hwmon: (pmbus/adm1266) reject short block-read responses in the GPIO accessors

adm1266_gpio_get() and adm1266_gpio_get_multiple() both compose the
pin-status word as

	pins_status = read_buf[0] + (read_buf[1] << 8);

right after i2c_smbus_read_block_data(), guarding only against an
error return.  A well-behaved device returns 2 bytes for
GPIO_STATUS/PDIO_STATUS, but the helper happily reports a 0- or
1-byte response too.  If the device returns 0 bytes, both read_buf
slots are uninitialized stack memory; if it returns 1 byte, read_buf[1]
is.

The composed value then flows through set_bit() into the caller's
*bits in adm1266_gpio_get_multiple(), or into the return value of
adm1266_gpio_get(), and ends up in userspace via gpiolib (sysfs and
the char-dev ioctls).  That leaks a few bits of kernel stack per
request on any device whose firmware glitch, bus error, or hostile
slave produces a short block-read response.

Add the missing length check to both call sites and surface a short
response as -EIO.

Fixes: d98dfad ("hwmon: (pmbus/adm1266) Add support for GPIOs")
Cc: stable@vger.kernel.org
Signed-off-by: Abdurrahman Hussain <abdurrahman@nexthop.ai>
Reviewed-by: Bartosz Golaszewski <bartosz.golaszewski@oss.qualcomm.com>
Link: https://lore.kernel.org/r/20260518-adm1266-gpio-fixes-v3-3-e425e4f88139@nexthop.ai
Signed-off-by: Guenter Roeck <linux@roeck-us.net>

Author: abdurrahman-nexthop

drivers/hwmon/pmbus/adm1266.c

@@ -176,6 +176,8 @@ static int adm1266_gpio_get(struct gpio_chip *chip, unsigned int offset)
 	ret = i2c_smbus_read_block_data(data->client, pmbus_cmd, read_buf);
 	if (ret < 0)
 		return ret;
+	if (ret < 2)
+		return -EIO;
 
 	pins_status = read_buf[0] + (read_buf[1] << 8);
 	if (offset < ADM1266_GPIO_NR)
@@ -196,6 +198,8 @@ static int adm1266_gpio_get_multiple(struct gpio_chip *chip, unsigned long *mask
 	ret = i2c_smbus_read_block_data(data->client, ADM1266_GPIO_STATUS, read_buf);
 	if (ret < 0)
 		return ret;
+	if (ret < 2)
+		return -EIO;
 
 	status = read_buf[0] + (read_buf[1] << 8);
 
@@ -208,6 +212,8 @@ static int adm1266_gpio_get_multiple(struct gpio_chip *chip, unsigned long *mask
 	ret = i2c_smbus_read_block_data(data->client, ADM1266_PDIO_STATUS, read_buf);
 	if (ret < 0)
 		return ret;
+	if (ret < 2)
+		return -EIO;
 
 	status = read_buf[0] + (read_buf[1] << 8);