[NETFILTER]: nf_conntrack: fix helper module unload races

When a helper module is unloaded all conntracks refering to it have their
helper pointer NULLed out, leading to lots of races. In most places this
can be fixed by proper use of RCU (they do already check for != NULL,
but in a racy way), additionally nf_conntrack_expect_related needs to
bail out when no helper is present.

Also remove two paranoid BUG_ONs in nf_conntrack_proto_gre that are racy
and not worth fixing.

Signed-off-by: Patrick McHarrdy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 8 insertions, 5 deletions

diff --git a/net/ipv4/netfilter/nf_conntrack_l3proto_ipv4.c b/net/ipv4/netfilter/nf_conntrack_l3proto_ipv4.c
index fd62a41d69c..6dc72a815f7 100644
--- a/net/ipv4/netfilter/nf_conntrack_l3proto_ipv4.c
+++ b/net/ipv4/netfilter/nf_conntrack_l3proto_ipv4.c
@@ -133,6 +133,7 @@ static unsigned int ipv4_conntrack_help(unsigned int hooknum,
         struct nf_conn *ct;
         enum ip_conntrack_info ctinfo;
         struct nf_conn_help *help;
+        struct nf_conntrack_helper *helper;
 
         /* This is where we call the helper: as the packet goes out. */
         ct = nf_ct_get(*pskb, &ctinfo);
@@ -140,12 +141,14 @@ static unsigned int ipv4_conntrack_help(unsigned int hooknum,
                 return NF_ACCEPT;
 
         help = nfct_help(ct);
-        if (!help || !help->helper)
+        if (!help)
                 return NF_ACCEPT;
-
-        return help->helper->help(pskb,
-                                  skb_network_offset(*pskb) + ip_hdrlen(*pskb),
-                                  ct, ctinfo);
+        /* rcu_read_lock()ed by nf_hook_slow */
+        helper = rcu_dereference(help->helper);
+        if (!helper)
+                return NF_ACCEPT;
+        return helper->help(pskb, skb_network_offset(*pskb) + ip_hdrlen(*pskb),
+                            ct, ctinfo);
 }
 
 static unsigned int ipv4_conntrack_defrag(unsigned int hooknum,