Re: [PATCH net-next] sctp: check ep asocs list before access

[Pietro Borrello]

On Wed, 8 Feb 2023 at 20:21, Xin Long <lucien.xin@xxxxxxxxx> wrote:
>
> [...]
> > We suspect a race condition between a connecting sctp socket
> > and the diag query.
> As it commented in sctp_transport_traverse_process():
>
> "asoc can be peeled off " before callinsctp_sock_filter(). Actually,

Ah, thank you for clarifying! I misunderstood the comment, and read it
like "we hold the ep, otherwise ascoc can be peeled off".

> the asoc can be peeled off from the ep anytime during it by another
> thread, and placing a list_empty(&ep->asocs) check and returning
> won't avoid it completely, as peeling off the asoc can happen after
> your check.
>
> We actually don't care about the asoc peeling off during the dump,
> as sctp diag can not work that accurately. There also shouldn't be

Agree. This makes a lot of sense.

> problems caused so far, as the "assoc" won't be used anywhere after
> that check.
>
> To avoid the "type confused pointer" thing,  maybe you can try to use
> list_is_first() there:
>
> -       struct sctp_association *assoc =
> -               list_entry(ep->asocs.next, struct sctp_association, asocs);
>
>         /* find the ep only once through the transports by this condition */
> -       if (tsp->asoc != assoc)
> +       if (!list_is_first(&tsp->asoc->asocs, &ep->asocs))
>                 return 0;
>

This is a very nice suggestion, which also avoids future issues in
case assoc would be used. I'll do that in v2. Thank you!

Best regards,
Pietro