cgroup dentry insufficiently initialized prior to calling d_instantiate.

[Casey Schaufler <casey-iSGtlc1asvQWG2LlvL+J4A@public.gmane.org>]

In kernel/cgroup.c in cgroup_add_file() we have:


        dentry = lookup_one_len(name, dir, strlen(name));
        if (IS_ERR(dentry)) {
                error = PTR_ERR(dentry);
                goto out;
        }

        mode = cgroup_file_mode(cft);
        error = cgroup_create_file(dentry, mode | S_IFREG, cgrp->root->sb);
        if (!error) {
                cfe->type = (void *)cft;
                cfe->dentry = dentry;
                dentry->d_fsdata = cfe;
                simple_xattrs_init(&cfe->xattrs);
                list_add_tail(&cfe->node, &parent->files);
                cfe = NULL;
        }
        dput(dentry);

cgroup_create_file() calls d_instantiate, which may
decide to look at the xattrs on the file. Smack always
does this and SELinux can be configured to do so, although
no one seems to be using that option. Since the dentry
has not been initialized panics in __d_xattr ensue. See
bugzilla 57791.