net: drop UFO packets in udp_rcv_segment()
CVE: CVE-2025-38622
Component: kernel
CWE: CWE-122
Severity: Moderate
CVSS v3: 5.6/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H
Release date: 2025-08-22T00:00:00+00:00

In the Linux kernel, the following vulnerability has been resolved:

net: drop UFO packets in udp_rcv_segment()

When sending a packet with virtio_net_hdr to tun device, if the gso_type
in virtio_net_hdr is SKB_GSO_UDP and the gso_size is less than udphdr
size, below crash may happen.

  ------------[ cut here ]------------
  kernel BUG at net/core/skbuff.c:4572!
  Oops: invalid opcode: 0000 [#1] SMP NOPTI
  CPU: 0 UID: 0 PID: 62 Comm: mytest Not tainted 6.16.0-rc7 #203 PREEMPT(voluntary)
  Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.15.0-1 04/01/2014
  RIP: 0010:skb_pull_rcsum+0x8e/0xa0
  Code: 00 00 5b c3 cc cc cc cc 8b 93 88 00 00 00 f7 da e8 37 44 38 00 f7 d8 89 83 88 00 00 00 48 8b 83 c8 00 00 00 5b c3 cc cc cc cc <0f> 0b 0f 0b 66 66 2e 0f 1f 84 00 000
  RSP: 0018:ffffc900001fba38 EFLAGS: 00000297
  RAX: 0000000000000004 RBX: ffff8880040c1000 RCX: ffffc900001fb948
  RDX: ffff888003e6d700 RSI: 0000000000000008 RDI: ffff88800411a062
  RBP: ffff8880040c1000 R08: 0000000000000000 R09: 0000000000000001
  R10: ffff888003606c00 R11: 0000000000000001 R12: 0000000000000000
  R13: ffff888004060900 R14: ffff888004050000 R15: ffff888004060900
  FS:  000000002406d3c0(0000) GS:ffff888084a19000(0000) knlGS:0000000000000000
  CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
  CR2: 0000000020000040 CR3: 0000000004007000 CR4: 00000000000006f0
  Call Trace:
   <TASK>
   udp_queue_rcv_one_skb+0x176/0x4b0 net/ipv4/udp.c:2445
   udp_queue_rcv_skb+0x155/0x1f0 net/ipv4/udp.c:2475
   udp_unicast_rcv_skb+0x71/0x90 net/ipv4/udp.c:2626
   __udp4_lib_rcv+0x433/0xb00 net/ipv4/udp.c:2690
   ip_protocol_deliver_rcu+0xa6/0x160 net/ipv4/ip_input.c:205
   ip_local_deliver_finish+0x72/0x90 net/ipv4/ip_input.c:233
   ip_sublist_rcv_finish+0x5f/0x70 net/ipv4/ip_input.c:579
   ip_sublist_rcv+0x122/0x1b0 net/ipv4/ip_input.c:636
   ip_list_rcv+0xf7/0x130 net/ipv4/ip_input.c:670
   __netif_receive_skb_list_core+0x21d/0x240 net/core/dev.c:6067
   netif_receive_skb_list_internal+0x186/0x2b0 net/core/dev.c:6210
   napi_complete_done+0x78/0x180 net/core/dev.c:6580
   tun_get_user+0xa63/0x1120 drivers/net/tun.c:1909
   tun_chr_write_iter+0x65/0xb0 drivers/net/tun.c:1984
   vfs_write+0x300/0x420 fs/read_write.c:593
   ksys_write+0x60/0xd0 fs/read_write.c:686
   do_syscall_64+0x50/0x1c0 arch/x86/entry/syscall_64.c:63
   </TASK>

To trigger gso segment in udp_queue_rcv_skb(), we should also set option
UDP_ENCAP_ESPINUDP to enable udp_sk(sk)->encap_rcv. When the encap_rcv
hook return 1 in udp_queue_rcv_one_skb(), udp_csum_pull_header() will try
to pull udphdr, but the skb size has been segmented to gso size, which
leads to this crash.

Previous commit cf329aa42b66 ("udp: cope with UDP GRO packet misdirection")
introduces segmentation in UDP receive path only for GRO, which was never
intended to be used for UFO, so drop UFO packets in udp_rcv_segment().

Statement: A bug in the UDP receive path allowed malformed UFO (UDP Fragmentation Offload) packets with too-small gso_size to reach udp_queue_rcv_skb(), leading to invalid header pulls and kernel crashes. An attacker with the ability to inject crafted virtio_net frames (e.g. to a TUN/TAP device) could trigger this condition if UDP encapsulation is enabled, resulting in a denial of service.
Exploitation requires at least low privileges, as an attacker must have access to a tun/tap or similar interface; in most systems this is not available to unprivileged users by default and typically requires elevated rights or specific configuration.

Remediation: