netfilter: ctnetlink: make it safer when checking the ct helper name

One CPU is doing ctnetlink_change_helper(), while another CPU is doing
unhelp() at the same time. So even if help->helper is not NULL at first,
the later statement strcmp(help->helper->name, ...) may still access
the NULL pointer.

So we must use rcu_read_lock and rcu_dereference to avoid such _bad_
thing happen.

Fixes: f95d7a46bc57 ("netfilter: ctnetlink: Fix regression in CTA_HELP processing")
Signed-off-by: Liping Zhang <zlpnobody@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

diff --git a/net/netfilter/nf_conntrack_netlink.c b/net/netfilter/nf_conntrack_netlink.c
index 06d28ac663df4f4061fa380fde47a89efaea6529..f9c643bc1a8e2ff1de2c2775a2ef0bc2147c6cb3 100644 (file)
--- a/net/netfilter/nf_conntrack_netlink.c
+++ b/net/netfilter/nf_conntrack_netlink.c
@@ -1488,11 +1488,16 @@ static int ctnetlink_change_helper(struct nf_conn *ct,
                 * treat the second attempt as a no-op instead of returning
                 * an error.
                 */
-               if (help && help->helper &&
-                   !strcmp(help->helper->name, helpname))
-                       return 0;
-               else
-                       return -EBUSY;
+               err = -EBUSY;
+               if (help) {
+                       rcu_read_lock();
+                       helper = rcu_dereference(help->helper);
+                       if (helper && !strcmp(helper->name, helpname))
+                               err = 0;
+                       rcu_read_unlock();
+               }
+
+               return err;
        }
 
        if (!strcmp(helpname, "")) {