fs: kernfs: Fix possible null-pointer dereferences in kernfs_path_from_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 <baijiaju1990@gmail.com>
Link: https://lore.kernel.org/r/20190724022242.27505-1-baijiaju1990@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/fs/kernfs/dir.c b/fs/kernfs/dir.c
index ddf537923a0aff320e48bc6e48c350833807dd91..6ebae6bbe6a5e6f127aab0318f3d06045bed837e 100644 (file)
--- 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,