netfilter: conntrack: re-fetch conntrack after insertion

commit 56b14ecec97f39118bf85c9ac2438c5a949509ed upstream.

In case the conntrack is clashing, insertion can free skb->_nfct and
set skb->_nfct to the already-confirmed entry.

This wasn't found before because the conntrack entry and the extension
space used to free'd after an rcu grace period, plus the race needs
events enabled to trigger.

Reported-by: <syzbot+793a590957d9c1b96620@syzkaller.appspotmail.com>
Fixes: 71d8c47fc653 ("netfilter: conntrack: introduce clash resolution on insertion race")
Fixes: 2ad9d7747c10 ("netfilter: conntrack: free extension area immediately")
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/include/net/netfilter/nf_conntrack_core.h b/include/net/netfilter/nf_conntrack_core.h
index 2a3e0974a6af4029ecba39cf4bb41d2c46d77282..4e3fff9f929b16676e731dff24162ba8c997b63f 100644 (file)
--- a/include/net/netfilter/nf_conntrack_core.h
+++ b/include/net/netfilter/nf_conntrack_core.h
@@ -58,8 +58,13 @@ static inline int nf_conntrack_confirm(struct sk_buff *skb)
        int ret = NF_ACCEPT;
 
        if (ct) {
-               if (!nf_ct_is_confirmed(ct))
+               if (!nf_ct_is_confirmed(ct)) {
                        ret = __nf_conntrack_confirm(skb);
+
+                       if (ret == NF_ACCEPT)
+                               ct = (struct nf_conn *)skb_nfct(skb);
+               }
+
                if (likely(ret == NF_ACCEPT))
                        nf_ct_deliver_cached_events(ct);
        }