To generate the patch for the CWE-401 (Memory Leak) vulnerability at line 15, we need to follow a structured approach to identify the root cause and then apply an appropriate fix. Here are the detailed reasoning steps:

### Step 1: Root Cause Analysis

1. **Understanding the Code Context:**
   - The function `pci_acpi_scan_root` is responsible for scanning and setting up a PCI root.
   - It allocates memory for `ri` and `root_ops` using `kzalloc_node`.

2. **Memory Allocation:**
   - At line 7, memory is allocated for `ri` using `kzalloc_node`.
   - At line 12, memory is allocated for `root_ops` using `kzalloc_node`.

3. **Error Handling:**
   - If the allocation for `ri` fails (checked at line 8), the function returns `NULL` at line 10.
   - If the allocation for `root_ops` fails (checked at line 13), the function returns `NULL` at line 15.

4. **Memory Leak Identification:**
   - If the allocation for `root_ops` fails, the function returns `NULL` at line 15.
   - However, at this point, memory has already been allocated for `ri` at line 7.
   - The allocated memory for `ri` is not freed before returning `NULL`, leading to a memory leak.

### Step 2: Fixing Strategy

To fix the memory leak, we need to ensure that any allocated memory is properly freed before the function returns, especially in error handling paths.

1. **Freeing Allocated Memory:**
   - Before returning `NULL` at line 15, we should free the memory allocated for `ri`.

2. **Applying the Fix:**
   - Add a call to `kfree(ri)` before returning `NULL` at line 15.

### Conclusion

The patch should ensure that the memory allocated for `ri` is freed if the allocation for `root_ops` fails. This prevents the memory leak.

### Patch Implementation

The patch modifies the error handling at line 15 to free the allocated memory for `ri` before returning `NULL`.

```diff
<     {
<         return NULL;
---
>     {
>         kfree(ri);
>         return NULL;
```

### Final Code with Patch

```c
1 struct pci_bus *pci_acpi_scan_root(struct acpi_pci_root *root)
2 {
3     int node = acpi_get_node(root->device->handle);
4     struct acpi_pci_generic_root_info *ri;
5     struct pci_bus *bus, *child;
6     struct acpi_pci_root_ops *root_ops;
7     ri = kzalloc_node(sizeof(*ri), GFP_KERNEL, node);
8     if (!ri)
9     {
10         return NULL;
11     }
12     root_ops = kzalloc_node(sizeof(*root_ops), GFP_KERNEL, node);
13     if (!root_ops)
14     {
15         kfree(ri);  // Free the allocated memory for ri
16         return NULL;
17     }
18     ri->cfg = pci_acpi_setup_ecam_mapping(root);
19     if (!ri->cfg)
20     {
21         kfree(ri);
22         kfree(root_ops);
23         return NULL;
24     }
25     root_ops->release_info = pci_acpi_generic_release_info;
26     root_ops->prepare_resources = pci_acpi_root_prepare_resources;
27     root_ops->pci_ops = &ri->cfg->ops->pci_ops;
28     bus = acpi_pci_root_create(root, root_ops, &ri->common, ri->cfg);
29     if (!bus)
30     {
31         return NULL;
32     }
33     pci_bus_size_bridges(bus);
34     pci_bus_assign_resources(bus);
35     list_for_each_entry(, , ) pcie_bus_configure_settings(child);
36     return bus;
}
```

By following these reasoning steps, we ensure that the memory leak is properly addressed, and the function handles memory allocation failures gracefully.