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

[Chengfeng Ye <nicoyip.dev@gmail.com>]

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: stable@vger.kernel.org
Assisted-by: Codex:gpt-5
Signed-off-by: Chengfeng Ye <nicoyip.dev@gmail.com>
---
 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;
 
 	skb_push(skb, hdrlen);
 
-- 
2.43.0