[PATCH 1/5] netfilter: Fix potential null pointer dereference

[Pablo Neira Ayuso <pablo@netfilter.org>]

From: Aaron Conole <aconole@bytheb.org>

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>
---
 net/netfilter/core.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/netfilter/core.c b/net/netfilter/core.c
index fa6715db4581..e3f68a786afe 100644
--- 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;
-- 
2.1.4