Subject: Re: [PATCH net] sctp: walk the list of asoc safely

From: Marcelo Ricardo Leitner <[email-masked]>
Date: Fri, 1 Feb 2019 12:20:37 -0200

On Fri, Feb 01, 2019 at 03:15:22PM +0100, Greg Kroah-Hartman wrote:
> In sctp_sendmesg(), when walking the list of endpoint associations, the
> association can be dropped from the list, making the list corrupt.
> Properly handle this by using list_for_each_entry_safe()
> 
> Fixes: 4910280503f3 ("sctp: add support for snd flag SCTP_SENDALL process in sendmsg")
> Reported-by: Secunia Research <[email-masked]>
> Tested-by: Secunia Research <[email-masked]>
> Signed-off-by: Greg Kroah-Hartman <[email-masked]>
> 
> diff --git a/net/sctp/socket.c b/net/sctp/socket.c
> index f93c3cf9e567..65d6d04546ae 100644
> --- a/net/sctp/socket.c
> +++ b/net/sctp/socket.c
> @@ -2027,7 +2027,7 @@ static int sctp_sendmsg(struct sock *sk, struct msghdr *msg, size_t msg_len)
>  	struct sctp_endpoint *ep = sctp_sk(sk)->ep;
>  	struct sctp_transport *transport = NULL;
>  	struct sctp_sndrcvinfo _sinfo, *sinfo;
> -	struct sctp_association *asoc;
> +	struct sctp_association *asoc, *tmp;
>  	struct sctp_cmsgs cmsgs;
>  	union sctp_addr *daddr;
>  	bool new = false;
> @@ -2053,7 +2053,7 @@ static int sctp_sendmsg(struct sock *sk, struct msghdr *msg, size_t msg_len)

Extending the context here by 1 line:
        lock_sock(sk);
>  
>  	/* SCTP_SENDALL process */
>  	if ((sflags & SCTP_SENDALL) && sctp_style(sk, UDP)) {
> -		list_for_each_entry(asoc, &ep->asocs, asocs) {
> +		list_for_each_entry_safe(asoc, tmp, &ep->asocs, asocs) {

With the socket being locked by here, how can an asoc be removed
while the list is being traversed? The socket lock should be
protecting from it.

>  			err = sctp_sendmsg_check_sflags(asoc, sflags, msg,
>  							msg_len);
>  			if (err == 0)