[PATCH net] ip6_gre: let collect_md ip6gretap send from the unspecified address

[Anton Danilov <littlesmilingcloud@gmail.com>]

ip6gre_xmit_ipv6() drops an IPv6 packet whose source address equals the
tunnel remote, to avoid a trivial tunneling loop. On a collect_md
ip6gretap device t->parms.raddr is not the exit point: the outer
destination comes from the per-packet tunnel metadata. Such devices are
normally created without a remote, so raddr is ::, and the check never
matches a real tunnel endpoint. It matches every frame sent from the
unspecified address instead.

On an L2 tunnel those frames are regular link traffic. Duplicate Address
Detection sends its Neighbor Solicitations from :: (RFC 4862, section
5.4.2), and MLD reports are sent from :: while an interface has no
link-local address yet (RFC 3590, section 4). Frames bridged into a
collect_md ip6gretap device, e.g. with tc tunnel_key and mirred, are
dropped even though they carry valid metadata, so DAD cannot detect a
duplicate address on the other side of the tunnel.

Skip the check for collect_md devices of type ARPHRD_ETHER. It stays for
L3 ip6gre, where sending a packet with an unspecified source means
forwarding it (RFC 4291, section 2.5.2), and for ip6gretap without
collect_md. ip6erspan does not run this check in collect_md mode either.

Fixes: 6712abc168eb ("ip6_gre: add ip6 gre and gretap collect_md mode")
Cc: stable@vger.kernel.org
Assisted-by: LLM
Signed-off-by: Anton Danilov <littlesmilingcloud@gmail.com>

---
 net/ipv6/ip6_gre.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/net/ipv6/ip6_gre.c b/net/ipv6/ip6_gre.c
index e61cb10b50dc..ba1ed459ee79 100644
--- a/net/ipv6/ip6_gre.c
+++ b/net/ipv6/ip6_gre.c
@@ -831,7 +831,12 @@ static inline int ip6gre_xmit_ipv6(struct sk_buff *skb, struct net_device *dev)
 	__u32 mtu;
 	int err;
 
-	if (ipv6_addr_equal(&t->parms.raddr, &ipv6h->saddr))
+	/* The outer destination of a collect_md tunnel comes from the
+	 * metadata, so raddr is not its exit point, and hosts on the link
+	 * bridged into an L2 one do send from :: (DAD, early MLD).
+	 */
+	if (!(t->parms.collect_md && dev->type == ARPHRD_ETHER) &&
+	    ipv6_addr_equal(&t->parms.raddr, &ipv6h->saddr))
 		return -1;
 
 	if (!t->parms.collect_md &&
-- 
2.47.3