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 raspberrypi#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 cf329aa ("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().

Link: https://lore.kernel.org/netdev/20250724083005.3918375-1-wangliang74@huawei.com/
Link: https://lore.kernel.org/netdev/20250729123907.3318425-1-wangliang74@huawei.com/
Fixes: cf329aa ("udp: cope with UDP GRO packet misdirection")
Suggested-by: Willem de Bruijn <willemdebruijn.kernel@gmail.com>
Signed-off-by: Wang Liang <wangliang74@huawei.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Link: https://patch.msgid.link/20250730101458.3470788-1-wangliang74@huawei.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: Wang Liang

include/net/udp.h

@@ -586,6 +586,16 @@ static inline struct sk_buff *udp_rcv_segment(struct sock *sk,
 {
 	netdev_features_t features = NETIF_F_SG;
 	struct sk_buff *segs;
+	int drop_count;
+
+	/*
+	 * Segmentation in UDP receive path is only for UDP GRO, drop udp
+	 * fragmentation offload (UFO) packets.
+	 */
+	if (skb_shinfo(skb)->gso_type & SKB_GSO_UDP) {
+		drop_count = 1;
+		goto drop;
+	}
 
 	/* Avoid csum recalculation by skb_segment unless userspace explicitly
 	 * asks for the final checksum values
@@ -609,16 +619,18 @@ static inline struct sk_buff *udp_rcv_segment(struct sock *sk,
 	 */
 	segs = __skb_gso_segment(skb, features, false);
 	if (IS_ERR_OR_NULL(segs)) {
-		int segs_nr = skb_shinfo(skb)->gso_segs;
-
-		atomic_add(segs_nr, &sk->sk_drops);
-		SNMP_ADD_STATS(__UDPX_MIB(sk, ipv4), UDP_MIB_INERRORS, segs_nr);
-		kfree_skb(skb);
-		return NULL;
+		drop_count = skb_shinfo(skb)->gso_segs;
+		goto drop;
 	}
 
 	consume_skb(skb);
 	return segs;
+
+drop:
+	atomic_add(drop_count, &sk->sk_drops);
+	SNMP_ADD_STATS(__UDPX_MIB(sk, ipv4), UDP_MIB_INERRORS, drop_count);
+	kfree_skb(skb);
+	return NULL;
 }
 
 static inline void udp_post_segment_fix_csum(struct sk_buff *skb)