Subject: Re: [(RFC) PATCH ] NULL pointer dereference on rmmod iptable_mangle.

From: Florian Westphal <[email-masked]>
Date: Thu, 4 Jun 2020 00:05:02 +0200

David Wilder <[email-masked]> wrote:
> This crash happened on a ppc64le system running ltp network tests when ltp script ran "rmmod iptable_mangle".
> 
> [213425.602369] BUG: Kernel NULL pointer dereference at 0x00000010
> [213425.602388] Faulting instruction address: 0xc008000000550bdc
[..]

> In the crash we find in iptable_mangle_hook() that state->net->ipv4.iptable_mangle=NULL causing a NULL pointer dereference. net->ipv4.iptable_mangle is set to NULL in iptable_mangle_net_exit() and called when ip_mangle modules is unloaded. A rmmod task was found in the crash dump.  A 2nd crash showed the same problem when running "rmmod iptable_filter" (net->ipv4.iptable_filter=NULL).
> 
> Once a hook is registered packets will picked up a pointer from: net->ipv4.iptable_$table. The patch adds a call to synchronize_net() in ipt_unregister_table() to insure no packets are in flight that have picked up the pointer before completing the un-register.
> 
> This change has has prevented the problem in our testing.  However, we have concerns with this change as it would mean that on netns cleanup, we would need one synchronize_net() call for every table in use. Also, on module unload, there would be one synchronize_net() for every existing netns.

Yes, I agree with the analysis.

> Signed-off-by: David Wilder <[email-masked]>
> ---
>  net/ipv4/netfilter/ip_tables.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/net/ipv4/netfilter/ip_tables.c b/net/ipv4/netfilter/ip_tables.c
> index c2670ea..97c4121 100644
> --- a/net/ipv4/netfilter/ip_tables.c
> +++ b/net/ipv4/netfilter/ip_tables.c
> @@ -1800,8 +1800,10 @@ int ipt_register_table(struct net *net, const struct xt_table *table,
>  void ipt_unregister_table(struct net *net, struct xt_table *table,
>  			  const struct nf_hook_ops *ops)
>  {
> -	if (ops)
> +	if (ops) {
>  		nf_unregister_net_hooks(net, ops, hweight32(table->valid_hooks));
> +		synchronize_net();
> +	}

I'd wager ebtables, arptables and ip6tables have the same bug.

The extra synchronize_net() isn't ideal.  We could probably do it this
way and then improve in a second patch.

One way to fix this without a new synchronize_net() is to switch all
iptable_foo.c to use ".pre_exit" hook as well.

pre_exit would unregister the underlying hook and .exit would to the
table freeing.

Since the netns core already does an unconditional synchronize_rcu after
the pre_exit hooks this would avoid the problem as well.