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Message-ID: <643dbfdd431ac_2d6665294a0@willemb.c.googlers.com.notmuch>
Date: Mon, 17 Apr 2023 17:53:33 -0400
From: Willem de Bruijn <willemdebruijn.kernel@...il.com>
To: Kuniyuki Iwashima <kuniyu@...zon.com>,
willemdebruijn.kernel@...il.com
Cc: davem@...emloft.net, edumazet@...gle.com, kuba@...nel.org,
kuni1840@...il.com, kuniyu@...zon.com, netdev@...r.kernel.org,
pabeni@...hat.com, syzkaller@...glegroups.com, willemb@...gle.com
Subject: RE: [PATCH v1 net] udp: Fix memleaks of sk and zerocopy skbs with TX
timestamp.
Kuniyuki Iwashima wrote:
> From: Willem de Bruijn <willemdebruijn.kernel@...il.com>
> Date: Mon, 17 Apr 2023 16:36:20 -0400
> > Kuniyuki Iwashima wrote:
> > > syzkaller reported [0] memory leaks of an UDP socket and ZEROCOPY
> > > skbs. We can reproduce the problem with these sequences:
> > >
> > > sk = socket(AF_INET, SOCK_DGRAM, 0)
> > > sk.setsockopt(SOL_SOCKET, SO_TIMESTAMPING, SOF_TIMESTAMPING_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 skb, 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 clone 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 skb, but struct
> > > ubuf_info_msgzc still has a refcnt, so __msg_zerocopy_callback() is
> > > not called.
> > >
> > > The last refcnt is not released unless we retrieve the TX timestamped
> > > skb by recvmsg(). When we close() the socket holding such skb, we
> > > never call sock_put() and leak the count, skb, and sk.
> > >
> > > To avoid this problem, we must call skb_queue_purge() while we close()
> > > UDP sockets.
> > >
> > > Note that TCP does not have this problem because skb_queue_purge() is
> > > called by sk_stream_kill_queues() during close().
> >
> > Thanks for the clear description.
> >
> > So the issue is that the tx timestamp notification is still queued on
> > the error queue and this is not freed on socket destruction.
> >
> > That surprises me. The error definitely needs to be purged on socket
> > destruction. And it is, called in inet_sock_destruct, which is called
> > udp_destruct_sock.
> >
> > The issue here is that there is a circular dependency, where the
> > sk_destruct is not called until the ref count drops to zero?
>
> Yes, right.
>
> >
> > sk_stream_kill_queues is called from inet_csk_destroy_sock, from
> > __tcp_close (and thus tcp_prot.close) among others.
> >
> > purging the error queue for other sockets on .close rather than
> > .destroy sounds good to me.
> >
> > But SOF_TIMESTAMPING_TX_SOFTWARE and MSG_ZEROCOPY are not limited to
> > TCP and UDP. So we probably need this in a more protocol independent
> > close.
>
> At least, we limit SO_ZEROCOPY to TCP, UDP and RDS for now. Also, RDS
> seems to just use TCP. [0]
>
> ---8<---
> case SO_ZEROCOPY:
> if (sk->sk_family == PF_INET || sk->sk_family == PF_INET6) {
> if (!(sk_is_tcp(sk) ||
> (sk->sk_type == SOCK_DGRAM &&
> sk->sk_protocol == IPPROTO_UDP)))
> ret = -EOPNOTSUPP;
> } else if (sk->sk_family != PF_RDS) {
> ret = -EOPNOTSUPP;
> }
> if (!ret) {
> if (val < 0 || val > 1)
> ret = -EINVAL;
> else
> sock_valbool_flag(sk, SOCK_ZEROCOPY, valbool);
> }
> break;
> ---8<---
>
> [0]: https://lore.kernel.org/netdev/cover.1518718761.git.sowmini.varadhan@oracle.com/
Good point, thanks. I used to experiment with more protocols, as can
be seen from msg_zerocopy.c. But those are thankfully not relevant.
>
> Thanks,
> Kuniyuki
>
>
> >
> > > [0]:
> > > BUG: memory leak
> > > unreferenced object 0xffff88800c6d2d00 (size 1152):
> > > comm "syz-executor392", pid 264, jiffies 4294785440 (age 13.044s)
> > > hex dump (first 32 bytes):
> > > 00 00 00 00 00 00 00 00 cd af e8 81 00 00 00 00 ................
> > > 02 00 07 40 00 00 00 00 00 00 00 00 00 00 00 00 ...@............
> > > backtrace:
> > > [<0000000055636812>] sk_prot_alloc+0x64/0x2a0 net/core/sock.c:2024
> > > [<0000000054d77b7a>] sk_alloc+0x3b/0x800 net/core/sock.c:2083
> > > [<0000000066f3c7e0>] inet_create net/ipv4/af_inet.c:319 [inline]
> > > [<0000000066f3c7e0>] inet_create+0x31e/0xe40 net/ipv4/af_inet.c:245
> > > [<000000009b83af97>] __sock_create+0x2ab/0x550 net/socket.c:1515
> > > [<00000000b9b11231>] sock_create net/socket.c:1566 [inline]
> > > [<00000000b9b11231>] __sys_socket_create net/socket.c:1603 [inline]
> > > [<00000000b9b11231>] __sys_socket_create net/socket.c:1588 [inline]
> > > [<00000000b9b11231>] __sys_socket+0x138/0x250 net/socket.c:1636
> > > [<000000004fb45142>] __do_sys_socket net/socket.c:1649 [inline]
> > > [<000000004fb45142>] __se_sys_socket net/socket.c:1647 [inline]
> > > [<000000004fb45142>] __x64_sys_socket+0x73/0xb0 net/socket.c:1647
> > > [<0000000066999e0e>] do_syscall_x64 arch/x86/entry/common.c:50 [inline]
> > > [<0000000066999e0e>] do_syscall_64+0x38/0x90 arch/x86/entry/common.c:80
> > > [<0000000017f238c1>] entry_SYSCALL_64_after_hwframe+0x63/0xcd
> > >
> > > BUG: memory leak
> > > unreferenced object 0xffff888017633a00 (size 240):
> > > comm "syz-executor392", pid 264, jiffies 4294785440 (age 13.044s)
> > > hex dump (first 32 bytes):
> > > 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
> > > 00 00 00 00 00 00 00 00 00 2d 6d 0c 80 88 ff ff .........-m.....
> > > backtrace:
> > > [<000000002b1c4368>] __alloc_skb+0x229/0x320 net/core/skbuff.c:497
> > > [<00000000143579a6>] alloc_skb include/linux/skbuff.h:1265 [inline]
> > > [<00000000143579a6>] sock_omalloc+0xaa/0x190 net/core/sock.c:2596
> > > [<00000000be626478>] msg_zerocopy_alloc net/core/skbuff.c:1294 [inline]
> > > [<00000000be626478>] msg_zerocopy_realloc+0x1ce/0x7f0 net/core/skbuff.c:1370
> > > [<00000000cbfc9870>] __ip_append_data+0x2adf/0x3b30 net/ipv4/ip_output.c:1037
> > > [<0000000089869146>] ip_make_skb+0x26c/0x2e0 net/ipv4/ip_output.c:1652
> > > [<00000000098015c2>] udp_sendmsg+0x1bac/0x2390 net/ipv4/udp.c:1253
> > > [<0000000045e0e95e>] inet_sendmsg+0x10a/0x150 net/ipv4/af_inet.c:819
> > > [<000000008d31bfde>] sock_sendmsg_nosec net/socket.c:714 [inline]
> > > [<000000008d31bfde>] sock_sendmsg+0x141/0x190 net/socket.c:734
> > > [<0000000021e21aa4>] __sys_sendto+0x243/0x360 net/socket.c:2117
> > > [<00000000ac0af00c>] __do_sys_sendto net/socket.c:2129 [inline]
> > > [<00000000ac0af00c>] __se_sys_sendto net/socket.c:2125 [inline]
> > > [<00000000ac0af00c>] __x64_sys_sendto+0xe1/0x1c0 net/socket.c:2125
> > > [<0000000066999e0e>] do_syscall_x64 arch/x86/entry/common.c:50 [inline]
> > > [<0000000066999e0e>] do_syscall_64+0x38/0x90 arch/x86/entry/common.c:80
> > > [<0000000017f238c1>] entry_SYSCALL_64_after_hwframe+0x63/0xcd
> > >
> > > Fixes: b5947e5d1e71 ("udp: msg_zerocopy")
> > > Reported-by: syzbot <syzkaller@...glegroups.com>
> > > Signed-off-by: Kuniyuki Iwashima <kuniyu@...zon.com>
> > > ---
> > > include/net/udp.h | 5 +++++
> > > 1 file changed, 5 insertions(+)
> > >
> > > diff --git a/include/net/udp.h b/include/net/udp.h
> > > index de4b528522bb..b9182f166b2f 100644
> > > --- a/include/net/udp.h
> > > +++ b/include/net/udp.h
> > > @@ -195,6 +195,11 @@ void udp_lib_rehash(struct sock *sk, u16 new_hash);
> > >
> > > static inline void udp_lib_close(struct sock *sk, long timeout)
> > > {
> > > + /* A zerocopy skb has a refcnt of sk and may be
> > > + * put into sk_error_queue with TX timestamp
> > > + */
> > > + skb_queue_purge(&sk->sk_error_queue);
> > > +
> > > sk_common_release(sk);
> > > }
Does this leave a race, where another completion may be queued between
skb_queue_purge and the eventual sock_put in sk_common_release?
Unfortunately even the sock_set_flag(sk, SOCK_DEAD) is not sufficient
to ensure that no tx completions are subsequently queued for packets
that are in transmission. sk_common_release has a comment
* receive queue, but transmitted packets will delay socket destruction
* until the last reference will be released.
*/
This should equally apply to TCP. Evidentlly it is not a problem
there. I don't entirely follow yet how this is avoided.
MSG_ZEROCOPY notifications are not queued if SOCK_DEAD (first branch
in __msg_zerocopy_callback). Perhaps we need the same in
sock_queue_err_skb?
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