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

[Flavio Leitner <fbl@redhat.com>]

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@redhat.com>
> > Acked-by: Florian Westphal <fw@strlen.de>
> > Signed-off-by: David S. Miller <davem@davemloft.net>
> > ---
> ...
> > --- 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?

>From a5f927e7f1368d753f87cb978d630d786d5adb62 Mon Sep 17 00:00:00 2001
From: Flavio Leitner <fbl@redhat.com>
Date: Thu, 27 Sep 2018 19:36:28 -0300
Subject: [PATCH] xt_socket: check sk before checking for netns.

Only check for the network namespace if the socket is available.

Fixes: f564650106a6 ("netfilter: check if the socket netns is correct.")
Reported-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Flavio Leitner <fbl@redhat.com>
---
 net/netfilter/xt_socket.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/net/netfilter/xt_socket.c b/net/netfilter/xt_socket.c
index 0472f3472842..ada144e5645b 100644
--- a/net/netfilter/xt_socket.c
+++ b/net/netfilter/xt_socket.c
@@ -56,7 +56,7 @@ 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)))
+	if (sk && !net_eq(xt_net(par), sock_net(sk)))
 		sk = NULL;
 
 	if (!sk)
@@ -117,7 +117,7 @@ socket_mt6_v1_v2_v3(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)))
+	if (sk && !net_eq(xt_net(par), sock_net(sk)))
 		sk = NULL;
 
 	if (!sk)
-- 
2.14.4