ipip_fill_forward_path() and ip6_tnl_fill_forward_path() look up the
route to the tunnel's remote endpoint and set ctx->dev to its device,
which is the tunnel itself when that route resolves back to the tunnel.
dev_fill_forward_path() then makes no progress and trips
WARN_ON_ONCE(last_dev == ctx->dev) as soon as a flowtable tries to
offload a flow through the tunnel. That routing loop is a configuration
any CAP_NET_ADMIN user can set up, and ip_tunnel_xmit() and
ip6_tnl_xmit() already treat it as a tx error, so remove the warning and
just fail the walk, as commit 008e7a7c293b ("net: remove WARN_ON_ONCE
when accessing forward path array") did for the path stack overflow.

Fixes: ab427db17885 ("netfilter: flowtable: Add IPIP rx sw acceleration")
Fixes: d98103575dcd ("netfilter: flowtable: Add IP6IP6 rx sw acceleration")
Closes: https://lore.kernel.org/all/CA+0ovCgaRvbd0Udj70b2xxG8Cx3CaCpNhnf1V4RWQuDveZYZhA@mail.gmail.com/
Suggested-by: Pablo Neira Ayuso <pablo@netfilter.org>
Assisted-by: Claude:claude-opus-5 syzkaller
Signed-off-by: Farhad Alemi <farhad.alemi@berkeley.edu>
---
v2:
 - Remove the WARN_ON_ONCE() in dev_fill_forward_path() instead of
   rejecting the looping route in the tunnel drivers (Pablo Neira Ayuso).
 - Add the Fixes: tags (Xuanqiang Luo).

v1: https://lore.kernel.org/all/CA+0ovCj+Pd5owGH+51rA8-RDu9YFx5oYUv8bV0zOE72QgVfY0A@mail.gmail.com/

diff --git a/net/core/dev.c b/net/core/dev.c
index ecfbd72d5d1a..c67900354fa6 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -789,7 +789,7 @@ int dev_fill_forward_path(struct net_device_path_ctx
 			goto err_out;

 		stack->num_paths++;
-		if (WARN_ON_ONCE(last_dev == ctx->dev))
+		if (last_dev == ctx->dev)
 			goto err_out;
 	}

-- 
2.53.0