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Date: Thu, 21 Sep 2017 10:12:14 -0700
From: Martin KaFai Lau <kafai@...com>
To: Eric Dumazet <eric.dumazet@...il.com>
CC: Paweł Staszewski <pstaszewski@...are.pl>,
Wei Wang <weiwan@...gle.com>,
Cong Wang <xiyou.wangcong@...il.com>,
"Linux Kernel Network Developers" <netdev@...r.kernel.org>,
Eric Dumazet <edumazet@...gle.com>
Subject: Re: [PATCH net] net: prevent dst uses after free
On Thu, Sep 21, 2017 at 04:15:46PM +0000, Eric Dumazet wrote:
> From: Eric Dumazet <edumazet@...gle.com>
>
> In linux-4.13, Wei worked hard to convert dst to a traditional
> refcounted model, removing GC.
>
> We now want to make sure a dst refcount can not transition from 0 back
> to 1.
>
> The problem here is that input path attached a not refcounted dst to an
> skb. Then later, because packet is forwarded and hits skb_dst_force()
> before exiting RCU section, we might try to take a refcount on one dst
> that is about to be freed, if another cpu saw 1 -> 0 transition in
> dst_release() and queued the dst for freeing after one RCU grace period.
>
> Lets unify skb_dst_force() and skb_dst_force_safe(), since we should
> always perform the complete check against dst refcount, and not assume
> it is not zero.
Acked-by: Martin KaFai Lau <kafai@...com>
>
> Bugzilla : https://bugzilla.kernel.org/show_bug.cgi?id=197005
>
> [ 989.919496] skb_dst_force+0x32/0x34
> [ 989.919498] __dev_queue_xmit+0x1ad/0x482
> [ 989.919501] ? eth_header+0x28/0xc6
> [ 989.919502] dev_queue_xmit+0xb/0xd
> [ 989.919504] neigh_connected_output+0x9b/0xb4
> [ 989.919507] ip_finish_output2+0x234/0x294
> [ 989.919509] ? ipt_do_table+0x369/0x388
> [ 989.919510] ip_finish_output+0x12c/0x13f
> [ 989.919512] ip_output+0x53/0x87
> [ 989.919513] ip_forward_finish+0x53/0x5a
> [ 989.919515] ip_forward+0x2cb/0x3e6
> [ 989.919516] ? pskb_trim_rcsum.part.9+0x4b/0x4b
> [ 989.919518] ip_rcv_finish+0x2e2/0x321
> [ 989.919519] ip_rcv+0x26f/0x2eb
> [ 989.919522] ? vlan_do_receive+0x4f/0x289
> [ 989.919523] __netif_receive_skb_core+0x467/0x50b
> [ 989.919526] ? tcp_gro_receive+0x239/0x239
> [ 989.919529] ? inet_gro_receive+0x226/0x238
> [ 989.919530] __netif_receive_skb+0x4d/0x5f
> [ 989.919532] netif_receive_skb_internal+0x5c/0xaf
> [ 989.919533] napi_gro_receive+0x45/0x81
> [ 989.919536] ixgbe_poll+0xc8a/0xf09
> [ 989.919539] ? kmem_cache_free_bulk+0x1b6/0x1f7
> [ 989.919540] net_rx_action+0xf4/0x266
> [ 989.919543] __do_softirq+0xa8/0x19d
> [ 989.919545] irq_exit+0x5d/0x6b
> [ 989.919546] do_IRQ+0x9c/0xb5
> [ 989.919548] common_interrupt+0x93/0x93
> [ 989.919548] </IRQ>
>
>
> Similarly dst_clone() can use dst_hold() helper to have additional
> debugging, as a follow up to commit 44ebe79149ff ("net: add debug
> atomic_inc_not_zero() in dst_hold()")
>
> In net-next we will convert dst atomic_t to refcount_t for peace of
> mind.
>
> Fixes: a4c2fd7f7891 ("net: remove DST_NOCACHE flag")
> Signed-off-by: Eric Dumazet <edumazet@...gle.com>
> Cc: Wei Wang <weiwan@...gle.com>
> Reported-by: Paweł Staszewski <pstaszewski@...are.pl>
> Bisected-by: Paweł Staszewski <pstaszewski@...are.pl>
> ---
> include/net/dst.h | 22 ++++------------------
> include/net/route.h | 2 +-
> include/net/sock.h | 2 +-
> 3 files changed, 6 insertions(+), 20 deletions(-)
>
> diff --git a/include/net/dst.h b/include/net/dst.h
> index 93568bd0a3520bb7402f04d90cf04ac99c81cfbe..06a6765da074449e6f1fe42ee05e711e898ad372 100644
> --- a/include/net/dst.h
> +++ b/include/net/dst.h
> @@ -271,7 +271,7 @@ static inline void dst_use_noref(struct dst_entry *dst, unsigned long time)
> static inline struct dst_entry *dst_clone(struct dst_entry *dst)
> {
> if (dst)
> - atomic_inc(&dst->__refcnt);
> + dst_hold(dst);
> return dst;
> }
>
> @@ -311,21 +311,6 @@ static inline void skb_dst_copy(struct sk_buff *nskb, const struct sk_buff *oskb
> __skb_dst_copy(nskb, oskb->_skb_refdst);
> }
>
> -/**
> - * skb_dst_force - makes sure skb dst is refcounted
> - * @skb: buffer
> - *
> - * If dst is not yet refcounted, let's do it
> - */
> -static inline void skb_dst_force(struct sk_buff *skb)
> -{
> - if (skb_dst_is_noref(skb)) {
> - WARN_ON(!rcu_read_lock_held());
> - skb->_skb_refdst &= ~SKB_DST_NOREF;
> - dst_clone(skb_dst(skb));
> - }
> -}
> -
> /**
> * dst_hold_safe - Take a reference on a dst if possible
> * @dst: pointer to dst entry
> @@ -339,16 +324,17 @@ static inline bool dst_hold_safe(struct dst_entry *dst)
> }
>
> /**
> - * skb_dst_force_safe - makes sure skb dst is refcounted
> + * skb_dst_force - makes sure skb dst is refcounted
> * @skb: buffer
> *
> * If dst is not yet refcounted and not destroyed, grab a ref on it.
> */
> -static inline void skb_dst_force_safe(struct sk_buff *skb)
> +static inline void skb_dst_force(struct sk_buff *skb)
> {
> if (skb_dst_is_noref(skb)) {
> struct dst_entry *dst = skb_dst(skb);
>
> + WARN_ON(!rcu_read_lock_held());
> if (!dst_hold_safe(dst))
> dst = NULL;
>
> diff --git a/include/net/route.h b/include/net/route.h
> index 1b09a9368c68d46f0c5ee8ce3cefe566000c1ec1..57dfc6850d378e4b96f13b140eef554d66c24cdf 100644
> --- a/include/net/route.h
> +++ b/include/net/route.h
> @@ -190,7 +190,7 @@ static inline int ip_route_input(struct sk_buff *skb, __be32 dst, __be32 src,
> rcu_read_lock();
> err = ip_route_input_noref(skb, dst, src, tos, devin);
> if (!err) {
> - skb_dst_force_safe(skb);
> + skb_dst_force(skb);
> if (!skb_dst(skb))
> err = -EINVAL;
> }
> diff --git a/include/net/sock.h b/include/net/sock.h
> index 03a362568357acc7278a318423dd3873103f90ca..a6b9a8d1a6df3f72df8f1aac0f577257fa6452d0 100644
> --- a/include/net/sock.h
> +++ b/include/net/sock.h
> @@ -856,7 +856,7 @@ void sk_stream_write_space(struct sock *sk);
> static inline void __sk_add_backlog(struct sock *sk, struct sk_buff *skb)
> {
> /* dont let skb dst not refcounted, we are going to leave rcu lock */
> - skb_dst_force_safe(skb);
> + skb_dst_force(skb);
>
> if (!sk->sk_backlog.tail)
> sk->sk_backlog.head = skb;
>
>
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