@@ -282,6 +282,12 @@ struct sk_buff *__udp_gso_segment(struct sk_buff *gso_skb,
skb_transport_header(gso_skb)))
return ERR_PTR(-EINVAL);
+ /* We don't know if egress device can segment and checksum the packet
+ * when IPv6 extension headers are present. Fall back to software GSO.
+ */
+ if (gso_skb->ip_summed != CHECKSUM_PARTIAL)
+ features &= ~(NETIF_F_GSO_UDP_L4 | NETIF_F_CSUM_MASK);
+
if (skb_gso_ok(gso_skb, features | NETIF_F_GSO_ROBUST)) {
/* Packet is from an untrusted source, reset gso_segs. */
skb_shinfo(gso_skb)->gso_segs = DIV_ROUND_UP(gso_skb->len - sizeof(*uh),
In commit 10154dbded6d ("udp: Allow GSO transmit from devices with no checksum offload") we have intentionally allowed UDP GSO packets marked CHECKSUM_NONE to pass to the GSO stack, so that they can be segmented and checksummed by a software fallback when the egress device lacks these features. What was not taken into consideration is that a CHECKSUM_NONE skb can be handed over to the GSO stack also when the egress device advertises the tx-udp-segmentation / NETIF_F_GSO_UDP_L4 feature. This will happen when there are IPv6 extension headers present, which we check for in __ip6_append_data(). Syzbot has discovered this scenario, producing a warning as below: ip6tnl0: caps=(0x00000006401d7869, 0x00000006401d7869) WARNING: CPU: 0 PID: 5112 at net/core/dev.c:3293 skb_warn_bad_offload+0x166/0x1a0 net/core/dev.c:3291 Modules linked in: CPU: 0 PID: 5112 Comm: syz-executor391 Not tainted 6.10.0-rc7-syzkaller-01603-g80ab5445da62 #0 Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 06/07/2024 RIP: 0010:skb_warn_bad_offload+0x166/0x1a0 net/core/dev.c:3291 [...] Call Trace: <TASK> __skb_gso_segment+0x3be/0x4c0 net/core/gso.c:127 skb_gso_segment include/net/gso.h:83 [inline] validate_xmit_skb+0x585/0x1120 net/core/dev.c:3661 __dev_queue_xmit+0x17a4/0x3e90 net/core/dev.c:4415 neigh_output include/net/neighbour.h:542 [inline] ip6_finish_output2+0xffa/0x1680 net/ipv6/ip6_output.c:137 ip6_finish_output+0x41e/0x810 net/ipv6/ip6_output.c:222 ip6_send_skb+0x112/0x230 net/ipv6/ip6_output.c:1958 udp_v6_send_skb+0xbf5/0x1870 net/ipv6/udp.c:1292 udpv6_sendmsg+0x23b3/0x3270 net/ipv6/udp.c:1588 sock_sendmsg_nosec net/socket.c:730 [inline] __sock_sendmsg+0xef/0x270 net/socket.c:745 ____sys_sendmsg+0x525/0x7d0 net/socket.c:2585 ___sys_sendmsg net/socket.c:2639 [inline] __sys_sendmmsg+0x3b2/0x740 net/socket.c:2725 __do_sys_sendmmsg net/socket.c:2754 [inline] __se_sys_sendmmsg net/socket.c:2751 [inline] __x64_sys_sendmmsg+0xa0/0xb0 net/socket.c:2751 do_syscall_x64 arch/x86/entry/common.c:52 [inline] do_syscall_64+0xf3/0x230 arch/x86/entry/common.c:83 entry_SYSCALL_64_after_hwframe+0x77/0x7f [...] </TASK> We are hitting the bad offload warning because when an egress device is capable of handling segmentation offload requested by skb_shinfo(skb)->gso_type, the chain of gso_segment callbacks won't produce any segment skbs and return NULL. See the skb_gso_ok() branch in {__udp,tcp,sctp}_gso_segment helpers. To fix it, force a fallback to software USO when processing a packet with IPv6 extension headers, since we don't know if these can checksummed by all devices which offer USO. Fixes: 10154dbded6d ("udp: Allow GSO transmit from devices with no checksum offload") Reported-by: syzbot+e15b7e15b8a751a91d9a@syzkaller.appspotmail.com Closes: https://lore.kernel.org/all/000000000000e1609a061d5330ce@google.com/ Reviewed-by: Willem de Bruijn <willemb@google.com> Signed-off-by: Jakub Sitnicki <jakub@cloudflare.com> --- net/ipv4/udp_offload.c | 6 ++++++ 1 file changed, 6 insertions(+)