netdev - [PATCHv2 net] sctp: hold transport before accessing its asoc in sctp_hash

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Date:   Thu, 29 Nov 2018 14:49:28 +0800
From:   Xin Long <lucien.xin@...il.com>
To:     network dev <netdev@...r.kernel.org>, linux-sctp@...r.kernel.org
Cc:     davem@...emloft.net,
        Marcelo Ricardo Leitner <marcelo.leitner@...il.com>,
        Neil Horman <nhorman@...driver.com>
Subject: [PATCHv2 net] sctp: hold transport before accessing its asoc in sctp_hash_transport

In sctp_hash_transport, it dereferences a transport's asoc only under
rcu_read_lock. Without holding the transport, its asoc could be freed
already, which leads to a use-after-free panic.

A similar fix as Commit bab1be79a516 ("sctp: hold transport before
accessing its asoc in sctp_transport_get_next") is needed to hold
the transport before accessing its asoc in sctp_hash_transport.

Note that as rhlist keeps the lists to a small size, this extra
atomic operation won't cause a noticeable latency on inserting
a transport. Yet it's not in a datapath.

v1->v2:
  - improve the changelog.

Fixes: cd2b70875058 ("sctp: check duplicate node before inserting a new transport")
Reported-by: syzbot+0b05d8aa7cb185107483@...kaller.appspotmail.com
Signed-off-by: Xin Long <lucien.xin@...il.com>
---
 net/sctp/input.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/net/sctp/input.c b/net/sctp/input.c
index ce7351c..c2c0816 100644
--- a/net/sctp/input.c
+++ b/net/sctp/input.c
@@ -896,11 +896,16 @@ int sctp_hash_transport(struct sctp_transport *t)
 	list = rhltable_lookup(&sctp_transport_hashtable, &arg,
 			       sctp_hash_params);
 
-	rhl_for_each_entry_rcu(transport, tmp, list, node)
+	rhl_for_each_entry_rcu(transport, tmp, list, node) {
+		if (!sctp_transport_hold(transport))
+			continue;
 		if (transport->asoc->ep == t->asoc->ep) {
+			sctp_transport_put(transport);
 			rcu_read_unlock();
 			return -EEXIST;
 		}
+		sctp_transport_put(transport);
+	}
 	rcu_read_unlock();
 
 	err = rhltable_insert_key(&sctp_transport_hashtable, &arg,
-- 
2.1.0