Re: [PATCH net v3] sctp: fix use-after-free of cached ASCONF chunk

On Sun, Aug 9, 2026 at 12:38 AM Yuxiang Yang
<[email protected]> wrote:
>
> addip_last_asconf caches the outstanding outbound ASCONF chunk. The normal
> ASCONF-ACK completion path releases the chunk and clears the pointer.
>
> However, sctp_asconf_queue_teardown() releases the cached chunk without
> clearing addip_last_asconf. During peer restart handling,
> sctp_sf_do_dupcook_a() queues SCTP_CMD_PURGE_ASCONF_QUEUE, which invokes
> sctp_asconf_queue_teardown() while the association remains alive and leaves
> the pointer dangling.
>
> A delayed authenticated ASCONF-ACK can then reach sctp_sf_do_asconf_ack(),
> which accesses the stale chunk and passes it to sctp_process_asconf_ack(),
> causing a use-after-free and a second release.
>
> Clearing the pointer exposes a race with T4 expiry. Peer restart handling
> queues the timer stop before the purge, but SCTP_CMD_TIMER_STOP uses
> timer_delete(), which does not wait for a callback already running on
> another CPU. Such a callback can reach sctp_sf_t4_timer_expire() after
> the purge and dereference NULL.
>
> Clear addip_last_asconf after releasing the cached chunk, and make
> sctp_sf_t4_timer_expire() consume a stale T4 expiry if no outstanding
> ASCONF remains.
>
> Fixes: a000c01e60e4 ("sctp: stop pending timers and purge queues when peer restart asoc")
> Cc: [email protected]
> Suggested-by: Xin Long <[email protected]>
> Assisted-by: Claude-Code:GLM-5.2
> Signed-off-by: Yuxiang Yang <[email protected]>
> ---
> Changes in v3:
> - Resend as a new, independent thread as requested by pv-bot. No code
>   changes.
> - Link to v2:
>   https://lore.kernel.org/netdev/[email protected]/
>
> Changes in v2:
> - Guard sctp_sf_t4_timer_expire() against a cleared cached ASCONF pointer,
>   as requested by Xin Long.
> - Explain why the T4 callback may run after the restart purge.
>
>  net/sctp/associola.c    | 4 +++-
>  net/sctp/sm_statefuns.c | 6 +++++-
>  2 files changed, 8 insertions(+), 2 deletions(-)
>
> diff --git a/net/sctp/associola.c b/net/sctp/associola.c
> index 5b0ae61..737f8ea 100644
> --- a/net/sctp/associola.c
> +++ b/net/sctp/associola.c
> @@ -1713,6 +1713,8 @@ void sctp_asconf_queue_teardown(struct sctp_association *asoc)
>         sctp_assoc_free_asconf_queue(asoc);
>
>         /* Free any cached ASCONF chunk. */
> -       if (asoc->addip_last_asconf)
> +       if (asoc->addip_last_asconf) {
>                 sctp_chunk_free(asoc->addip_last_asconf);
> +               asoc->addip_last_asconf = NULL;
> +       }
>  }
> diff --git a/net/sctp/sm_statefuns.c b/net/sctp/sm_statefuns.c
> index 708fa07..3a8e16b 100644
> --- a/net/sctp/sm_statefuns.c
> +++ b/net/sctp/sm_statefuns.c
> @@ -6145,8 +6145,12 @@ enum sctp_disposition sctp_sf_t4_timer_expire(
>                                         struct sctp_cmd_seq *commands)
>  {
>         struct sctp_chunk *chunk = asoc->addip_last_asconf;
> -       struct sctp_transport *transport = chunk->transport;
> +       struct sctp_transport *transport;
> +
> +       if (!chunk)
> +               return SCTP_DISPOSITION_CONSUME;
>
> +       transport = chunk->transport;
>         SCTP_INC_STATS(net, SCTP_MIB_T4_RTO_EXPIREDS);
>
>         /* ADDIP 4.1 B1) Increment the error counters and perform path failure
> --
> 2.25.1
>

Acked-by: Xin Long <[email protected]>