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

[Håkon Bugge]

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 4.14, 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 1af25d5..37f4313 100644
--- a/net/core/net_namespace.c
+++ b/net/core/net_namespace.c
@@ -181,9 +181,9 @@ 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)
 {
@@ -207,7 +207,7 @@ 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;
@@ -240,9 +240,10 @@ 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);
@@ -761,6 +762,7 @@ 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;
@@ -791,9 +793,9 @@ 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