hwmon: (pmbus/adm1266) bounce blackbox records through a protocol-sized buffer

adm1266_pmbus_block_xfer() copies the device-supplied block payload
into the caller-provided buffer using the device-supplied length:

	memcpy(data_r, &msgs[1].buf[1], msgs[1].buf[0]);

The helper does not know how large data_r is and trusts the device to
return at most one record's worth of bytes.  adm1266_nvmem_read_blackbox()
violates that contract: it advances read_buff inside data->dev_mem in
ADM1266_BLACKBOX_SIZE (64-byte) strides while the helper is willing to
write up to ADM1266_PMBUS_BLOCK_MAX (255) bytes.  A device that returns
more than 64 bytes on the trailing record (read_buff offset 1984 in
the 2048-byte dev_mem allocation) overflows dev_mem by up to 191 bytes
before the post-call

	if (ret != ADM1266_BLACKBOX_SIZE)
		return -EIO;

can reject the response.

Contain the fix in the caller without changing the helper signature:
read each record into a 255-byte local bounce buffer that matches the
helper's maximum output, validate the returned length, and only then
copy exactly ADM1266_BLACKBOX_SIZE bytes into the dev_mem slot.

Fixes: 407dc80 ("hwmon: (pmbus/adm1266) Add Block process call")
Cc: stable@vger.kernel.org
Signed-off-by: Abdurrahman Hussain <abdurrahman@nexthop.ai>
Link: https://lore.kernel.org/r/20260515-adm1266-fixes-v1-5-1c1ea1349cfe@nexthop.ai
Signed-off-by: Guenter Roeck <linux@roeck-us.net>

Author: abdurrahman-nexthop

drivers/hwmon/pmbus/adm1266.c

@@ -349,6 +349,7 @@ static void adm1266_init_debugfs(struct adm1266_data *data)
 
 static int adm1266_nvmem_read_blackbox(struct adm1266_data *data, u8 *read_buff)
 {
+	u8 record[ADM1266_PMBUS_BLOCK_MAX];
 	int record_count;
 	char index;
 	u8 buf[I2C_SMBUS_BLOCK_MAX];
@@ -366,13 +367,14 @@ static int adm1266_nvmem_read_blackbox(struct adm1266_data *data, u8 *read_buff)
 		return -EIO;
 
 	for (index = 0; index < record_count; index++) {
-		ret = adm1266_pmbus_block_xfer(data, ADM1266_READ_BLACKBOX, 1, &index, read_buff);
+		ret = adm1266_pmbus_block_xfer(data, ADM1266_READ_BLACKBOX, 1, &index, record);
 		if (ret < 0)
 			return ret;
 
 		if (ret != ADM1266_BLACKBOX_SIZE)
 			return -EIO;
 
+		memcpy(read_buff, record, ADM1266_BLACKBOX_SIZE);
 		read_buff += ADM1266_BLACKBOX_SIZE;
 	}