[PATCH 5.18 226/231] power: supply: core: Fix boundary conditions in interpolation

Greg Kroah-Hartman
2022-07-19T13:12:16Z

From: Dorian Rudolph <mail@dorianrudolph.com>

commit 093d27bb6f2d1963f927ef59c9a2d37059175426 upstream.

The functions power_supply_temp2resist_simple and power_supply_ocv2cap_simple
handle boundary conditions incorrectly.
The change was introduced in a4585ba2050f460f749bbaf2b67bd56c41e30283
("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: a4585ba2050f ("power: supply: core: Use library interpolation")
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
drivers/power/supply/power_supply_core.c | 24 ++++++++++++------------
1 file changed, 12 insertions(+), 12 deletions(-)

--- a/drivers/power/supply/power_supply_core.c
+++ b/drivers/power/supply/power_supply_core.c
@@ -846,17 +846,17 @@ int power_supply_temp2resist_simple(stru
{
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 p
{
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,