Re: [PATCH net] net: reject a forward path that loops back to the tunnel

[Pablo Neira Ayuso]

Hi,

On Fri, Sep 11, 2026 at 12:17:08AM +0000, Farhad Alemi wrote:
> 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.

I think we can remove this WARN_ON_ONCE() from dev_fill_forward_path(), ie.

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 *ctx,
                        goto err_out;
 
                stack->num_paths++;
-               if (WARN_ON_ONCE(last_dev == ctx->dev))
+               if (last_dev == ctx->dev)
                        goto err_out;
        }
 
> Closes: https://lore.kernel.org/all/CA+0ovCgaRvbd0Udj70b2xxG8Cx3CaCpNhnf1V4RWQuDveZYZhA@xxxxxxxxxxxxxx/
> Signed-off-by: Farhad Alemi <farhad.alemi@xxxxxxxxxxxx>
> ---
> --- 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);