struct inv_mpu6050_state *st, int fifo_rate)
  */
 int inv_mpu_magn_set_orient(struct inv_mpu6050_state *st)
 {
+	struct device *dev = regmap_get_device(st->map);
 	const char *orient;
 	char *str;
 	int i;
@@ -279,22 +280,27 @@ int inv_mpu_magn_set_orient(struct inv_mpu6050_state *st)
 		st->magn_orient.rotation[4] = st->orientation.rotation[1];
 		st->magn_orient.rotation[5] = st->orientation.rotation[2];
 		/* z <- -z */
-		for (i = 0; i < 3; ++i) {
-			orient = st->orientation.rotation[6 + i];
-			/* use length + 2 for adding minus sign if needed */
-			str = devm_kzalloc(regmap_get_device(st->map),
-					   strlen(orient) + 2, GFP_KERNEL);
-			if (str == NULL)
+		for (i = 6; i < 9; ++i) {
+			orient = st->orientation.rotation[i];
+
+			/*
+			 * The value is negated according to one of the following
+			 * rules:
+			 *
+			 * 1) Drop leading minus.
+			 * 2) Leave 0 as is.
+			 * 3) Add leading minus.
+			 */
+			if (orient[0] == '-')
+				str = devm_kstrdup(dev, orient + 1, GFP_KERNEL);
+			else if (!strcmp(orient, "0"))
+				str = devm_kstrdup(dev, orient, GFP_KERNEL);
+			else
+				str = devm_kasprintf(dev, GFP_KERNEL, "-%s", orient);
+			if (!str)
 				return -ENOMEM;
-			if (strcmp(orient, "0") == 0) {
-				strcpy(str, orient);
-			} else if (orient[0] == '-') {
-				strcpy(str, &orient[1]);
-			} else {
-				str[0] = '-';
-				strcpy(&str[1], orient);
-			}
-			st->magn_orient.rotation[6 + i] = str;
+
+			st->magn_orient.rotation[i] = str;
 		}
 		break;
 	default:
--
2.25.1