Re: [PATCH 5.4] netns: protect netns ID lookups with RCU

[Haakon Bugge]

Please disregard, wrong version in $Subject.


Thxs, Håkon



> On 11 Sep 2021, at 15:58, Håkon Bugge <haakon.bugge@xxxxxxxxxx> wrote:
> 
> From: Guillaume Nault <gnault@xxxxxxxxxx>
> 
> __peernet2id() can be protected by RCU as it only calls idr_for_each(),
> which is RCU-safe, and never modifies the nsid table.
> 
> rtnl_net_dumpid() can also do lockless lookups. It does two nested
> idr_for_each() calls on nsid tables (one direct call and one indirect
> call because of rtnl_net_dumpid_one() calling __peernet2id()). The
> netnsid tables are never updated. Therefore it is safe to not take the
> nsid_lock and run within an RCU-critical section instead.
> 
> Signed-off-by: Guillaume Nault <gnault@xxxxxxxxxx>
> Signed-off-by: David S. Miller <davem@xxxxxxxxxxxxx>
> 
> A nice side-effect of replacing spin_{lock,unlock}_bh() with
> rcu_spin_{lock,unlock}() in peernet2id() is that it avoids the
> situation where SoftIRQs get enabled whilst IRQs are turned off.
> 
> From bugzilla.redhat.com/show_bug.cgi?id=1384179 (an ancient
> 4.9.0-0.rc0 kernel):
> 
> dump_stack+0x86/0xc3
> __warn+0xcb/0xf0
> warn_slowpath_null+0x1d/0x20
> __local_bh_enable_ip+0x9d/0xc0
> _raw_spin_unlock_bh+0x35/0x40
> peernet2id+0x54/0x80
> netlink_broadcast_filtered+0x220/0x3c0
> netlink_broadcast+0x1d/0x20
> audit_log+0x6a/0x90
> security_set_bools+0xee/0x200
> []
> 
> Note, security_set_bools() calls write_lock_irq(). peernet2id() calls
> spin_unlock_bh().
> 
> From an internal (UEK) stack trace based on the v4.14.35 kernel (LTS
> 4.14.231):
> 
> queued_spin_lock_slowpath+0xb/0xf
> _raw_spin_lock_irqsave+0x46/0x48
> send_mad+0x3d2/0x590 [ib_core]
> ib_sa_path_rec_get+0x223/0x4d0 [ib_core]
> path_rec_start+0xa3/0x140 [ib_ipoib]
> ipoib_start_xmit+0x2b0/0x6a0 [ib_ipoib]
> dev_hard_start_xmit+0xb2/0x237
> sch_direct_xmit+0x114/0x1bf
> __dev_queue_xmit+0x592/0x818
> dev_queue_xmit+0x10/0x12
> arp_xmit+0x38/0xa6
> arp_send_dst.part.16+0x61/0x84
> arp_process+0x825/0x889
> arp_rcv+0x140/0x1c9
> __netif_receive_skb_core+0x401/0xb39
> __netif_receive_skb+0x18/0x59
> netif_receive_skb_internal+0x45/0x119
> napi_gro_receive+0xd8/0xf6
> ipoib_ib_handle_rx_wc+0x1ca/0x520 [ib_ipoib]
> ipoib_poll+0xcd/0x150 [ib_ipoib]
> net_rx_action+0x289/0x3f4
> __do_softirq+0xe1/0x2b5
> do_softirq_own_stack+0x2a/0x35
> </IRQ>
> do_softirq+0x4d/0x6a
> __local_bh_enable_ip+0x57/0x59
> _raw_spin_unlock_bh+0x23/0x25
> peernet2id+0x51/0x73
> netlink_broadcast_filtered+0x223/0x41b
> netlink_broadcast+0x1d/0x1f
> rdma_nl_multicast+0x22/0x30 [ib_core]
> send_mad+0x3e5/0x590 [ib_core]
> ib_sa_path_rec_get+0x223/0x4d0 [ib_core]
> rdma_resolve_route+0x287/0x810 [rdma_cm]
> rds_rdma_cm_event_handler_cmn+0x311/0x7d0 [rds_rdma]
> rds_rdma_cm_event_handler_worker+0x22/0x30 [rds_rdma]
> process_one_work+0x169/0x3a6
> worker_thread+0x4d/0x3e5
> kthread+0x105/0x138
> ret_from_fork+0x24/0x49
> 
> Here, pay attention to ib_nl_make_request() which calls
> spin_lock_irqsave() on a global lock just before calling
> rdma_nl_multicast(). Thereafter, peernet2id() enables SoftIRQs, and
> ipoib starts and calls the same path and ends up trying to acquire the
> same global lock again.
> 
> (cherry picked from commit 2dce224f469f060b9998a5a869151ef83c08ce77)
> 
> Fixes: fba143c66abb ("netns: avoid disabling irq for netns id")
> Signed-off-by: Håkon Bugge <haakon.bugge@xxxxxxxxxx>
> 
> Conflicts:
> 	net/core/net_namespace.c
> 
> 		* rtnl_valid_dump_net_req() has a very minimal
>                  implementation in 4.19, hence only a simple
>                  substituting of spin_{lock,unlock}_bh() with
>                  rcu_spin_{lock,unlock}() was required
> ---
> net/core/net_namespace.c | 18 ++++++++++--------
> 1 file changed, 10 insertions(+), 8 deletions(-)
> 
> diff --git a/net/core/net_namespace.c b/net/core/net_namespace.c
> index 939d8a3..26d70c0 100644
> --- a/net/core/net_namespace.c
> +++ b/net/core/net_namespace.c
> @@ -192,9 +192,9 @@ static int net_eq_idr(int id, void *net, void *peer)
> 	return 0;
> }
> 
> -/* Should be called with nsid_lock held. If a new id is assigned, the bool alloc
> - * is set to true, thus the caller knows that the new id must be notified via
> - * rtnl.
> +/* Must be called from RCU-critical section or with nsid_lock held. If
> + * a new id is assigned, the bool alloc is set to true, thus the
> + * caller knows that the new id must be notified via rtnl.
>  */
> static int __peernet2id_alloc(struct net *net, struct net *peer, bool *alloc)
> {
> @@ -218,7 +218,7 @@ static int __peernet2id_alloc(struct net *net, struct net *peer, bool *alloc)
> 	return NETNSA_NSID_NOT_ASSIGNED;
> }
> 
> -/* should be called with nsid_lock held */
> +/* Must be called from RCU-critical section or with nsid_lock held */
> static int __peernet2id(struct net *net, struct net *peer)
> {
> 	bool no = false;
> @@ -261,9 +261,10 @@ int peernet2id(struct net *net, struct net *peer)
> {
> 	int id;
> 
> -	spin_lock_bh(&net->nsid_lock);
> +	rcu_read_lock();
> 	id = __peernet2id(net, peer);
> -	spin_unlock_bh(&net->nsid_lock);
> +	rcu_read_unlock();
> +
> 	return id;
> }
> EXPORT_SYMBOL(peernet2id);
> @@ -837,6 +838,7 @@ struct rtnl_net_dump_cb {
> 	int s_idx;
> };
> 
> +/* Runs in RCU-critical section. */
> static int rtnl_net_dumpid_one(int id, void *peer, void *data)
> {
> 	struct rtnl_net_dump_cb *net_cb = (struct rtnl_net_dump_cb *)data;
> @@ -867,9 +869,9 @@ static int rtnl_net_dumpid(struct sk_buff *skb, struct netlink_callback *cb)
> 		.s_idx = cb->args[0],
> 	};
> 
> -	spin_lock_bh(&net->nsid_lock);
> +	rcu_read_lock();
> 	idr_for_each(&net->netns_ids, rtnl_net_dumpid_one, &net_cb);
> -	spin_unlock_bh(&net->nsid_lock);
> +	rcu_read_unlock();
> 
> 	cb->args[0] = net_cb.idx;
> 	return skb->len;
> -- 
> 1.8.3.1
>