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

[Stefano Brivio <sbrivio@redhat.com>]

On Fri, 20 Dec 2024 15:16:42 +0100
Eric Dumazet <edumazet@google.com> wrote:

> 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.

I was actually a bit undecided because, conceptually, the race condition
itself was added by 30fff9231fad. On the other hand, it can't really be
called a race without 4cdeeee9252a, because by itself it was a mere
optimisation not affecting the result of the lookup.

And on a second thought, perhaps more relevant for backports, there's
no issue without 4cdeeee9252a. So yeah, I guess you're right, the tag
should be amended to:

Fixes: 4cdeeee9252a ("net: udp: prefer listeners bound to an address")

> 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()

Ah, right, thanks, those are potentially nasty as well.

-- 
Stefano