firmware: xilinx: fix debugfs write handler

- Userspace wants to write a string with `len` bytes, not counting the
   terminating NULL, so we should allocate `len+1` bytes. It looks like the
   current code relied on having a nullbyte directly behind `kern_buff`,
   which happens to work reliably as long as `len` isn't one of the kmalloc
   size classes.
 - strncpy_from_user() is completely wrong here; userspace is giving us a
   (not necessarily null-terminated) buffer and its length.
   strncpy_from_user() is for cases in which we don't know the length.
 - Don't let broken userspace allocate arbitrarily big kmalloc allocations.

Just use memdup_user_nul(), which is designed precisely for things like
this.

Signed-off-by: Jann Horn <jannh@google.com>
Acked-by: Jolly Shah <jolly.shah@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>

drivers/firmware/xilinx/zynqmp-debug.c

@@ -163,21 +163,14 @@ static ssize_t zynqmp_pm_debugfs_api_write(struct file *file,
 
 	strcpy(debugfs_buf, "");
 
-	if (*off != 0 || len == 0)
+	if (*off != 0 || len <= 1 || len > PAGE_SIZE - 1)
 		return -EINVAL;
 
-	kern_buff = kzalloc(len, GFP_KERNEL);
-	if (!kern_buff)
-		return -ENOMEM;
-
+	kern_buff = memdup_user_nul(ptr, len);
+	if (IS_ERR(kern_buff))
+		return PTR_ERR(kern_buff);
 	tmp_buff = kern_buff;
 
-	ret = strncpy_from_user(kern_buff, ptr, len);
-	if (ret < 0) {
-		ret = -EFAULT;
-		goto err;
-	}
-
 	/* Read the API name from a user request */
 	pm_api_req = strsep(&kern_buff, " ");