hwmon: (pmbus/q54sj108a2) fix stack overflow in debugfs read

The q54sj108a2_debugfs_read function suffers from a stack buffer overflow
due to incorrect arguments passed to bin2hex(). The function currently
passes 'data' as the destination and 'data_char' as the source.

Because bin2hex() converts each input byte into two hex characters, a
32-byte block read results in 64 bytes of output. Since 'data' is only
34 bytes (I2C_SMBUS_BLOCK_MAX + 2), this writes 30 bytes past the end
of the buffer onto the stack.

Additionally, the arguments were swapped: it was reading from the
zero-initialized 'data_char' and writing to 'data', resulting in
all-zero output regardless of the actual I2C read.

Fix this by:
1. Expanding 'data_char' to 66 bytes to safely hold the hex output.
2. Correcting the bin2hex() argument order and using the actual read count.
3. Using a pointer to select the correct output buffer for the final
   simple_read_from_buffer call.

Fixes: d014538 ("hwmon: (pmbus) Driver for Delta power supplies Q54SJ108A2")
Cc: stable@vger.kernel.org
Signed-off-by: Sanman Pradhan <psanman@juniper.net>
Link: https://lore.kernel.org/r/20260304235116.1045-1-sanman.p211993@gmail.com
Signed-off-by: Guenter Roeck <linux@roeck-us.net>

Author: Sanman Pradhan

drivers/hwmon/pmbus/q54sj108a2.c

@@ -79,7 +79,8 @@ static ssize_t q54sj108a2_debugfs_read(struct file *file, char __user *buf,
 	int idx = *idxp;
 	struct q54sj108a2_data *psu = to_psu(idxp, idx);
 	char data[I2C_SMBUS_BLOCK_MAX + 2] = { 0 };
-	char data_char[I2C_SMBUS_BLOCK_MAX + 2] = { 0 };
+	char data_char[I2C_SMBUS_BLOCK_MAX * 2 + 2] = { 0 };
+	char *out = data;
 	char *res;
 
 	switch (idx) {
@@ -150,27 +151,27 @@ static ssize_t q54sj108a2_debugfs_read(struct file *file, char __user *buf,
 		if (rc < 0)
 			return rc;
 
-		res = bin2hex(data, data_char, 32);
-		rc = res - data;
-
+		res = bin2hex(data_char, data, rc);
+		rc = res - data_char;
+		out = data_char;
 		break;
 	case Q54SJ108A2_DEBUGFS_FLASH_KEY:
 		rc = i2c_smbus_read_block_data(psu->client, PMBUS_FLASH_KEY_WRITE, data);
 		if (rc < 0)
 			return rc;
 
-		res = bin2hex(data, data_char, 4);
-		rc = res - data;
-
+		res = bin2hex(data_char, data, rc);
+		rc = res - data_char;
+		out = data_char;
 		break;
 	default:
 		return -EINVAL;
 	}
 
-	data[rc] = '\n';
+	out[rc] = '\n';
 	rc += 2;
 
-	return simple_read_from_buffer(buf, count, ppos, data, rc);
+	return simple_read_from_buffer(buf, count, ppos, out, rc);
 }
 
 static ssize_t q54sj108a2_debugfs_write(struct file *file, const char __user *buf,