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Date: Tue, 18 Apr 2023 20:33:44 +0200
From: Eric Dumazet <edumazet@...gle.com>
To: Kuniyuki Iwashima <kuniyu@...zon.com>
Cc: "David S. Miller" <davem@...emloft.net>,
Jakub Kicinski <kuba@...nel.org>,
Paolo Abeni <pabeni@...hat.com>,
Willem de Bruijn <willemb@...gle.com>,
Kuniyuki Iwashima <kuni1840@...il.com>, netdev@...r.kernel.org,
syzbot <syzkaller@...glegroups.com>
Subject: Re: [PATCH v2 net] tcp/udp: Fix memleaks of sk and zerocopy skbs with
TX timestamp.
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;
+
skb_orphan(skb);
skb->sk = sk;
skb->destructor = sock_rmem_free;
@@ -4993,8 +4996,7 @@ int sock_queue_err_skb(struct sock *sk, struct
sk_buff *skb)
skb_dst_force(skb);
skb_queue_tail(&sk->sk_error_queue, skb);
- if (!sock_flag(sk, SOCK_DEAD))
- sk_error_report(sk);
+ sk_error_report(sk);
return 0;
}
EXPORT_SYMBOL(sock_queue_err_skb);
> + spin_unlock_irqrestore(&sk->sk_error_queue.lock, flags);
> + return -EINVAL;
> + }
> + __skb_queue_tail(&sk->sk_error_queue, skb);
> + spin_unlock_irqrestore(&sk->sk_error_queue.lock, flags);
> +
> + sk_error_report(sk);
> +
> return 0;
> }
> EXPORT_SYMBOL(sock_queue_err_skb);
> diff --git a/net/ipv4/udp.c b/net/ipv4/udp.c
> index c605d171eb2d..7060a5cda711 100644
> --- a/net/ipv4/udp.c
> +++ b/net/ipv4/udp.c
> @@ -2674,6 +2674,11 @@ void udp_destroy_sock(struct sock *sk)
> if (up->encap_enabled)
> static_branch_dec(&udp_encap_needed_key);
> }
> +
> + /* 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);
> }
>
> /*
> --
> 2.30.2
>
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