ACPI: custom_method: fix a possible memory leak

In cm_write(), if the 'buf' is allocated memory but not fully consumed,
it is possible to reallocate the buffer without freeing it by passing
'*ppos' as 0 on a subsequent call.

Add an explicit kfree() before kzalloc() to prevent the possible memory
leak.

Fixes: 526b4af47f44 ("ACPI: Split out custom_method functionality into an own driver")
Signed-off-by: Mark Langsdorf <[e-mail]>
Cc: 5.4+ <[e-mail]> # 5.4+
Signed-off-by: Rafael J. Wysocki <[e-mail]>

drivers/acpi/custom_method.c

···
 				   sizeof(struct acpi_table_header)))
 			return -EFAULT;
 		uncopied_bytes = max_size = table.length;
+		/* make sure the buf is not allocated */
+		kfree(buf);
 		buf = kzalloc(max_size, GFP_KERNEL);
 		if (!buf)
 			return -ENOMEM;