netdev - Re: [PATCHv2 net] sctp: hold the transport before using it in sctp_hash

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Message-Id: <20160913.114535.186471066793791952.davem@davemloft.net>
Date:   Tue, 13 Sep 2016 11:45:35 -0400 (EDT)
From:   David Miller <davem@...emloft.net>
To:     lucien.xin@...il.com
Cc:     netdev@...r.kernel.org, linux-sctp@...r.kernel.org,
        marcelo.leitner@...il.com, vyasevich@...il.com,
        daniel@...earbox.net
Subject: Re: [PATCHv2 net] sctp: hold the transport before using it in
 sctp_hash_cmp

From: Xin Long <lucien.xin@...il.com>
Date: Sat, 10 Sep 2016 23:11:23 +0800

> Since commit 4f0087812648 ("sctp: apply rhashtable api to send/recv
> path"), sctp uses transport rhashtable with .obj_cmpfn sctp_hash_cmp,
> in which it compares the members of the transport with the rhashtable
> args to check if it's the right transport.
> 
> But sctp uses the transport without holding it in sctp_hash_cmp, it can
> cause a use-after-free panic. As after it gets transport from hashtable,
> another CPU may close the sk and free the asoc. In sctp_association_free,
> it frees all the transports, meanwhile, the assoc's refcnt may be reduced
> to 0, assoc can be destroyed by sctp_association_destroy.
> 
> So after that, transport->assoc is actually an unavailable memory address
> in sctp_hash_cmp. Although sctp_hash_cmp is under rcu_read_lock, it still
> can not avoid this, as assoc is not freed by RCU.
> 
> This patch is to hold the transport before checking it's members with
> sctp_transport_hold, in which it checks the refcnt first, holds it if
> it's not 0.
> 
> Fixes: 4f0087812648 ("sctp: apply rhashtable api to send/recv path")
> Signed-off-by: Xin Long <lucien.xin@...il.com>

Applied and queued up for -stable.