hw/net/net_tx_pkt: Fix overrun in update_sctp_checksum()

hw/net/net_tx_pkt: Fix overrun in update_sctp_checksum()

If a fragmented packet size is too short, do not try to
calculate its checksum.

Reproduced using:

  $ cat << EOF | qemu-system-i386 -display none -nodefaults \
                                  -machine q35,accel=qtest -m 32M \
                                  -device igb,netdev=net0 \
                                  -netdev user,id=net0 \
                                  -qtest stdio
  outl 0xcf8 0x80000810
  outl 0xcfc 0xe0000000
  outl 0xcf8 0x80000804
  outw 0xcfc 0x06
  write 0xe0000403 0x1 0x02
  writel 0xe0003808 0xffffffff
  write 0xe000381a 0x1 0x5b
  write 0xe000381b 0x1 0x00
  EOF
  Assertion failed: (offset == 0), function iov_from_buf_full, file util/iov.c, line 39.
  #1 0x5575e81e952a in iov_from_buf_full qemu/util/iov.c:39:5
  #2 0x5575e6500768 in net_tx_pkt_update_sctp_checksum qemu/hw/net/net_tx_pkt.c:144:9
  #3 0x5575e659f3e1 in igb_setup_tx_offloads qemu/hw/net/igb_core.c:478:11
  #4 0x5575e659f3e1 in igb_tx_pkt_send qemu/hw/net/igb_core.c:552:10
  #5 0x5575e659f3e1 in igb_process_tx_desc qemu/hw/net/igb_core.c:671:17
  #6 0x5575e659f3e1 in igb_start_xmit qemu/hw/net/igb_core.c:903:9
  #7 0x5575e659f3e1 in igb_set_tdt qemu/hw/net/igb_core.c:2812:5
  #8 0x5575e657d6a4 in igb_core_write qemu/hw/net/igb_core.c:4248:9

Fixes: CVE-2024-3567
Cc: qemu-stable@nongnu.org
Reported-by: Zheyu Ma <zheyuma97@gmail.com>
Fixes: f199b13bc1 ("igb: Implement Tx SCTP CSO")
Resolves: https://gitlab.com/qemu-project/qemu/-/issues/2273
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Akihiko Odaki <akihiko.odaki@daynix.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Message-Id: <20240410070459.49112-1-philmd@linaro.org>