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Date: Wed, 19 Apr 2023 11:34:11 +0200
From: Eric Dumazet <edumazet@...gle.com>
To: Kuniyuki Iwashima <kuniyu@...zon.com>
Cc: davem@...emloft.net, kuba@...nel.org, kuni1840@...il.com,
netdev@...r.kernel.org, pabeni@...hat.com,
syzkaller@...glegroups.com, willemb@...gle.com
Subject: Re: [PATCH v2 net] tcp/udp: Fix memleaks of sk and zerocopy skbs with
TX timestamp.
On Tue, Apr 18, 2023 at 9:25 PM Kuniyuki Iwashima <kuniyu@...zon.com> wrote:
>
> From: Eric Dumazet <edumazet@...gle.com>
> Date: Tue, 18 Apr 2023 21:04:32 +0200
> > On Tue, Apr 18, 2023 at 8:44 PM Kuniyuki Iwashima <kuniyu@...zon.com> wrote:
> > >
> > > From: Eric Dumazet <edumazet@...gle.com>
> > > Date: Tue, 18 Apr 2023 20:33:44 +0200
> > > > On Tue, Apr 18, 2023 at 8:09 PM Kuniyuki Iwashima <kuniyu@...zon.com> 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 (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, SOCK_DEAD)
> > > > > skb_queue_purge()
> > > > >
> > > > > skb_queue_tail()
> > > > >
> > > > > [0]:
> > > >
> > > > > Fixes: f214f915e7db ("tcp: enable MSG_ZEROCOPY")
> > > > > Fixes: b5947e5d1e71 ("udp: msg_zerocopy")
> > > > > Reported-by: syzbot <syzkaller@...glegroups.com>
> > > > > Signed-off-by: Kuniyuki Iwashima <kuniyu@...zon.com>
> > > > > ---
> > > > > 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(struct 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->truesize >=
> > > > > (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 refcounted */
> > > > > 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..fda05cb44f95821e98f8c5c05fba840a9d276abb
> > > > 100644
> > > > --- a/net/core/skbuff.c
> > > > +++ b/net/core/skbuff.c
> > > > @@ -4983,6 +4983,9 @@ int sock_queue_err_skb(struct sock *sk, 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 changelog)
> > >
> > > I thought we can guarantee the ordering by taking the same lock.
> > >
> >
> > 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 atomic 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 issues
in the past,
see for instance [1]
We better make sure error queue is purged at socket dismantle (after
refcnt reached 0)
I suggest we generalize fix that went in this patch :
dd4f10722aeb10f4f582948839f066bebe44e5fb net: fix socket refcounting
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: purge
sk_error_queue in sk_stream_kill_queues()
(A leak in TCP would be caught in a few hours by syzbot really)
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