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

udp: Deal with race between UDP socket address change and rehash

If a UDP socket changes its local address while it's receiving
datagrams, as a result of connect(), there is a period during which
a lookup operation might fail to find it, after the address is changed
but before the secondary hash (port and address) and the four-tuple
hash (local and remote ports and addresses) are updated.

Secondary hash chains were introduced by commit 30fff9231fad ("udp:
bind() optimisation") and, as a result, a rehash operation became
needed to make a bound socket reachable again after a connect().

This operation was introduced by commit 719f835853a9 ("udp: add
rehash on connect()") which isn't however a complete fix: the
socket will be found once the rehashing completes, but not while
it's pending.

This is noticeable with a socat(1) server in UDP4-LISTEN mode, and a
client sending datagrams to it. After the server receives the first
datagram (cf. _xioopen_ipdgram_listen()), it issues a connect() to
the address of the sender, in order to set up a directed flow.

Now, if the client, running on a different CPU thread, happens to
send a (subsequent) datagram while the server's socket changes its
address, but is not rehashed yet, this will result in a failed
lookup and a port unreachable error delivered to the client, as
apparent from the following reproducer:

  LEN=$(($(cat /proc/sys/net/core/wmem_default) / 4))
  dd if=/dev/urandom bs=1 count=${LEN} of=tmp.in

  while :; do
  	taskset -c 1 socat UDP4-LISTEN:1337,null-eof OPEN:tmp.out,create,trunc &
  	sleep 0.1 || sleep 1
  	taskset -c 2 socat OPEN:tmp.in UDP4:localhost:1337,shut-null
  	wait
  done

where the client will eventually get ECONNREFUSED on a write()
(typically the second or third one of a given iteration):

  2024/11/13 21:28:23 socat[46901] E write(6, 0x556db2e3c000, 8192): Connection refused

This issue was first observed as a seldom failure in Podman's tests
checking UDP functionality while using pasta(1) to connect the
container's network namespace, which leads us to a reproducer with
the lookup error resulting in an ICMP packet on a tap device:

  LOCAL_ADDR="$(ip -j -4 addr show|jq -rM '.[] | .addr_info[0] | select(.scope == "global").local')"

  while :; do
  	./pasta --config-net -p pasta.pcap -u 1337 socat UDP4-LISTEN:1337,null-eof OPEN:tmp.out,create,trunc &
  	sleep 0.2 || sleep 1
  	socat OPEN:tmp.in UDP4:${LOCAL_ADDR}:1337,shut-null
  	wait
  	cmp tmp.in tmp.out
  done

Once this fails:

  tmp.in tmp.out differ: char 8193, line 29

we can finally have a look at what's going on:

  $ tshark -r pasta.pcap
      1   0.000000           :: ? ff02::16     ICMPv6 110 Multicast Listener Report Message v2
      2   0.168690 88.198.0.161 ? 88.198.0.164 UDP 8234 60260 ? 1337 Len=8192
      3   0.168767 88.198.0.161 ? 88.198.0.164 UDP 8234 60260 ? 1337 Len=8192
      4   0.168806 88.198.0.161 ? 88.198.0.164 UDP 8234 60260 ? 1337 Len=8192
      5   0.168827 c6:47:05:8d:dc:04 ? Broadcast    ARP 42 Who has 88.198.0.161? Tell 88.198.0.164
      6   0.168851 9a:55:9a:55:9a:55 ? c6:47:05:8d:dc:04 ARP 42 88.198.0.161 is at 9a:55:9a:55:9a:55
      7   0.168875 88.198.0.161 ? 88.198.0.164 UDP 8234 60260 ? 1337 Len=8192
      8   0.168896 88.198.0.164 ? 88.198.0.161 ICMP 590 Destination unreachable (Port unreachable)
      9   0.168926 88.198.0.161 ? 88.198.0.164 UDP 8234 60260 ? 1337 Len=8192
     10   0.168959 88.198.0.161 ? 88.198.0.164 UDP 8234 60260 ? 1337 Len=8192
     11   0.168989 88.198.0.161 ? 88.198.0.164 UDP 4138 60260 ? 1337 Len=4096
     12   0.169010 88.198.0.161 ? 88.198.0.164 UDP 42 60260 ? 1337 Len=0

On the third datagram received, the network namespace of the container
initiates an ARP lookup to deliver the ICMP message.

In another variant of this reproducer, starting the client with:

  strace -f pasta --config-net -u 1337 socat UDP4-LISTEN:1337,null-eof OPEN:tmp.out,create,tru
---truncated---