Re: [PATCH net v4] ip_tunnel: reserve FOU/GUE headroom before encapsulation

[Ido Schimmel]

On Sat, Sep 05, 2026 at 12:55:44AM +0800, Chengfeng Ye wrote:
> ip_tunnel_encap() expects its callers to reserve headroom based on
> ip_encap_hlen(). ip_tunnel_xmit() currently pushes FOU and GUE headers
> before it grows the skb headroom.
> 
> That becomes visible when ipgre_changelink() publishes UDP
> encapsulation before it updates the device 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
> 
> Snapshot tunnel->encap, route and perform PMTU handling first, then
> reserve the final headroom before ip_tunnel_encap() builds the UDP
> tunnel headers. Because encapsulation is now delayed, pass the
> snapshotted encap length into tnl_update_pmtu() so the inner packet
> size stays skb->len + encap_hlen - tunnel_hlen.
> 
> ip_md_tunnel_xmit() is left unchanged. It takes FOU/GUE parameters
> from the skb metadata dst, not from the device configuration, so it
> is not exposed to this race. tnl_update_pmtu() still runs after
> encapsulation there, so that path passes 0 for the extra length.
> 
> The IPv6 analogue of this headroom reservation is still work in
> progress.

Please wait for the IPv6 part to be accepted (some changes are needed
there and here as well) and then mention its sha1 in the commit message:

https://lore.kernel.org/netdev/2ce8f3e4bbed6060afb0aee33dc4e1d9ae021280.1788262122.git.petalzu987@xxxxxxxxx/

> 
> Link: https://lore.kernel.org/netdev/b58876297f7d45de008f2e94b6ecab8b2ed84d21.1786088695.git.petalzu987@xxxxxxxxx/
> Fixes: dd9d598c6657 ("ip_gre: add the support for i/o_flags update via netlink")

Looks like a more suitable Fixes tag:

Fixes: 56328486539d ("net: Changes to ip_tunnel to support foo-over-udp encapsulation")

> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Chengfeng Ye <nicoyip.dev@xxxxxxxxx>
> ---
> Changes in v4:
> - Not restructure ip_md_tunnel_xmit(); only pass 0 into the new
>   tnl_update_pmtu() argument.
> - Snapshot tunnel->encap with data_race() and reuse the copy for both
>   ip_encap_hlen() and ip_tunnel_encap().
> - Fix tnl_update_pmtu() inner packet size after delaying the encap
>   push: pkt_size = skb->len + encap_hlen - tunnel_hlen.
> - Keep reverse xmas tree for the new locals.
> - Note that the IPv6 FOU/GUE headroom fix is still WIP.
> 
> Changes in v3:
> - Move the headroom reservation into ip_tunnel_xmit() instead of
>   growing the skb inside the FOU/GUE builders.
> - Use ip_encap_hlen() to reserve the final caller-side headroom before
>   ip_tunnel_encap().
> - Drop the IPv4 raw-pointer refreshes that were only needed when
>   skb_cow_head() could run inside the encapsulation builders.
> 
> Link: https://lore.kernel.org/netdev/20260824111944.187200-1-nicoyip.dev@xxxxxxxxx/ [v3]
> Link: https://lore.kernel.org/netdev/20260808005956.3761487-1-nicoyip.dev@xxxxxxxxx/ [v2]
> Link: https://lore.kernel.org/netdev/20260801060115.3538849-1-nicoyip.dev@xxxxxxxxx/ [v1]
> ---
>  net/ipv4/ip_tunnel.c | 24 ++++++++++++++++++------
>  1 file changed, 18 insertions(+), 6 deletions(-)

ipip6_tunnel_xmit() also calls ip_tunnel_encap() and seems to have the
same problem as ip6_tnl_xmit().