netdev - Re: Possible problem with e6afc8ac ("udp: remove headers from UDP packets before queueing")

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Date:	Wed, 01 Jun 2016 12:18:15 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Paul Moore <paul@...l-moore.com>
Cc:	samanthakumar@...gle.com, netdev@...r.kernel.org,
	linux-security-module@...r.kernel.org, selinux@...ho.nsa.gov
Subject: Re: Possible problem with e6afc8ac ("udp: remove headers from UDP
 packets before queueing")

On Wed, 2016-06-01 at 15:01 -0400, Paul Moore wrote:
> Hello,
> 
> I'm currently trying to debug a problem with 4.7-rc1 and labeled
> networking over UDP.  I'm having some difficulty with the latest
> 4.7-rc1 builds on my test system at the moment so I haven't been able
> to concisely identify the problem, but looking through the commits in
> 4.7-rc1 I think there may be a problem with the following:
> 
>   commit e6afc8ace6dd5cef5e812f26c72579da8806f5ac
>   Author: samanthakumar <samanthakumar@...gle.com>
>   Date:   Tue Apr 5 12:41:15 2016 -0400
> 
>    udp: remove headers from UDP packets before queueing
> 
>    Remove UDP transport headers before queueing packets for reception.
>    This change simplifies a follow-up patch to add MSG_PEEK support.
> 
>    Signed-off-by: Sam Kumar <samanthakumar@...gle.com>
>    Signed-off-by: Willem de Bruijn <willemb@...gle.com>
>    Signed-off-by: David S. Miller <davem@...emloft.net>
> 
> ... it appears that this commit changes things so that sk_filter() is
> only called when sk->sk_filter is not NULL.  While this is fine for
> the traditional socket filter case, it causes problems with LSMs that
> make use of security_sock_rcv_skb() to enforce per-packet access
> controls.
> 
> Hopefully I'll get 4.7-rc1 booting soon and I can do a proper
> bisection test around this patch, but I wanted to mention this now in
> case others are seeing the same problem.
> 

Thanks for the report. Please try following fix.

sk_filter() got additional features like the skb_pfmemalloc() things and
security_sock_rcv_skb()

diff --git a/net/ipv4/udp.c b/net/ipv4/udp.c
index d56c0559b477..0ff31d97d485 100644
--- a/net/ipv4/udp.c
+++ b/net/ipv4/udp.c
@@ -1618,12 +1618,12 @@ int udp_queue_rcv_skb(struct sock *sk, struct sk_buff *skb)
 		}
 	}
 
-	if (rcu_access_pointer(sk->sk_filter)) {
-		if (udp_lib_checksum_complete(skb))
+	if (rcu_access_pointer(sk->sk_filter) &&
+	    udp_lib_checksum_complete(skb))
 			goto csum_error;
-		if (sk_filter(sk, skb))
-			goto drop;
-	}
+
+	if (sk_filter(sk, skb))
+		goto drop;
 
 	udp_csum_pull_header(skb);
 	if (sk_rcvqueues_full(sk, sk->sk_rcvbuf)) {
diff --git a/net/ipv6/udp.c b/net/ipv6/udp.c
index 2da1896af934..f421c9f23c5b 100644
--- a/net/ipv6/udp.c
+++ b/net/ipv6/udp.c
@@ -653,12 +653,12 @@ int udpv6_queue_rcv_skb(struct sock *sk, struct sk_buff *skb)
 		}
 	}
 
-	if (rcu_access_pointer(sk->sk_filter)) {
-		if (udp_lib_checksum_complete(skb))
-			goto csum_error;
-		if (sk_filter(sk, skb))
-			goto drop;
-	}
+	if (rcu_access_pointer(sk->sk_filter) &&
+	    udp_lib_checksum_complete(skb))
+		goto csum_error;
+
+	if (sk_filter(sk, skb))
+		goto drop;
 
 	udp_csum_pull_header(skb);
 	if (sk_rcvqueues_full(sk, sk->sk_rcvbuf)) {