Re: conntrack, suspicious RCU usage

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Le mercredi 11 janvier 2012 à 14:33 +0100, Eric Dumazet a écrit :
> Le mercredi 11 janvier 2012 à 14:24 +0100, Hans Schillstrom a écrit :
> > On Wednesday 11 January 2012 11:01:51 Eric Dumazet wrote:
> 
> > > Hmm, we either need to take rcu_read_lock() while calling
> > > __nf_ct_l3proto_find(), or define a variant using
> > > rcu_dereference_protected() in places we hold nf_conntrack_lock
> > > 
> > I made a qick test with locks /unlocks in
> > __nf_ct_l3proto_find() and __nf_ct_l4proto_find()
> > 
> > 	rcu_read_lock();
> > ...
> > 	rcu_read_unlock();
> > 	return retp;
> > 
> > It seems to help, I cant see the dump anymore and everything else that I run works ...
> > 
> > 
> 
> You cant do that, its just a brown paper bag :)
> 
> If "retp" is returned, then the caller must handle the rcu_read_unlock()
> itself, after all possible "retp" dereferences.
> 
> But really adding rcu_read_lock() should not be necessary on paths we
> own the conntrack lock. We should use rcu_dereference_protected()
> instead.
> 

Well, __nf_ct_l4proto_find() being out of line and the way we already
use rcu_read_lock() in this code, it seems following patch is
the most natural way to cope with these lockdep warnings.

Thanks

[PATCH] netfilter: ctnetlink: fix lockep splats

net/netfilter/nf_conntrack_proto.c:70 suspicious rcu_dereference_check() usage!
 
other info that might help us debug this:
 
 
rcu_scheduler_active = 1, debug_locks = 0
3 locks held by conntrack/3235:
 #0:  (nfnl_mutex){+.+.+.}, at: [<ffffffff81603537>]
nfnl_lock+0x17/0x20
 #1:  (nlk->cb_mutex){+.+.+.}, at: [<ffffffff815fbd72>]
netlink_dump+0x32/0x240
 #2:  (nf_conntrack_lock){+.-...}, at: [<ffffffffa0115d2e>]
ctnetlink_dump_table+0x3e/0x170 [nf_conntrack_netlink]
 
stack backtrace:
Pid: 3235, comm: conntrack Tainted: G        W    3.2.0+ #511
Call Trace:
 [<ffffffff8108ce45>] lockdep_rcu_suspicious+0xe5/0x100
 [<ffffffffa00ec6e1>] __nf_ct_l4proto_find+0x81/0xb0 [nf_conntrack]
 [<ffffffffa0115675>] ctnetlink_fill_info+0x215/0x5f0 [nf_conntrack_netlink]
 [<ffffffffa0115dc1>] ctnetlink_dump_table+0xd1/0x170 [nf_conntrack_netlink]
 [<ffffffff815fbdbf>] netlink_dump+0x7f/0x240
 [<ffffffff81090f9d>] ? trace_hardirqs_on+0xd/0x10
 [<ffffffff815fd34f>] netlink_dump_start+0xdf/0x190
 [<ffffffffa0111490>] ? ctnetlink_change_nat_seq_adj+0x160/0x160 [nf_conntrack_netlink]
 [<ffffffffa0115cf0>] ? ctnetlink_get_conntrack+0x2a0/0x2a0 [nf_conntrack_netlink]
 [<ffffffffa0115ad9>] ctnetlink_get_conntrack+0x89/0x2a0 [nf_conntrack_netlink]
 [<ffffffff81603a47>] nfnetlink_rcv_msg+0x467/0x5f0
 [<ffffffff81603a7c>] ? nfnetlink_rcv_msg+0x49c/0x5f0
 [<ffffffff81603922>] ? nfnetlink_rcv_msg+0x342/0x5f0
 [<ffffffff81071b21>] ? get_parent_ip+0x11/0x50
 [<ffffffff816035e0>] ? nfnetlink_subsys_register+0x60/0x60
 [<ffffffff815fed49>] netlink_rcv_skb+0xa9/0xd0
 [<ffffffff81603475>] nfnetlink_rcv+0x15/0x20
 [<ffffffff815fe70e>] netlink_unicast+0x1ae/0x1f0
 [<ffffffff815fea16>] netlink_sendmsg+0x2c6/0x320
 [<ffffffff815b2a87>] sock_sendmsg+0x117/0x130
 [<ffffffff81125093>] ? might_fault+0x53/0xb0
 [<ffffffff811250dc>] ? might_fault+0x9c/0xb0
 [<ffffffff81125093>] ? might_fault+0x53/0xb0
 [<ffffffff815b5991>] ? move_addr_to_kernel+0x71/0x80
 [<ffffffff815b644e>] sys_sendto+0xfe/0x130
 [<ffffffff815b5c94>] ? sys_bind+0xb4/0xd0
 [<ffffffff817a8a0e>] ? retint_swapgs+0xe/0x13
 [<ffffffff817afcd2>] system_call_fastpath+0x16/0x1b


Reported-by: Hans Schillstrom <hans.schillstrom@xxxxxxxxxxxx>
Signed-off-by: Eric Dumazet <eric.dumazet@xxxxxxxxx>
---
 net/netfilter/nf_conntrack_netlink.c |   31 ++++++++++++++-----------
 1 file changed, 18 insertions(+), 13 deletions(-)

diff --git a/net/netfilter/nf_conntrack_netlink.c b/net/netfilter/nf_conntrack_netlink.c
index e07dc3a..14840d9 100644
--- a/net/netfilter/nf_conntrack_netlink.c
+++ b/net/netfilter/nf_conntrack_netlink.c
@@ -110,15 +110,15 @@ ctnetlink_dump_tuples(struct sk_buff *skb,
 	struct nf_conntrack_l3proto *l3proto;
 	struct nf_conntrack_l4proto *l4proto;
 
+	rcu_read_lock();
 	l3proto = __nf_ct_l3proto_find(tuple->src.l3num);
 	ret = ctnetlink_dump_tuples_ip(skb, tuple, l3proto);
 
-	if (unlikely(ret < 0))
-		return ret;
-
-	l4proto = __nf_ct_l4proto_find(tuple->src.l3num, tuple->dst.protonum);
-	ret = ctnetlink_dump_tuples_proto(skb, tuple, l4proto);
-
+	if (ret >= 0) {
+		l4proto = __nf_ct_l4proto_find(tuple->src.l3num, tuple->dst.protonum);
+		ret = ctnetlink_dump_tuples_proto(skb, tuple, l4proto);
+	}
+	rcu_read_unlock();
 	return ret;
 }
 
@@ -703,6 +703,7 @@ ctnetlink_dump_table(struct sk_buff *skb, struct netlink_callback *cb)
 	struct hlist_nulls_node *n;
 	struct nfgenmsg *nfmsg = nlmsg_data(cb->nlh);
 	u_int8_t l3proto = nfmsg->nfgen_family;
+	int res;
 
 	spin_lock_bh(&nf_conntrack_lock);
 	last = (struct nf_conn *)cb->args[1];
@@ -723,11 +724,14 @@ restart:
 					continue;
 				cb->args[1] = 0;
 			}
-			if (ctnetlink_fill_info(skb, NETLINK_CB(cb->skb).pid,
+			rcu_read_lock();
+			res = ctnetlink_fill_info(skb, NETLINK_CB(cb->skb).pid,
 						cb->nlh->nlmsg_seq,
 						NFNL_MSG_TYPE(
 							cb->nlh->nlmsg_type),
-						ct) < 0) {
+						ct);
+			rcu_read_unlock();
+			if (res < 0) {
 				nf_conntrack_get(&ct->ct_general);
 				cb->args[1] = (unsigned long)ct;
 				goto out;
@@ -1626,17 +1630,18 @@ ctnetlink_exp_dump_mask(struct sk_buff *skb,
 	if (!nest_parms)
 		goto nla_put_failure;
 
+	rcu_read_lock();
 	l3proto = __nf_ct_l3proto_find(tuple->src.l3num);
 	ret = ctnetlink_dump_tuples_ip(skb, &m, l3proto);
+	if (ret >= 0) {
+		l4proto = __nf_ct_l4proto_find(tuple->src.l3num, tuple->dst.protonum);
+		ret = ctnetlink_dump_tuples_proto(skb, &m, l4proto);
+	}
+	rcu_read_unlock();
 
 	if (unlikely(ret < 0))
 		goto nla_put_failure;
 
-	l4proto = __nf_ct_l4proto_find(tuple->src.l3num, tuple->dst.protonum);
-	ret = ctnetlink_dump_tuples_proto(skb, &m, l4proto);
-	if (unlikely(ret < 0))
-		goto nla_put_failure;
-
 	nla_nest_end(skb, nest_parms);
 
 	return 0;


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