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[2a10:fc81:a806:d6a9::1]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-4364b1516bbsm75716325e9.1.2024.12.20.07.05.52 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 20 Dec 2024 07:05:52 -0800 (PST) Date: Fri, 20 Dec 2024 16:05:50 +0100 From: Stefano Brivio To: Eric Dumazet Cc: Willem de Bruijn , Paolo Abeni , netdev@vger.kernel.org, Kuniyuki Iwashima , Mike Manning , David Gibson , Paul Holzinger , Philo Lu , Cambda Zhu , Fred Chen , Yubing Qiu , Peter Oskolkov Subject: Re: [PATCH net-next v2] udp: Deal with race between UDP socket address change and rehash Message-ID: <20241220160550.74c1c7e0@elisabeth> In-Reply-To: References: <20241218162116.681734-1-sbrivio@redhat.com> Organization: Red Hat X-Mailer: Claws Mail 4.2.0 (GTK 3.24.41; x86_64-pc-linux-gnu) Precedence: bulk X-Mailing-List: netdev@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable On Fri, 20 Dec 2024 15:16:42 +0100 Eric Dumazet wrote: > On Wed, Dec 18, 2024 at 5:21=E2=80=AFPM Stefano Brivio 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=3D$(($(cat /proc/sys/net/core/wmem_default) / 4)) > > dd if=3D/dev/urandom bs=3D1 count=3D${LEN} of=3Dtmp.in > > > > while :; do > > taskset -c 1 socat UDP4-LISTEN:1337,null-eof OPEN:tmp.out,creat= e,trunc & > > sleep 0.1 || sleep 1 > > taskset -c 2 socat OPEN:tmp.in UDP4:localhost:1337,shut-null > > wait > > done > > > > where the client will eventually get ECONNREFUSED on a write() > > (typically the second or third one of a given iteration): > > > > 2024/11/13 21:28:23 socat[46901] E write(6, 0x556db2e3c000, 8192): Co= nnection refused > > > > This issue was first observed as a seldom failure in Podman's tests > > checking UDP functionality while using pasta(1) to connect the > > container's network namespace, which leads us to a reproducer with > > the lookup error resulting in an ICMP packet on a tap device: > > > > LOCAL_ADDR=3D"$(ip -j -4 addr show|jq -rM '.[] | .addr_info[0] | sele= ct(.scope =3D=3D "global").local')" > > > > while :; do > > ./pasta --config-net -p pasta.pcap -u 1337 socat UDP4-LISTEN:13= 37,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 Lis= tener Report Message v2 > > 2 0.168690 88.198.0.161 ? 88.198.0.164 UDP 8234 60260 ? 1337 Le= n=3D8192 > > 3 0.168767 88.198.0.161 ? 88.198.0.164 UDP 8234 60260 ? 1337 Le= n=3D8192 > > 4 0.168806 88.198.0.161 ? 88.198.0.164 UDP 8234 60260 ? 1337 Le= n=3D8192 > > 5 0.168827 c6:47:05:8d:dc:04 ? Broadcast ARP 42 Who has 88.1= 98.0.161? 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 Le= n=3D8192 > > 8 0.168896 88.198.0.164 ? 88.198.0.161 ICMP 590 Destination unr= eachable (Port unreachable) > > 9 0.168926 88.198.0.161 ? 88.198.0.164 UDP 8234 60260 ? 1337 Le= n=3D8192 > > 10 0.168959 88.198.0.161 ? 88.198.0.164 UDP 8234 60260 ? 1337 Le= n=3D8192 > > 11 0.168989 88.198.0.161 ? 88.198.0.164 UDP 4138 60260 ? 1337 Le= n=3D4096 > > 12 0.169010 88.198.0.161 ? 88.198.0.164 UDP 42 60260 ? 1337 Len= =3D0 > > > > 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) =3D 0 > > [...] > > [pid 278012] recvmmsg(173, 0x7fff96c96020, 1024, MSG_DONTWAIT, NULL) = =3D -1 EAGAIN (Resource temporarily unavailable) > > [pid 278012] sendmmsg(173, 0x561c5ad0a720, 1, MSG_NOSIGNAL) =3D 1 > > [...] > > [pid 278012] sendmmsg(173, 0x561c5ad0a720, 1, MSG_NOSIGNAL) =3D -1 EC= ONNREFUSED (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=3Dn: 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 > > Link: https://github.com/containers/podman/issues/24147 > > Analysed-by: David Gibson > > Fixes: 30fff9231fad ("udp: bind() optimisation") > > Signed-off-by: Stefano Brivio > > --- =20 >=20 > I think this should work. Another solution would have been to add a > sequence to each UDP socket. >=20 > 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 >=20 > 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. --=20 Stefano