CVE-2024-57974 Information

Description

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 ( p: 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 ( p: 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:1337null-eof OPEN:tmp.outcreatetrunc & sleep 0.1 || sleep 1 taskset -c 2 socat OPEN:tmp.in UDP4:localhost:1337shut-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=$(ip -j -4 addr show|jq -rM ‘.[] | .addr_info[0] | select(.scope == \global).local’)\n while :; do ./pasta –config-net -p pasta.pcap -u 1337 socat UDP4-LISTEN:1337null-eof OPEN:tmp.outcreatetrunc & sleep 0.2 || sleep 1 socat OPEN:tmp.in UDP4:$LOCAL_ADDR:1337shut-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:1337null-eof OPEN:tmp.outcreatetru

truncated—

Reference

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