[PATCH] fs: kernfs: Fix possible null-pointer dereferences in kernfs_path_from_node_locked()

[Jia-Ju Bai]

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 <baijiaju1...@gmail.com>
---
 fs/kernfs/dir.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/fs/kernfs/dir.c b/fs/kernfs/dir.c
index a387534c9577..9b01f6ce1d79 100644
--- a/fs/kernfs/dir.c
+++ b/fs/kernfs/dir.c
@@ -137,6 +137,9 @@ 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;
@@ -144,8 +147,7 @@ static int kernfs_path_from_node_locked(struct kernfs_node 
*kn_to,
        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,
-- 
2.17.0