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[net-next] netfilter: bridge: reset skb->pkt_type after NF_INET_POST_ROUTING traversal

Message ID 20201123174902.622102-1-atenart@kernel.org (mailing list archive)
State Accepted
Delegated to: Netdev Maintainers
Headers show
Series [net-next] netfilter: bridge: reset skb->pkt_type after NF_INET_POST_ROUTING traversal | expand

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Commit Message

Antoine Tenart Nov. 23, 2020, 5:49 p.m. UTC
Netfilter changes PACKET_OTHERHOST to PACKET_HOST before invoking the
hooks as, while it's an expected value for a bridge, routing expects
PACKET_HOST. The change is undone later on after hook traversal. This
can be seen with pairs of functions updating skb>pkt_type and then
reverting it to its original value:

For hook NF_INET_PRE_ROUTING:
  setup_pre_routing / br_nf_pre_routing_finish

For hook NF_INET_FORWARD:
  br_nf_forward_ip / br_nf_forward_finish

But the third case where netfilter does this, for hook
NF_INET_POST_ROUTING, the packet type is changed in br_nf_post_routing
but never reverted. A comment says:

  /* We assume any code from br_dev_queue_push_xmit onwards doesn't care
   * about the value of skb->pkt_type. */

But when having a tunnel (say vxlan) attached to a bridge we have the
following call trace:

  br_nf_pre_routing
  br_nf_pre_routing_ipv6
     br_nf_pre_routing_finish
  br_nf_forward_ip
     br_nf_forward_finish
  br_nf_post_routing           <- pkt_type is updated to PACKET_HOST
     br_nf_dev_queue_xmit      <- but not reverted to its original value
  vxlan_xmit
     vxlan_xmit_one
        skb_tunnel_check_pmtu  <- a check on pkt_type is performed

In this specific case, this creates issues such as when an ICMPv6 PTB
should be sent back. When CONFIG_BRIDGE_NETFILTER is enabled, the PTB
isn't sent (as skb_tunnel_check_pmtu checks if pkt_type is PACKET_HOST
and returns early).

If the comment is right and no one cares about the value of
skb->pkt_type after br_dev_queue_push_xmit (which isn't true), resetting
it to its original value should be safe.

Signed-off-by: Antoine Tenart <atenart@kernel.org>
---
 net/bridge/br_netfilter_hooks.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

Comments

Florian Westphal Nov. 23, 2020, 6:32 p.m. UTC | #1
Antoine Tenart <atenart@kernel.org> wrote:
> Netfilter changes PACKET_OTHERHOST to PACKET_HOST before invoking the
> hooks as, while it's an expected value for a bridge, routing expects
> PACKET_HOST. The change is undone later on after hook traversal. This
> can be seen with pairs of functions updating skb>pkt_type and then
> reverting it to its original value:
> 
> For hook NF_INET_PRE_ROUTING:
>   setup_pre_routing / br_nf_pre_routing_finish
> 
> For hook NF_INET_FORWARD:
>   br_nf_forward_ip / br_nf_forward_finish
> 
> But the third case where netfilter does this, for hook
> NF_INET_POST_ROUTING, the packet type is changed in br_nf_post_routing
> but never reverted. A comment says:
> 
>   /* We assume any code from br_dev_queue_push_xmit onwards doesn't care
>    * about the value of skb->pkt_type. */

[..]
> But when having a tunnel (say vxlan) attached to a bridge we have the
> following call trace:

> In this specific case, this creates issues such as when an ICMPv6 PTB
> should be sent back. When CONFIG_BRIDGE_NETFILTER is enabled, the PTB
> isn't sent (as skb_tunnel_check_pmtu checks if pkt_type is PACKET_HOST
> and returns early).
> 
> If the comment is right and no one cares about the value of
> skb->pkt_type after br_dev_queue_push_xmit (which isn't true), resetting
> it to its original value should be safe.

That comment is 18 years old, safe bet noone thought of
ipv6-in-tunnel-interface-added-as-bridge-port back then.

Reviewed-by: Florian Westphal <fw@strlen.de>
Jakub Kicinski Nov. 28, 2020, 12:06 a.m. UTC | #2
On Mon, 23 Nov 2020 19:32:53 +0100 Florian Westphal wrote:
> Antoine Tenart <atenart@kernel.org> wrote:
> > Netfilter changes PACKET_OTHERHOST to PACKET_HOST before invoking the
> > hooks as, while it's an expected value for a bridge, routing expects
> > PACKET_HOST. The change is undone later on after hook traversal. This
> > can be seen with pairs of functions updating skb>pkt_type and then
> > reverting it to its original value:
> > 
> > For hook NF_INET_PRE_ROUTING:
> >   setup_pre_routing / br_nf_pre_routing_finish
> > 
> > For hook NF_INET_FORWARD:
> >   br_nf_forward_ip / br_nf_forward_finish
> > 
> > But the third case where netfilter does this, for hook
> > NF_INET_POST_ROUTING, the packet type is changed in br_nf_post_routing
> > but never reverted. A comment says:
> > 
> >   /* We assume any code from br_dev_queue_push_xmit onwards doesn't care
> >    * about the value of skb->pkt_type. */  
> 
> [..]
> > But when having a tunnel (say vxlan) attached to a bridge we have the
> > following call trace:  
> 
> > In this specific case, this creates issues such as when an ICMPv6 PTB
> > should be sent back. When CONFIG_BRIDGE_NETFILTER is enabled, the PTB
> > isn't sent (as skb_tunnel_check_pmtu checks if pkt_type is PACKET_HOST
> > and returns early).
> > 
> > If the comment is right and no one cares about the value of
> > skb->pkt_type after br_dev_queue_push_xmit (which isn't true), resetting
> > it to its original value should be safe.  
> 
> That comment is 18 years old, safe bet noone thought of
> ipv6-in-tunnel-interface-added-as-bridge-port back then.
> 
> Reviewed-by: Florian Westphal <fw@strlen.de>

Sounds like a fix. Probably hard to pin point which commit to blame,
but this should go to net, not net-next, right?
Florian Westphal Nov. 28, 2020, 9:59 a.m. UTC | #3
Jakub Kicinski <kuba@kernel.org> wrote:
> On Mon, 23 Nov 2020 19:32:53 +0100 Florian Westphal wrote:
> > That comment is 18 years old, safe bet noone thought of
> > ipv6-in-tunnel-interface-added-as-bridge-port back then.
> > 
> > Reviewed-by: Florian Westphal <fw@strlen.de>
> 
> Sounds like a fix. Probably hard to pin point which commit to blame,
> but this should go to net, not net-next, right?

The commit predates git history, so probably a good idea to add
Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")

... and apply it to net tree.
Jakub Kicinski Nov. 28, 2020, 7:49 p.m. UTC | #4
On Sat, 28 Nov 2020 10:59:29 +0100 Florian Westphal wrote:
> Jakub Kicinski <kuba@kernel.org> wrote:
> > On Mon, 23 Nov 2020 19:32:53 +0100 Florian Westphal wrote:  
> > > That comment is 18 years old, safe bet noone thought of
> > > ipv6-in-tunnel-interface-added-as-bridge-port back then.
> > > 
> > > Reviewed-by: Florian Westphal <fw@strlen.de>  
> > 
> > Sounds like a fix. Probably hard to pin point which commit to blame,
> > but this should go to net, not net-next, right?  
> 
> The commit predates git history, so probably a good idea to add
> Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
> 
> ... and apply it to net tree.

Done, thanks!
diff mbox series

Patch

diff --git a/net/bridge/br_netfilter_hooks.c b/net/bridge/br_netfilter_hooks.c
index 04c3f9a82650..8edfb98ae1d5 100644
--- a/net/bridge/br_netfilter_hooks.c
+++ b/net/bridge/br_netfilter_hooks.c
@@ -735,6 +735,11 @@  static int br_nf_dev_queue_xmit(struct net *net, struct sock *sk, struct sk_buff
 	mtu_reserved = nf_bridge_mtu_reduction(skb);
 	mtu = skb->dev->mtu;
 
+	if (nf_bridge->pkt_otherhost) {
+		skb->pkt_type = PACKET_OTHERHOST;
+		nf_bridge->pkt_otherhost = false;
+	}
+
 	if (nf_bridge->frag_max_size && nf_bridge->frag_max_size < mtu)
 		mtu = nf_bridge->frag_max_size;
 
@@ -835,8 +840,6 @@  static unsigned int br_nf_post_routing(void *priv,
 	else
 		return NF_ACCEPT;
 
-	/* We assume any code from br_dev_queue_push_xmit onwards doesn't care
-	 * about the value of skb->pkt_type. */
 	if (skb->pkt_type == PACKET_OTHERHOST) {
 		skb->pkt_type = PACKET_HOST;
 		nf_bridge->pkt_otherhost = true;