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

[Xin Long]

Without holding transport to dereference its asoc, a use after
free panic can be caused in sctp_epaddr_lookup_transport. Note
that a sock lock can't protect these transports that belong to
other socks.

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_epaddr_lookup_transport.

Note that this extra atomic operation is on the datapath,
but as rhlist keeps the lists to a small size, it won't
see a noticeable performance hurt.

v1->v2:
  - improve the changelog.

Fixes: 7fda702f9315 ("sctp: use new rhlist interface on sctp transport 
rhashtable")
Reported-by: syzbot+aad231d51b1923158...@syzkaller.appspotmail.com
Signed-off-by: Xin Long <lucien....@gmail.com>
---
 net/sctp/input.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/net/sctp/input.c b/net/sctp/input.c
index 5c36a99..ce7351c 100644
--- a/net/sctp/input.c
+++ b/net/sctp/input.c
@@ -967,9 +967,15 @@ struct sctp_transport *sctp_epaddr_lookup_transport(
        list = rhltable_lookup(&sctp_transport_hashtable, &arg,
                               sctp_hash_params);
 
-       rhl_for_each_entry_rcu(t, tmp, list, node)
-               if (ep == t->asoc->ep)
+       rhl_for_each_entry_rcu(t, tmp, list, node) {
+               if (!sctp_transport_hold(t))
+                       continue;
+               if (ep == t->asoc->ep) {
+                       sctp_transport_put(t);
                        return t;
+               }
+               sctp_transport_put(t);
+       }
 
        return NULL;
 }
-- 
2.1.0