power: supply: core: Fix boundary conditions in interpolation

The functions power_supply_temp2resist_simple and power_supply_ocv2cap_simple
handle boundary conditions incorrectly.
The change was introduced in a4585ba
("power: supply: core: Use library interpolation").
There are two issues: First, the lines "high = i - 1" and "high = i" in ocv2cap
have the wrong order compared to temp2resist. As a consequence, ocv2cap
sets high=-1 if ocv>table[0].ocv, which causes an out-of-bounds read.
Second, the logic of temp2resist is also not correct.
Consider the case table[] = {{20, 100}, {10, 80}, {0, 60}}.
For temp=5, we expect a resistance of 70% by interpolation.
However, temp2resist sets high=low=2 and returns 60.

Cc: stable@vger.kernel.org
Signed-off-by: Dorian Rudolph <mail@dorianrudolph.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Fixes: a4585ba ("power: supply: core: Use library interpolation")
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>

Author: DorianRudolph

drivers/power/supply/power_supply_core.c

@@ -846,17 +846,17 @@ int power_supply_temp2resist_simple(struct power_supply_resistance_temp_table *t
 {
 	int i, high, low;
 
-	/* Break loop at table_len - 1 because that is the highest index */
-	for (i = 0; i < table_len - 1; i++)
+	for (i = 0; i < table_len; i++)
 		if (temp > table[i].temp)
 			break;
 
 	/* The library function will deal with high == low */
-	if ((i == 0) || (i == (table_len - 1)))
-		high = i;
+	if (i == 0)
+		high = low = i;
+	else if (i == table_len)
+		high = low = i - 1;
 	else
-		high = i - 1;
-	low = i;
+		high = (low = i) - 1;
 
 	return fixp_linear_interpolate(table[low].temp,
 				       table[low].resistance,
@@ -958,17 +958,17 @@ int power_supply_ocv2cap_simple(struct power_supply_battery_ocv_table *table,
 {
 	int i, high, low;
 
-	/* Break loop at table_len - 1 because that is the highest index */
-	for (i = 0; i < table_len - 1; i++)
+	for (i = 0; i < table_len; i++)
 		if (ocv > table[i].ocv)
 			break;
 
 	/* The library function will deal with high == low */
-	if ((i == 0) || (i == (table_len - 1)))
-		high = i - 1;
+	if (i == 0)
+		high = low = i;
+	else if (i == table_len)
+		high = low = i - 1;
 	else
-		high = i; /* i.e. i == 0 */
-	low = i;
+		high = (low = i) - 1;
 
 	return fixp_linear_interpolate(table[low].ocv,
 				       table[low].capacity,