ipip_fill_forward_path() and ip6_tnl_fill_forward_path() assign the outer
route's device to ctx->dev without checking that it differs from the tunnel
device itself.  When a tunnel's outer route resolves back to that same
tunnel, the walk in dev_fill_forward_path() makes no progress and trips its
loop check WARN_ON_ONCE(last_dev == ctx->dev). Release the route and
return -EOPNOTSUPP when the resolved dst device equals ctx->dev, so the
path walk either advances to a different device or fails cleanly.

Closes: https://lore.kernel.org/all/CA+0ovCgaRvbd0Udj70b2xxG8Cx3CaCpNhnf1V4RWQuDveZYZhA@mail.gmail.com/
Signed-off-by: Farhad Alemi <farhad.alemi@berkeley.edu>
---
--- a/net/ipv4/ipip.c
+++ b/net/ipv4/ipip.c
@@ -375,6 +375,12 @@ static int ipip_fill_forward_path(struct
net_device_path_ctx *ctx,
 	if (IS_ERR(rt))
 		return PTR_ERR(rt);

+	/* The path walk must advance: a route back into the tunnel is a loop. */
+	if (rt->dst.dev == ctx->dev) {
+		ip_rt_put(rt);
+		return -EOPNOTSUPP;
+	}
+
 	path->type = DEV_PATH_TUN;
 	path->tun.src_v4.s_addr = tiph->saddr;
 	path->tun.dst_v4.s_addr = tiph->daddr;
--- a/net/ipv6/ip6_tunnel.c
+++ b/net/ipv6/ip6_tunnel.c
@@ -1863,7 +1863,12 @@ static int ip6_tnl_fill_forward_path(struct
net_device_path_ctx *ctx,
 	fl6.flowi6_proto = 0;

 	dst = ip6_route_output(dev_net(ctx->dev), NULL, &fl6);
-	if (!dst->error) {
+	err = dst->error;
+	/* The path walk must advance: a route back into the tunnel is a loop. */
+	if (!err && dst->dev == ctx->dev)
+		err = -EOPNOTSUPP;
+
+	if (!err) {
 		path->type = DEV_PATH_TUN;
 		path->tun.src_v6 = fl6.saddr;
 		path->tun.dst_v6 = fl6.daddr;
@@ -1873,7 +1878,6 @@ static int ip6_tnl_fill_forward_path(struct
net_device_path_ctx *ctx,
 		ctx->dev = dst->dev;
 	}

-	err = dst->error;
 	if (err)
 		dst_release(dst);