[PATCH] selinux: don't label bridged packets in selinux_ip_forward()

With br_netfilter and bridge-nf-call-ip{,6}tables enabled, bridged IP
packets go through the NF_INET_FORWARD hook, where selinux_ip_forward()
calls selinux_netlbl_skbuff_setsid() and NetLabel may add or remove a
CIPSO or CALIPSO option.  cipso_v4_skbuff_setattr(),
calipso_skbuff_setattr() and calipso_skbuff_delattr() move the IP
header and reset the network header, but the Ethernet header stays
where it was; when the bridge pushes it back in front of the packet it
picks up stale bytes and the frame goes out with a garbage destination
MAC and EtherType.

Labeling a bridged packet based on its IP addresses is not what we want
here anyway, so leave bridged packets alone.  Add a stub of
nf_bridge_info_exists() for CONFIG_BRIDGE_NETFILTER=n.

Tested with QEMU, two veths enslaved to a bridge with br_netfilter in
the middle, SELinux and a NetLabel DOI 16 configuration, 5 UDP packets
per case:

  case                                        before        after
  CALIPSO packet, unlabeled destination       MAC garbage   5/5
  unlabeled IPv6, CALIPSO destination         MAC garbage   5/5
  unlabeled IPv4, CIPSO destination           MAC garbage   5/5

Routed packets are still labeled (an unlabeled packet forwarded towards
a CALIPSO destination gets the option, 5/5).

Fixes: 948bf85c1bc9 ("netlabel: Add functionality to set the security attributes of a packet")
Link: https://lore.kernel.org/netdev/CAHC9VhS+bdL_YnLQtd0sPmKKfy9BvS-LWV_6EhAuu4qfUFe_mw@mail.gmail.com/ (local)
Suggested-by: Paul Moore <paul@paul-moore.com>
Signed-off-by: Joas Antonio dos Santos <redacted>
Assisted-by: Claude:claude-opus-5-5
---
 include/linux/netfilter_bridge.h |  5 +++++
 security/selinux/hooks.c         | 13 ++++++++-----
 2 files changed, 13 insertions(+), 5 deletions(-)

diff --git a/include/linux/netfilter_bridge.h b/include/linux/netfilter_bridge.h
index 743475ca7..c1f7c3cb2 100644
--- a/include/linux/netfilter_bridge.h
+++ b/include/linux/netfilter_bridge.h

@@ -79,6 +79,11 @@ static inline bool nf_bridge_in_prerouting(const struct sk_buff *skb)
 }
 #else
 #define br_drop_fake_rtable(skb)	        do { } while (0)
+static inline bool nf_bridge_info_exists(const struct sk_buff *skb)
+{
+	return false;
+}
+
 static inline bool nf_bridge_in_prerouting(const struct sk_buff *skb)
 {
 	return false;

diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
index 30776b010..1e4f02f99 100644
--- a/security/selinux/hooks.c
+++ b/security/selinux/hooks.c

@@ -49,6 +49,7 @@
 #include <linux/fs_parser.h>
 #include <linux/netfilter_ipv4.h>
 #include <linux/netfilter_ipv6.h>
+#include <linux/netfilter_bridge.h>
 #include <linux/tty.h>
 #include <net/icmp.h>
 #include <net/ip.h>		/* for local_port_range[] */

@@ -6145,11 +6146,13 @@ static unsigned int selinux_ip_forward(void *priv, struct sk_buff *skb,
 				 SECCLASS_PACKET, PACKET__FORWARD_IN, &ad))
 			return NF_DROP;
 
-	if (netlbl_enabled())
-		/* we do this in the FORWARD path and not the POST_ROUTING
-		 * path because we want to make sure we apply the necessary
-		 * labeling before IPsec is applied so we can leverage AH
-		 * protection */
+	/* we do this in the FORWARD path and not the POST_ROUTING path
+	 * because we want to make sure we apply the necessary labeling
+	 * before IPsec is applied so we can leverage AH protection; bridged
+	 * packets are skipped as changing their IP options would leave the
+	 * Ethernet header behind
+	 */
+	if (netlbl_enabled() && !nf_bridge_info_exists(skb))
 		if (selinux_netlbl_skbuff_setsid(skb, family, peer_sid) != 0)
 			return NF_DROP;
 

-- 
2.43.0