Re: [PATCH net-next v2] udp: Deal with race between UDP socket address change and rehash

Eric Dumazet
2024-12-20T14:16:55Z

On Wed, Dec 18, 2024 at 5:21 PM Stefano Brivio <sbrivio@redhat.com> wrote:
>
> 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,trunc 2>strace.log &
>
> and connecting to the socat server using a loopback address:
>
> socat OPEN:tmp.in UDP4:localhost:1337,shut-null
>
> we can more clearly observe a sendmmsg() call failing after the
> first datagram is delivered:
>
> [pid 278012] connect(173, 0x7fff96c95fc0, 16) = 0
> [...]
> [pid 278012] recvmmsg(173, 0x7fff96c96020, 1024, MSG_DONTWAIT, NULL) = -1 EAGAIN (Resource temporarily unavailable)
> [pid 278012] sendmmsg(173, 0x561c5ad0a720, 1, MSG_NOSIGNAL) = 1
> [...]
> [pid 278012] sendmmsg(173, 0x561c5ad0a720, 1, MSG_NOSIGNAL) = -1 ECONNREFUSED (Connection refused)
>
> and, somewhat confusingly, after a connect() on the same socket
> succeeded.
>
> Until commit 4cdeeee9252a ("net: udp: prefer listeners bound to an
> address"), the race between receive address change and lookup didn't
> actually cause visible issues, because, once the lookup based on the
> secondary hash chain failed, we would still attempt a lookup based on
> the primary hash (destination port only), and find the socket with the
> outdated secondary hash.
>
> That change, however, dropped port-only lookups altogether, as side
> effect, making the race visible.
>
> To fix this, while avoiding the need to make address changes and
> rehash atomic against lookups, reintroduce primary hash lookups as
> fallback, if lookups based on four-tuple and secondary hashes fail.
>
> To this end, introduce a simplified lookup implementation, which
> doesn't take care of SO_REUSEPORT groups: if we have one, there are
> multiple sockets that would match the four-tuple or secondary hash,
> meaning that we can't run into this race at all.
>
> v2:
> - instead of synchronising lookup operations against address change
> plus rehash, reintroduce a simplified version of the original
> primary hash lookup as fallback
>
> v1:
> - fix build with CONFIG_IPV6=n: add ifdef around sk_v6_rcv_saddr
> usage (Kuniyuki Iwashima)
> - directly use sk_rcv_saddr for IPv4 receive addresses instead of
> fetching inet_rcv_saddr (Kuniyuki Iwashima)
> - move inet_update_saddr() to inet_hashtables.h and use that
> to set IPv4/IPv6 addresses as suitable (Kuniyuki Iwashima)
> - rebase onto net-next, update commit message accordingly
>
> Reported-by: Ed Santiago <santiago@redhat.com>
> Link: https://github.com/containers/podman/issues/24147
> Analysed-by: David Gibson <david@gibson.dropbear.id.au>
> Fixes: 30fff9231fad ("udp: bind() optimisation")
> Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
> ---

I think this should work. Another solution would have been to add a
sequence to each UDP socket.

Fixes: tag probably could refer to 4cdeeee9252a ("net: udp: prefer
listeners bound to an address"), because your patch
is partially kind-of reverting it.

Reviewed-by: Eric Dumazet <edumazet@google.com>

I will post additional patches for net-next to better take care of
data-races in compute_score()