fs: kernfs: Fix possible null-pointer dereferences in kernfs_path_fro…

…m_node_locked()

In kernfs_path_from_node_locked(), there is an if statement on line 147
to check whether buf is NULL:
    if (buf)

When buf is NULL, it is used on line 151:
    len += strlcpy(buf + len, parent_str, ...)
and line 158:
    len += strlcpy(buf + len, "/", ...)
and line 160:
    len += strlcpy(buf + len, kn->name, ...)

Thus, possible null-pointer dereferences may occur.

To fix these possible bugs, buf is checked before being used.
If it is NULL, -EINVAL is returned.

These bugs are found by a static analysis tool STCheck written by us.

Signed-off-by: Jia-Ju Bai <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Greg Kroah-Hartman <[email protected]>

fs/kernfs/dir.c

@@ -137,15 +137,17 @@ static int kernfs_path_from_node_locked(struct kernfs_node *kn_to,
  if (kn_from == kn_to)
  return strlcpy(buf, "/", buflen);
 
+ if (!buf)
+ return -EINVAL;
+
  common = kernfs_common_ancestor(kn_from, kn_to);
  if (WARN_ON(!common))
  return -EINVAL;
 
  depth_to = kernfs_depth(common, kn_to);
  depth_from = kernfs_depth(common, kn_from);
 
- if (buf)
- buf[0] = '\0';
+ buf[0] = '\0';
 
  for (i = 0; i < depth_from; i++)
  len += strlcpy(buf + len, parent_str,