Re: [PATCH net] fou: ensure GUE headers have enough headroom

On Sat, Aug 01, 2026 at 02:01:15PM +0800, Chengfeng Ye wrote:
> ipgre_changelink() installs GUE encapsulation before it publishes the
> new GRE header length and updates dev->needed_headroom.  The transmit
> path does not serialize with RTNL, so it can interleave as follows:
> 
>   CPU 0 (ipgre_changelink)        CPU 1 (ipgre_xmit)
>   install GUE encapsulation
>                                   reserve the old needed_headroom
>   publish larger GRE flags
>   update tunnel->tun_hlen
>                                   push the larger GRE header
>                                   push the GUE and UDP headers
>   update dev->needed_headroom
> 
> With REMCSUM, the new layout can push 16 bytes of GRE and 20 bytes of
> GUE/UDP headers into an skb with only 32 bytes of actual headroom.  The
> final UDP push writes four bytes before skb->head.
> 
> With the update window widened, the kernel reported:
> 
>   skbuff: skb_under_panic: ... len:128 put:8 ... dev:gre0poc
>   kernel BUG at net/core/skbuff.c:214!
>   Oops: invalid opcode: 0000 [#1] SMP KASAN NOPTI
>   Call Trace:
>    skb_push
>    fou_build_udp
>    gue_build_header
>    ip_tunnel_xmit
>    __gre_xmit
>    ipgre_xmit
> 
> Make __gue_build_header() ensure space for both the GUE header it is
> about to push and the UDP header that follows.  On normally sized skbs
> the check is a no-op.  If configuration changes race with transmission,
> skb_cow_head() expands the head before either GUE write, or returns an
> error without modifying the packet.
> 
> Fixes: dd9d598c6657 ("ip_gre: add the support for i/o_flags update via netlink")
> Cc: [email protected]
> Assisted-by: Codex:gpt-5
> Signed-off-by: Chengfeng Ye <[email protected]>
> ---
>  net/ipv4/fou_core.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/net/ipv4/fou_core.c b/net/ipv4/fou_core.c
> index ab09dfcdecbd..8cf0d43acb41 100644
> --- a/net/ipv4/fou_core.c
> +++ b/net/ipv4/fou_core.c
> @@ -980,6 +980,8 @@ int __gue_build_header(struct sk_buff *skb, struct ip_tunnel_encap *e,
>  						skb, 0, 0, false);
>  
>  	hdrlen = sizeof(struct guehdr) + optlen;
> +	if (skb_cow_head(skb, hdrlen + sizeof(struct udphdr)))
> +		return -ENOMEM;

My understanding is this takes into account the UDP header pushed in
fou_build_udp. Isn't fou_build_header also affected by the same issue
then? (If so check the IPv6 paths too).

Also please check Sashiko's output,
https://sashiko.dev/#/patchset/20260801060115.3538849-1-nicoyip.dev%40gmail.com