[PATCH v2 2/2] iio: proximity: sx9360: reject out-of-range hardware gain values

[Arnav Kapoor <kapoorarnav43@gmail.com>]

sx9360_write_gain() passes the value from userspace to ilog2() and then
FIELD_PREP() into the 3-bit PROX_CTRL0 gain field without checking it,
so some values silently program a gain unrelated to the one requested,
and the write still reports success:

  - 16 to 255 write gain codes 4 to 7, which the driver does not
    advertise, and the next read reports gains of 16 to 128;
  - 256 and larger wrap around in the field, e.g. 256 sets a gain of 1;
  - 0 and negative values write code 7 (read back as 128), because
    ilog2(0) is -1 and a negative int becomes a large unsigned value.

Return -EINVAL for values <= 0 and for values above the gain of 8 code.
FIELD_FIT() is not enough here, as the field also holds codes 4 to 7,
which the driver doesn't support. Values between the supported gains
of 1, 2, 4 and 8 are still rounded down, as before.

Fixes: 1cdb4c47f7f5 ("iio:proximity:sx9360: Add sx9360 support")
Signed-off-by: Arnav Kapoor <kapoorarnav43@gmail.com>
---
 drivers/iio/proximity/sx9360.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/drivers/iio/proximity/sx9360.c b/drivers/iio/proximity/sx9360.c
index 2a9b37df8f62..ede20dfcdd80 100644
--- a/drivers/iio/proximity/sx9360.c
+++ b/drivers/iio/proximity/sx9360.c
@@ -68,6 +68,7 @@
 #define SX9360_REG_PROX_CTRL0_PHM	0x41
 #define SX9360_REG_PROX_CTRL0_GAIN_MASK	GENMASK(5, 3)
 #define SX9360_REG_PROX_CTRL0_GAIN_1		0x80
+#define SX9360_REG_PROX_CTRL0_GAIN_8		0x3
 #define SX9360_REG_PROX_CTRL0_RAWFILT_MASK	GENMASK(2, 0)
 #define SX9360_REG_PROX_CTRL0_RAWFILT_1P50	0x01
 #define SX9360_REG_PROX_CTRL1		0x42
@@ -625,7 +626,13 @@ static int sx9360_write_gain(struct sx_common_data *data,
 {
 	unsigned int gain, reg;
 
+	if (val <= 0)
+		return -EINVAL;
+
 	gain = ilog2(val);
+	if (gain > SX9360_REG_PROX_CTRL0_GAIN_8)
+		return -EINVAL;
+
 	reg = SX9360_REG_PROX_CTRL0_PHR + chan->channel;
 	gain = FIELD_PREP(SX9360_REG_PROX_CTRL0_GAIN_MASK, gain);
 
-- 
2.53.0