CVE-2024-57974

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 its 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 isnt however a complete fix: the socket will be found once the rehashing completes, but not while its 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 servers 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 Podmans tests checking UDP functionality while using pasta(1) to connect the containers 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 whats 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—

Affected Software

References

• https://git.kernel.org/stable/c/4f8344fce91c5766d368edb0ad80142eacd805c7
• https://git.kernel.org/stable/c/a502ea6fa94b1f7be72a24bcf9e3f5f6b7e6e90c
• https://git.kernel.org/stable/c/d65d3bf309b2649d27b24efd0d8784da2d81f2a6