Re: [PATCH] netfilter: check if the socket netns is correct.

[Flavio Leitner]

On Thu, Sep 27, 2018 at 01:46:29PM -0700, Guenter Roeck wrote:
> Hi Flavio,
> 
> On Wed, Jun 27, 2018 at 10:34:25AM -0300, Flavio Leitner wrote:
> > Netfilter assumes that if the socket is present in the skb, then
> > it can be used because that reference is cleaned up while the skb
> > is crossing netns.
> > 
> > We want to change that to preserve the socket reference in a future
> > patch, so this is a preparation updating netfilter to check if the
> > socket netns matches before use it.
> > 
> > Signed-off-by: Flavio Leitner <fbl@xxxxxxxxxx>
> > Acked-by: Florian Westphal <fw@xxxxxxxxx>
> > Signed-off-by: David S. Miller <davem@xxxxxxxxxxxxx>
> > ---
> ...
> > --- a/net/netfilter/xt_socket.c
> > +++ b/net/netfilter/xt_socket.c
> > @@ -56,8 +56,12 @@ socket_match(const struct sk_buff *skb, struct xt_action_param *par,
> >  	struct sk_buff *pskb = (struct sk_buff *)skb;
> >  	struct sock *sk = skb->sk;
> >  
> > +	if (!net_eq(xt_net(par), sock_net(sk)))
> > +		sk = NULL;
> > +
> 
> I am having trouble with this code. With CONFIG_NET_NS enabled, it crashes
> for me in read_pnet() because sk is NULL.
> 
> >  	if (!sk)
> >  		sk = nf_sk_lookup_slow_v4(xt_net(par), skb, xt_in(par));
> 
> The old code seems to suggest that sk == NULL was possible.
> 
> I see the problem with the Chrome OS kernel rebased to v4.19-rc5, so I
> can not guarantee that this really an upstream problem. The change seems
> odd, though. Are you sure that it is not (or, rather, no longer) necessary
> to check if sk == NULL before dereferencing it in sock_net() ?

Oops, it is necessary but if it's not and the netns doesn't match, we need
do the lookup. So, could you check if this fixes the problem for you?