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

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Date:   Thu, 9 Feb 2023 12:53:54 +0100
From:   Pietro Borrello <borrello@...g.uniroma1.it>
To:     Xin Long <lucien.xin@...il.com>
Cc:     Neil Horman <nhorman@...driver.com>,
        Marcelo Ricardo Leitner <marcelo.leitner@...il.com>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Cristiano Giuffrida <c.giuffrida@...nl>,
        "Bos, H.J." <h.j.bos@...nl>, Jakob Koschel <jkl820.git@...il.com>,
        linux-sctp@...r.kernel.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH net-next] sctp: check ep asocs list before access

On Wed, 8 Feb 2023 at 20:21, Xin Long <lucien.xin@...il.com> 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