[PATCH net v2] 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.

skb_cow_head() may move skb->head.  Refresh skb-derived inner_iph
pointers after encapsulation, and read the tunnel fragment and TTL
fields before ip_tunnel_encap() so ip_tunnel_xmit() does not dereference
tnl_params after a possible skb head reallocation.

Fixes: dd9d598c6657 ("ip_gre: add the support for i/o_flags update via netlink")
Cc: stable@vger.kernel.org
Signed-off-by: Chengfeng Ye <nicoyip.dev@gmail.com>
---
Changes in v2:
- Refresh skb-derived inner_iph pointers after ip_tunnel_encap() and read 
frag_off and ttl from tnl_params before ip_tunnel_encap(), because the new
skb_cow_head() in the GUE builder can move skb->head.

Link: https://lore.kernel.org/netdev/20260801060115.3538849-1-nicoyip.dev@gmail.com/ [v1]

 net/ipv4/fou_core.c  | 2 ++
 net/ipv4/ip_tunnel.c | 7 +++++--
 2 files changed, 7 insertions(+), 2 deletions(-)

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);
 
diff --git a/net/ipv4/ip_tunnel.c b/net/ipv4/ip_tunnel.c
index 9d114bd575f9..2bd684eb2ba2 100644
--- a/net/ipv4/ip_tunnel.c
+++ b/net/ipv4/ip_tunnel.c
@@ -606,6 +606,7 @@ void ip_md_tunnel_xmit(struct sk_buff *skb, struct net_device *dev,
 
 	if (ip_tunnel_encap(skb, &tun_info->encap, &proto, &fl4) < 0)
 		goto tx_error;
+	inner_iph = (const struct iphdr *)skb_inner_network_header(skb);
 
 	use_cache = ip_tunnel_dst_cache_usable(skb, tun_info);
 	if (use_cache)
@@ -765,8 +766,12 @@ void ip_tunnel_xmit(struct sk_buff *skb, struct net_device *dev,
 			    tunnel->net, READ_ONCE(tunnel->parms.link),
 			    tunnel->fwmark, skb_get_hash(skb), 0);
 
+	df = tnl_params->frag_off;
+	ttl = tnl_params->ttl;
+
 	if (ip_tunnel_encap(skb, &tunnel->encap, &protocol, &fl4) < 0)
 		goto tx_error;
+	inner_iph = (const struct iphdr *)skb_inner_network_header(skb);
 
 	if (connected && md) {
 		use_cache = ip_tunnel_dst_cache_usable(skb, tun_info);
@@ -799,7 +804,6 @@ void ip_tunnel_xmit(struct sk_buff *skb, struct net_device *dev,
 		goto tx_error;
 	}
 
-	df = tnl_params->frag_off;
 	if (payload_protocol == htons(ETH_P_IP) && !tunnel->ignore_df)
 		df |= (inner_iph->frag_off & htons(IP_DF));
 
@@ -821,7 +825,6 @@ void ip_tunnel_xmit(struct sk_buff *skb, struct net_device *dev,
 	}
 
 	tos = ip_tunnel_ecn_encap(tos, inner_iph, skb);
-	ttl = tnl_params->ttl;
 	if (ttl == 0) {
 		if (payload_protocol == htons(ETH_P_IP))
 			ttl = inner_iph->ttl;
-- 
2.43.0