CMNT: [Xenial SRU PATCH] sctp: deny peeloff operation on asocs with threads sleeping on it

[Brad Figg]

I don't see a buglink nor do I find a launchpad bug for this CVE. Both are
required for an SRU.

On Tue, Mar 21, 2017 at 04:54:15PM -0300, Thadeu Lima de Souza Cascardo wrote:
> From: Marcelo Ricardo Leitner <marcelo.leitner at gmail.com>
> 
> commit 2dcab5984841 ("sctp: avoid BUG_ON on sctp_wait_for_sndbuf")
> attempted to avoid a BUG_ON call when the association being used for a
> sendmsg() is blocked waiting for more sndbuf and another thread did a
> peeloff operation on such asoc, moving it to another socket.
> 
> As Ben Hutchings noticed, then in such case it would return without
> locking back the socket and would cause two unlocks in a row.
> 
> Further analysis also revealed that it could allow a double free if the
> application managed to peeloff the asoc that is created during the
> sendmsg call, because then sctp_sendmsg() would try to free the asoc
> that was created only for that call.
> 
> This patch takes another approach. It will deny the peeloff operation
> if there is a thread sleeping on the asoc, so this situation doesn't
> exist anymore. This avoids the issues described above and also honors
> the syscalls that are already being handled (it can be multiple sendmsg
> calls).
> 
> Joint work with Xin Long.
> 
> Fixes: 2dcab5984841 ("sctp: avoid BUG_ON on sctp_wait_for_sndbuf")
> Cc: Alexander Popov <alex.popov at linux.com>
> Cc: Ben Hutchings <ben at decadent.org.uk>
> Signed-off-by: Marcelo Ricardo Leitner <marcelo.leitner at gmail.com>
> Signed-off-by: Xin Long <lucien.xin at gmail.com>
> Signed-off-by: David S. Miller <davem at davemloft.net>
> (cherry picked from commit dfcb9f4f99f1e9a49e43398a7bfbf56927544af1)
> CVE-2017-6353
> Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo at canonical.com>
> ---
>  net/sctp/socket.c | 8 ++++++--
>  1 file changed, 6 insertions(+), 2 deletions(-)
> 
> diff --git a/net/sctp/socket.c b/net/sctp/socket.c
> index 138f2d6..5758818 100644
> --- a/net/sctp/socket.c
> +++ b/net/sctp/socket.c
> @@ -4422,6 +4422,12 @@ int sctp_do_peeloff(struct sock *sk, sctp_assoc_t id, struct socket **sockp)
>  	if (!asoc)
>  		return -EINVAL;
>  
> +	/* If there is a thread waiting on more sndbuf space for
> +	 * sending on this asoc, it cannot be peeled.
> +	 */
> +	if (waitqueue_active(&asoc->wait))
> +		return -EBUSY;
> +
>  	/* An association cannot be branched off from an already peeled-off
>  	 * socket, nor is this supported for tcp style sockets.
>  	 */
> @@ -6960,8 +6966,6 @@ static int sctp_wait_for_sndbuf(struct sctp_association *asoc, long *timeo_p,
>  		 */
>  		release_sock(sk);
>  		current_timeo = schedule_timeout(current_timeo);
> -		if (sk != asoc->base.sk)
> -			goto do_error;
>  		lock_sock(sk);
>  
>  		*timeo_p = current_timeo;
> -- 
> 2.9.3
> 
> 
> -- 
> kernel-team mailing list
> kernel-team at lists.ubuntu.com
> https://lists.ubuntu.com/mailman/listinfo/kernel-team

-- 
Brad Figg brad.figg at canonical.com http://www.canonical.com