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 --- 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