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[34.150.157.240]) by smtp.gmail.com with ESMTPSA id l11-20020a056214028b00b005ef4ad380cesm4554099qvv.10.2023.04.19.10.50.48 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 19 Apr 2023 10:50:48 -0700 (PDT) Date: Wed, 19 Apr 2023 13:50:47 -0400 From: Willem de Bruijn To: Kuniyuki Iwashima , willemdebruijn.kernel@gmail.com Cc: davem@davemloft.net, edumazet@google.com, kuba@kernel.org, kuni1840@gmail.com, kuniyu@amazon.com, netdev@vger.kernel.org, pabeni@redhat.com, syzkaller@googlegroups.com, willemb@google.com Message-ID: <644029f7e7814_38cc8429476@willemb.c.googlers.com.notmuch> In-Reply-To: <20230419170727.29740-1-kuniyu@amazon.com> References: <643ff7a7a551f_38347529475@willemb.c.googlers.com.notmuch> <20230419170727.29740-1-kuniyu@amazon.com> Subject: Re: [PATCH v2 net] tcp/udp: Fix memleaks of sk and zerocopy skbs with TX timestamp. Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable Precedence: bulk List-ID: X-Mailing-List: netdev@vger.kernel.org Kuniyuki Iwashima wrote: > From: Willem de Bruijn > Date: Wed, 19 Apr 2023 10:16:07 -0400 > > Eric Dumazet wrote: > > > On Tue, Apr 18, 2023 at 9:25=E2=80=AFPM Kuniyuki Iwashima wrote: > > > > > > > > From: Eric Dumazet > > > > Date: Tue, 18 Apr 2023 21:04:32 +0200 > > > > > On Tue, Apr 18, 2023 at 8:44=E2=80=AFPM Kuniyuki Iwashima wrote: > > > > > > > > > > > > From: Eric Dumazet > > > > > > Date: Tue, 18 Apr 2023 20:33:44 +0200 > > > > > > > On Tue, Apr 18, 2023 at 8:09=E2=80=AFPM Kuniyuki Iwashima <= kuniyu@amazon.com> wrote: > > > > > > > > > > > > > > > > syzkaller reported [0] memory leaks of an UDP socket and = ZEROCOPY > > > > > > > > skbs. We can reproduce the problem with these sequences:= > > > > > > > > > > > > > > > > sk =3D socket(AF_INET, SOCK_DGRAM, 0) > > > > > > > > sk.setsockopt(SOL_SOCKET, SO_TIMESTAMPING, SOF_TIMESTAM= PING_TX_SOFTWARE) > > > > > > > > sk.setsockopt(SOL_SOCKET, SO_ZEROCOPY, 1) > > > > > > > > sk.sendto(b'', MSG_ZEROCOPY, ('127.0.0.1', 53)) > > > > > > > > sk.close() > > > > > > > > > > > > > > > > sendmsg() calls msg_zerocopy_alloc(), which allocates a s= kb, sets > > > > > > > > skb->cb->ubuf.refcnt to 1, and calls sock_hold(). Here, = struct > > > > > > > > ubuf_info_msgzc indirectly holds a refcnt of the socket. = When the > > > > > > > > skb is sent, __skb_tstamp_tx() clones it and puts the clo= ne into > > > > > > > > the socket's error queue with the TX timestamp. > > > > > > > > > > > > > > > > When the original skb is received locally, skb_copy_ubufs= () calls > > > > > > > > skb_unclone(), and pskb_expand_head() increments skb->cb-= >ubuf.refcnt. > > > > > > > > This additional count is decremented while freeing the sk= b, but struct > > > > > > > > ubuf_info_msgzc still has a refcnt, so __msg_zerocopy_cal= lback() is > > > > > > > > not called. > > > > > > > > > > > > > > > > The last refcnt is not released unless we retrieve the TX= timestamped > > > > > > > > skb by recvmsg(). When we close() the socket holding suc= h skb, we > > > > > > > > never call sock_put() and leak the count, skb, and sk. > > > > > > > > > > > > > > > > To avoid this problem, we must (i) call skb_queue_purge()= after > > > > > > > > flagging SOCK_DEAD during close() and (ii) make sure that= TX tstamp > > > > > > > > skb is not queued when SOCK_DEAD is flagged. UDP lacks (= i) and (ii), > > > > > > > > and TCP lacks (ii). > > > > > > > > > > > > > > > > Without (ii), a skb queued in a qdisc or device could be = put into > > > > > > > > the error queue after skb_queue_purge(). > > > > > > > > > > > > > > > > sendmsg() /* return immediately, but packets > > > > > > > > * are queued in a qdisc or device > > > > > > > > */ > > > > > > > > close() > > > > > > > > skb_queue_purge() > > > > > > > > __skb_tstamp_tx() > > > > > > > > __skb_complete_tx_timestamp() > > > > > > > > sock_queue_err_skb() > > > > > > > > skb_queue_tail() > > > > > > > > > > > > > > > > Also, we need to check SOCK_DEAD under sk->sk_error_queue= .lock > > > > > > > > in sock_queue_err_skb() to avoid this race. > > > > > > > > > > > > > > > > if (!sock_flag(sk, SOCK_DEAD)) > > > > > > > > sock_set_flag(sk, SOC= K_DEAD) > > > > > > > > skb_queue_purge() > > > > > > > > > > > > > > > > skb_queue_tail() > > > > > > > > > > > > > > > > [0]: > > > > > > > > > > > > > > > Fixes: f214f915e7db ("tcp: enable MSG_ZEROCOPY") > > > > > > > > Fixes: b5947e5d1e71 ("udp: msg_zerocopy") > > > > > > > > Reported-by: syzbot > > > > > > > > Signed-off-by: Kuniyuki Iwashima > > > > > > > > --- > > > > > > > > v2: > > > > > > > > * Move skb_queue_purge() after setting SOCK_DEAD in udp= _destroy_sock() > > > > > > > > * Check SOCK_DEAD in sock_queue_err_skb() with sk_error= _queue.lock > > > > > > > > * Add Fixes tag for TCP > > > > > > > > > > > > > > > > v1: https://lore.kernel.org/netdev/20230417171155.22916-1= -kuniyu@amazon.com/ > > > > > > > > --- > > > > > > > > net/core/skbuff.c | 15 ++++++++++++--- > > > > > > > > net/ipv4/udp.c | 5 +++++ > > > > > > > > 2 files changed, 17 insertions(+), 3 deletions(-) > > > > > > > > > > > > > > > > diff --git a/net/core/skbuff.c b/net/core/skbuff.c > > > > > > > > index 4c0879798eb8..287b834df9c8 100644 > > > > > > > > --- a/net/core/skbuff.c > > > > > > > > +++ b/net/core/skbuff.c > > > > > > > > @@ -4979,6 +4979,8 @@ static void skb_set_err_queue(struc= t sk_buff *skb) > > > > > > > > */ > > > > > > > > int sock_queue_err_skb(struct sock *sk, struct sk_buff *= skb) > > > > > > > > { > > > > > > > > + unsigned long flags; > > > > > > > > + > > > > > > > > if (atomic_read(&sk->sk_rmem_alloc) + skb->truesi= ze >=3D > > > > > > > > (unsigned int)READ_ONCE(sk->sk_rcvbuf)) > > > > > > > > return -ENOMEM; > > > > > > > > @@ -4992,9 +4994,16 @@ int sock_queue_err_skb(struct sock= *sk, struct sk_buff *skb) > > > > > > > > /* before exiting rcu section, make sure dst is r= efcounted */ > > > > > > > > skb_dst_force(skb); > > > > > > > > > > > > > > > > - skb_queue_tail(&sk->sk_error_queue, skb); > > > > > > > > - if (!sock_flag(sk, SOCK_DEAD)) > > > > > > > > - sk_error_report(sk); > > > > > > > > + spin_lock_irqsave(&sk->sk_error_queue.lock, flags= ); > > > > > > > > + if (sock_flag(sk, SOCK_DEAD)) { > > > > > > > > > > > > > > SOCK_DEAD is set without holding sk_error_queue.lock, so I = wonder why you > > > > > > > want to add a confusing construct. > > > > > > > > > > > > > > Just bail early ? > > > > > > > > > > > > > > diff --git a/net/core/skbuff.c b/net/core/skbuff.c > > > > > > > index ef81452759be3fd251faaf76d89cfd002ee79256..fda05cb44f9= 5821e98f8c5c05fba840a9d276abb > > > > > > > 100644 > > > > > > > --- a/net/core/skbuff.c > > > > > > > +++ b/net/core/skbuff.c > > > > > > > @@ -4983,6 +4983,9 @@ int sock_queue_err_skb(struct sock *s= k, struct > > > > > > > sk_buff *skb) > > > > > > > (unsigned int)READ_ONCE(sk->sk_rcvbuf)) > > > > > > > return -ENOMEM; > > > > > > > > > > > > > > + if (sock_flag(sk, SOCK_DEAD)) > > > > > > > + return -EINVAL; > > > > > > > + > > > > > > > > > > > > Isn't it possible that these sequences happen > > > > > > > > > > > > close() > > > > > > sock_set_flag(sk, SOCK_DEAD); > > > > > > skb_queue_purge(&sk->sk_error_queue) > > > > > > > > > > > > between the skb_queue_tail() below ? (2nd race mentioned in c= hangelog) > > > > > > > > > > > > I thought we can guarantee the ordering by taking the same lo= ck. > > > > > > > > > > > > > > > > This is fragile. > > > > > > > > Yes, but I didn't have better idea to avoid the race... > > > > > > > > > > > > > > We could very well rewrite skb_queue_purge() to not acquire the= lock > > > > > in the common case. > > > > > I had the following in my tree for a while, to avoid many atomi= c and > > > > > irq masking operations... > > > > > > > > Cool, and it still works with my patch, no ? > > > > > > > = > > > = > > > Really the only thing that ensures a race is not possible is the > > > typical sk_refcnt acquisition. > > > = > > > But I do not see why an skb stored in error_queue should keep the > > > refcnt on the socket. > > > This seems like a chicken and egg problem, and caused various issue= s > > > in the past, > > > see for instance [1] > > > = > > > We better make sure error queue is purged at socket dismantle (afte= r > > > refcnt reached 0) > > = > > The problem here is that the timestamp queued on the error queue > > holds a reference on a ubuf if MSG_ZEROCOPY and that ubuf holds an > > sk_ref. > > = > > The timestamped packet may contain packet contents, so the ubuf > > ref is not superfluous. > > = > > Come to think of it, we've always maintained that zerocopy packets > > should not be looped to sockets where they can be queued indefinitely= , > > including packet sockets. > > = > > If we enforce that for these tx timestamps too, then that also > > solves this issue. > > = > > A process that wants efficient MSG_ZEROCOPY will have to request > > timestamping with SOF_TIMESTAMPING_OPT_TSONLY to avoid returning the > > data along with the timestamp. > = > Actually, my first attempt was similar to this that avoids skb_clone() > silently if MSG_ZEROCOPY, but this kind of way could break users who > were using tstamp and just added MSG_ZEROCOPY logic to their app, so > I placed skb_queue_purge() during close(). > = > ---8<--- > diff --git a/net/core/skbuff.c b/net/core/skbuff.c > index eb7d33b41e71..9318b438888e 100644 > --- a/net/core/skbuff.c > +++ b/net/core/skbuff.c > @@ -5135,7 +5149,7 @@ void __skb_tstamp_tx(struct sk_buff *orig_skb, > if (!skb_may_tx_timestamp(sk, tsonly)) > return; > = > - if (tsonly) { > + if (tsonly || skb_zcopy(orig_skb)) { > #ifdef CONFIG_INET > if ((sk->sk_tsflags & SOF_TIMESTAMPING_OPT_STATS) && > sk_is_tcp(sk)) { > ---8<--- Actually, the skb_clone in __skb_tstamp_tx should already release the reference on the ubuf. With the same mechanism that we rely on for packet sockets, e.g., in dev_queue_xmit_nit. skb_clone calls skb_orphan_frags calls skb_copy_ubufs for zerocopy skbs. Which creates a copy of the data and calls skb_zcopy_clear. The skb that gets queued onto the error queue should not have a reference on an ubuf: skb_zcopy(skb) should return NULL. > = > > = > > > I suggest we generalize fix that went in this patch : > > > = > > > dd4f10722aeb10f4f582948839f066bebe44e5fb net: fix socket refcountin= g > > > in skb_complete_wifi_ack() > > > = > > > Instead of adding yet another rule about a queue lock, and a socket= > > > flag, this would keep things tied to sk_refcnt. > > > = > > > Also I do not think TCP has an issue, please take a look at > > > = > > > [1] > > > commit e0c8bccd40fc1c19e1d246c39bcf79e357e1ada3 net: stream: pur= ge > > > sk_error_queue in sk_stream_kill_queues() > > > = > > > (A leak in TCP would be caught in a few hours by syzbot really) > = > I have tested TCP before posting v1 and found this skb_queue_purge() > was to prevent the same issue, but Willem's point sounds reasonable > that packets queued in qdisc or device could be put in the error queue > after close() completes. That's why I mentioned TCP since v2.