Re: [PATCH net-next] sctp: fix use-after-free in timeout event handling

[Xin Long]

On Tue, Aug 4, 2026 at 4:44 AM luoqing <l1138897701@xxxxxxx> wrote:
>
> From: Qing Luo <luoqing@xxxxxxxxxx>
>
> sctp_do_sm() in sctp_generate_timeout_event() may process an
> SCTP_CMD_ASSOC_FAILED command, which calls sctp_association_free() and
> drops the last reference, freeing the association and its socket while
> the timer callback is still running. The callback then dereferences the
> freed sk and asoc, a use-after-free.
>
I don't understand how this happened.

If the timer callback is still running, the asoc refcnt must be held until the
callback completes, and sctp_association_destroy() is not supposed to
put the refcnt any running timer holds.

Can you share the PoC and the call trace if any?

Thanks.

> Hold an extra reference on the association at the start of the callback
> so both it and its socket stay alive until we are done, and release it
> at out_unlock along with the timer's reference. Also only set sk->sk_err
> while the association is still alive.
>
> Assisted-by: LLM
> Signed-off-by: Qing Luo <luoqing@xxxxxxxxxx>
> ---
>  net/sctp/sm_sideeffect.c | 12 +++++++++---
>  1 file changed, 9 insertions(+), 3 deletions(-)
>
> diff --git a/net/sctp/sm_sideeffect.c b/net/sctp/sm_sideeffect.c
> index 424f10a6fdba..f384c2faf1c4 100644
> --- a/net/sctp/sm_sideeffect.c
> +++ b/net/sctp/sm_sideeffect.c
> @@ -275,6 +275,12 @@ static void sctp_generate_timeout_event(struct sctp_association *asoc,
>         int error = 0;
>
>         bh_lock_sock(sk);
> +
> +       /* Take an extra ref to keep asoc alive through sctp_do_sm().
> +        * The timer already holds one ref; we'll release both at out_unlock.
> +        */
> +       sctp_association_hold(asoc);
> +
>         if (sock_owned_by_user(sk)) {
>                 pr_debug("%s: sock is busy: timer %d\n", __func__,
>                          timeout_type);
> @@ -296,13 +302,13 @@ static void sctp_generate_timeout_event(struct sctp_association *asoc,
>                            SCTP_ST_TIMEOUT(timeout_type),
>                            asoc->state, asoc->ep, asoc,
>                            (void *)timeout_type, GFP_ATOMIC);
> -
> -       if (error)
> +       if (!asoc->base.dead && error)
>                 sk->sk_err = -error;
>
>  out_unlock:
>         bh_unlock_sock(sk);
> -       sctp_association_put(asoc);
> +       sctp_association_put(asoc);     /* release timer's ref */
> +       sctp_association_put(asoc);     /* release our extra ref */
>  }
>
>  static void sctp_generate_t1_cookie_event(struct timer_list *t)
> --
> 2.25.1
>