Subject: Re: [PATCH net] net: sctp: test if association is dead in sctp_wake_up_waiters

From: Vlad Yasevich <[email-masked]>
Date: Tue, 08 Apr 2014 19:10:56 -0400

On 04/08/2014 06:23 PM, Daniel Borkmann wrote:
> In function sctp_wake_up_waiters() we need to involve a test
> if the association is declared dead. If so, we don't have any
> reference to a possible sibling association anymore and need
> to invoke sctp_write_space() instead and normally walk the
> socket's associations and notify them of new wmem space. The
> reason for special casing is that, otherwise, we could run
> into the following issue:
> 
> sctp_association_free()
> `-> list_del(&asoc->asocs)         <-- poisons list pointer
>     asoc->base.dead = true
>     sctp_outq_free(&asoc->outqueue)
>     `-> __sctp_outq_teardown()
>      `-> sctp_chunk_free()
>       `-> consume_skb()
>        `-> sctp_wfree()
>         `-> sctp_wake_up_waiters() <-- dereferences poisoned pointers
>                                        if asoc->ep->sndbuf_policy=0
> 
> Therefore, only walk the list in an 'optimized' way if we find
> that the current association is still active. It's also more
> clean in that context to just use list_del_init() when we call
> sctp_association_free(). Stress-testing seems fine now.

One of the reasons that we don't use list_del_init() here is that
we want to be able to trap on uninitialized/corrupt list manipulation,
just like you did.  If it wasn't there, the bug would have been hidden.

Please keep it there.  The rest of the patch is fine.

Thanks
-vlad

> 
> Fixes: cd253f9f357d ("net: sctp: wake up all assocs if sndbuf policy is per socket")
> Signed-off-by: Daniel Borkmann <[email-masked]>
> Cc: Vlad Yasevich <[email-masked]>
> ---
>  net/sctp/associola.c | 2 +-
>  net/sctp/socket.c    | 6 ++++++
>  2 files changed, 7 insertions(+), 1 deletion(-)
> 
> diff --git a/net/sctp/associola.c b/net/sctp/associola.c
> index 4f6d6f9..0f8fa97 100644
> --- a/net/sctp/associola.c
> +++ b/net/sctp/associola.c
> @@ -331,7 +331,7 @@ void sctp_association_free(struct sctp_association *asoc)
>  	 * don't bother for if this is a temporary association.
>  	 */
>  	if (!asoc->temp) {
> -		list_del(&asoc->asocs);
> +		list_del_init(&asoc->asocs);
>  
>  		/* Decrement the backlog value for a TCP-style listening
>  		 * socket.
> diff --git a/net/sctp/socket.c b/net/sctp/socket.c
> index 5f83a6a..270d5bd 100644
> --- a/net/sctp/socket.c
> +++ b/net/sctp/socket.c
> @@ -6604,6 +6604,12 @@ static void sctp_wake_up_waiters(struct sock *sk,
>  	if (asoc->ep->sndbuf_policy)
>  		return __sctp_write_space(asoc);
>  
> +	/* If association goes down and is just flushing its
> +	 * outq, then just normally notify others.
> +	 */
> +	if (asoc->base.dead)
> +		return sctp_write_space(sk);
> +
>  	/* Accounting for the sndbuf space is per socket, so we
>  	 * need to wake up others, try to be fair and in case of
>  	 * other associations, let them have a go first instead
>