[PATCH 1/3] hwmon: (ltc4282) Avoid overflow in maximum power calculation

[Guenter Roeck <linux@roeck-us.net>]

During device initialization in ltc4282_set_max_limits(), the calculation
of the maximum power limit can suffer from a 32-bit integer overflow.

static int ltc4282_set_max_limits(struct ltc4282_state *st)
{
    ...
    st->power_max = DIV_ROUND_CLOSEST(st->vsense_max * DECA * MILLI,
                                      st->rsense) * st->vfs_out;
    ...
}

The result of DIV_ROUND_CLOSEST() evaluates to a 32-bit unsigned integer
on 32-bit architectures. This result is then multiplied by st->vfs_out,
which is a 16-bit unsigned integer. According to C promotion rules, since
both operands are 32-bit or smaller, the multiplication is performed in
32-bit precision.

If the device is configured with a low sense resistor value via the device
tree (for example, 100 nano-ohms, resulting in st->rsense = 1) and the
voltage is high, the division result can reach 343,750,000 and st->vfs_out
can be 33,280. The product of these values is approximately 11.44 trillion,
which exceeds the maximum capacity of a 32-bit integer and overflows
before being stored in st->power_max.

This overflow causes a truncated value to be assigned to st->power_max and
written to the hardware limit register. An incorrect maximum power limit
can trigger spurious power-bad faults or alarms, which may lead to the
shutdown of the monitored power rail.

Avoid the problem by calculating and storing the maximum power using 64-bit
variables.

Reported-by: Sashiko <sashiko-bot@kernel.org>
Fixes: cbc29538dbf7d ("hwmon: Add driver for LTC4282")
Cc: Nuno Sa <nuno.sa@analog.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
---
 drivers/hwmon/ltc4282.c | 12 ++++++------
 1 file changed, 6 insertions(+), 6 deletions(-)

diff --git a/drivers/hwmon/ltc4282.c b/drivers/hwmon/ltc4282.c
index cc698803f8bf..bb7f6727c44d 100644
--- a/drivers/hwmon/ltc4282.c
+++ b/drivers/hwmon/ltc4282.c
@@ -137,7 +137,7 @@ struct ltc4282_state {
 	 */
 	struct ltc4282_cache in0_1_cache[LTC4282_CHAN_VGPIO];
 	u32 vsense_max;
-	long power_max;
+	s64 power_max;
 	u32 rsense;
 	u16 vdd;
 	u16 vfs_out;
@@ -613,13 +613,12 @@ static int ltc4282_read(struct device *dev, enum hwmon_sensor_types type,
 }
 
 static int ltc4282_write_power_byte(const struct ltc4282_state *st, u32 reg,
-				    long val)
+				    s64 val)
 {
 	u32 power;
 	u64 temp;
 
-	if (val > st->power_max)
-		val = st->power_max;
+	val = clamp(val, 0, st->power_max);
 
 	temp = val * int_pow(U8_MAX, 2) * st->rsense;
 	power = DIV64_U64_ROUND_CLOSEST(temp,
@@ -629,7 +628,7 @@ static int ltc4282_write_power_byte(const struct ltc4282_state *st, u32 reg,
 }
 
 static int ltc4282_write_power_word(const struct ltc4282_state *st, u32 reg,
-				    long val)
+				    u64 val)
 {
 	u64 temp = int_pow(U16_MAX, 2) * st->rsense, temp_2;
 	__be16 __raw;
@@ -1222,7 +1221,8 @@ static int ltc4282_set_max_limits(struct ltc4282_state *st)
 		return ret;
 
 	/* Power is given by ISENSE * Vout. */
-	st->power_max = DIV_ROUND_CLOSEST(st->vsense_max * DECA * MILLI, st->rsense) * st->vfs_out;
+	st->power_max = DIV_ROUND_CLOSEST_ULL((u64)st->vsense_max * DECA * MILLI,
+					      st->rsense) * st->vfs_out;
 	ret = ltc4282_write_power_byte(st, LTC4282_POWER_MAX, st->power_max);
 	if (ret)
 		return ret;
-- 
2.45.2