bpf: Pin the start cgroup in cgroup_iter_seq_init()

bpf_iter_attach_cgroup() has already acquired an extra reference for the
start cgroup, but the reference may be released if the iterator link fd
is closed after the creation of iterator fd, and it may lead to
user-after-free problem when reading the iterator fd.

An alternative fix is pinning iterator link when opening iterator,
but it will make iterator link being still visible after the close of
iterator link fd and the behavior is different with other link types, so
just fixing it by acquiring another reference for the start cgroup.

Fixes: d4ccaf58 ("bpf: Introduce cgroup iter")
Signed-off-by: Hou Tao <houtao1@huawei.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/20221121073440.1828292-2-houtao@huaweicloud.com

kernel/bpf/cgroup_iter.c

@@ -164,16 +164,30 @@ static int cgroup_iter_seq_init(void *priv, struct bpf_iter_aux_info *aux)
 	struct cgroup_iter_priv *p = (struct cgroup_iter_priv *)priv;
 	struct cgroup *cgrp = aux->cgroup.start;
 
+	/* bpf_iter_attach_cgroup() has already acquired an extra reference
+	 * for the start cgroup, but the reference may be released after
+	 * cgroup_iter_seq_init(), so acquire another reference for the
+	 * start cgroup.
+	 */
 	p->start_css = &cgrp->self;
+	css_get(p->start_css);
 	p->terminate = false;
 	p->visited_all = false;
 	p->order = aux->cgroup.order;
 	return 0;
 }
 
+static void cgroup_iter_seq_fini(void *priv)
+{
+	struct cgroup_iter_priv *p = (struct cgroup_iter_priv *)priv;
+
+	css_put(p->start_css);
+}
+
 static const struct bpf_iter_seq_info cgroup_iter_seq_info = {
 	.seq_ops		= &cgroup_iter_seq_ops,
 	.init_seq_private	= cgroup_iter_seq_init,
+	.fini_seq_private	= cgroup_iter_seq_fini,
 	.seq_priv_size		= sizeof(struct cgroup_iter_priv),
 };