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

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Date:   Tue, 20 Nov 2018 07:52:48 -0500
From:   Neil Horman <nhorman@...driver.com>
To:     Xin Long <lucien.xin@...il.com>
Cc:     network dev <netdev@...r.kernel.org>, linux-sctp@...r.kernel.org,
        davem@...emloft.net,
        Marcelo Ricardo Leitner <marcelo.leitner@...il.com>
Subject: Re: [PATCH net] sctp: hold transport before accessing its asoc in
 sctp_hash_transport

On Tue, Nov 20, 2018 at 07:09:16PM +0800, Xin Long wrote:
> 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.
> 
> 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 5c36a99..69584e9 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
> 
> 

something doesn't feel at all right about this.  If we are inserting a transport
to an association, it would seem to me that we should have at least one user of
the association (i.e. non-zero refcount).  As such it seems something is wrong
with the association refcount here.  At the very least, if there is a case where
an association is being removed while a transport is being added, the better
solution would be to ensure that sctp_association_destroy goes through a
quiescent point prior to unhashing transports from the list, to ensure that
there is no conflict with the add operation above.

Neil