Re: [PATCH net 6/6] net/sched: act_ct: fix helper UAF due to extensions realloc

While calling the helpers, a raw pointer to the extensions area is
wired into expectations list:

  -> nf_ct_helper()
   -> helper->help()
    -> nf_ct_expect_related_report()
     -> nf_ct_expect_insert()
      -> hlist_add_head_rcu(&exp->lnode, &master_help->expectations)

In case the connection is not confirmed yet, more extensions can be
added afterwards with *_ext_add() calls reallocating the extension
space and leaving the now invalid pointer in the expectations list
that is later accessed while removing the expectation.

Make sure that helpers are called at the end after all the other
extensions are already added.

Note that the helper rejection now leaves the mark and labels set,
but that's not different from how the NAT was handled before or how
the mark and the labels were handled on confirmation failure.  And
there are no atomicity guarantees provided by the API anyway.

Fixes: a21b06e73191 ("net: sched: add helper support in act_ct")
Cc: stable@vger.kernel.org
Reported-by: Axel Mierczuk <redacted>
Signed-off-by: Ilya Maximets <i.maximets@ovn.org>
---
 net/sched/act_ct.c | 18 ++++++++++++------
 1 file changed, 12 insertions(+), 6 deletions(-)

diff --git a/net/sched/act_ct.c b/net/sched/act_ct.c
index f62051ec9d57d..411e3dd92d072 100644
--- a/net/sched/act_ct.c
+++ b/net/sched/act_ct.c

@@ -1102,19 +1102,25 @@ TC_INDIRECT_SCOPE int tcf_ct_act(struct sk_buff *skb, const struct tc_action *a,
                }
        }

-       if (nf_ct_is_confirmed(ct) ? (!cached && !skip_add) : commit) {
-               err = nf_ct_helper(skb, ct, ctinfo, family);
-               if (err != NF_ACCEPT)
-                       goto nf_error;
-       }
-
        if (commit) {
                tcf_ct_act_set_mark(ct, p->mark, p->mark_mask);
                tcf_ct_act_set_labels(ct, p->labels, p->labels_mask);

                if (!nf_ct_is_confirmed(ct))
                        nf_conn_act_ct_ext_add(skb, ct, ctinfo);
+       }

+       /* Run helpers for the connection if nf_conntrack_in() was executed
+        * or if we're about to commit.  This has to be done after all the
+        * extensions are already added.
+        */
+       if (nf_ct_is_confirmed(ct) ? (!cached && !skip_add) : commit) {
+               err = nf_ct_helper(skb, ct, ctinfo, family);
+               if (err != NF_ACCEPT)
+                       goto nf_error;
+       }
+
+       if (commit) {
                /* This will take care of sending queued events
                 * even if the connection is already confirmed.
                 */
--

2.55.0