netfilter: Fix potential null pointer dereference

It's possible for nf_hook_entry_head to return NULL.  If two
nf_unregister_net_hook calls happen simultaneously with a single hook
entry in the list, both will enter the nf_hook_mutex critical section.
The first will successfully delete the head, but the second will see
this NULL pointer and attempt to dereference.

This fix ensures that no null pointer dereference could occur when such
a condition happens.

Fixes: e3b37f11e6e4 ("netfilter: replace list_head with single linked list")
Signed-off-by: Aaron Conole <aconole@bytheb.org>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

diff --git a/net/netfilter/core.c b/net/netfilter/core.c
index fa6715db4581087c360e77cdbe9d5a497c5bce38..e3f68a786afe537b9cc0818d991f014358fc1668 100644 (file)
--- a/net/netfilter/core.c
+++ b/net/netfilter/core.c
@@ -160,7 +160,7 @@ void nf_unregister_net_hook(struct net *net, const struct nf_hook_ops *reg)
 
        mutex_lock(&nf_hook_mutex);
        hooks_entry = nf_hook_entry_head(net, reg);
-       if (hooks_entry->orig_ops == reg) {
+       if (hooks_entry && hooks_entry->orig_ops == reg) {
                nf_set_hooks_head(net, reg,
                                  nf_entry_dereference(hooks_entry->next));
                goto unlock;