[PATCHv2 net] sctp: hold transport before accessing its asoc in sctp_hash_transport

[Xin Long]

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+0b05d8aa7cb185107...@syzkaller.appspotmail.com
Signed-off-by: Xin Long <lucien....@gmail.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