Re: [PATCH net v3] sctp: fix NULL deref on untransmitted RECONF completion

On Sun, Aug 23, 2026 at 1:29 PM Weiming Shi <[email protected]> wrote:
>
> sctp_process_strreset_outreq(), sctp_process_strreset_addstrm_out() and
> sctp_process_strreset_resp() complete a pending stream reconfiguration
> request by stopping the reconf timer on the transport it was sent on:
>
>         t = asoc->strreset_chunk->transport;
>         if (timer_delete(&t->reconf_timer))
>                 sctp_transport_put(t);
>
> chunk->transport is assigned by __sctp_packet_append_chunk() when the
> chunk is appended to an outbound packet, and sctp_outq_flush_ctrl() arms
> the reconf timer at that same point. A request already published in
> asoc->strreset_chunk but not yet transmitted has neither, so completing
> it dereferences NULL.
>
> Two ways to get there. sctp_send_asconf_del_ip() sets
> asoc->src_out_of_asoc_ok without sending anything when the address being
> removed is the association's last one, and sctp_outq_flush_ctrl() then
> leaves every non-ASCONF control chunk queued; as only
> sctp_process_asconf_ack() clears that flag, it persists. An unprivileged
> process that removes such an address and then asks for a stream reset
> panics the kernel from softirq. A peer needs neither ASCONF nor local
> help: sctp_cmd_interpreter() uncorks the outqueue only once the whole
> packet has been processed, so a reply built while walking a RECONF chunk
> stays untransmitted for the rest of that walk, and one RECONF chunk
> carrying [Incoming SSN Reset Request, Outgoing SSN Reset Request,
> Response] -- or two RECONF chunks in one packet -- reaches the same
> dereference.
>
>   KASAN: null-ptr-deref in range [0x00000000000001e8-0x00000000000001ef]
>   RIP: 0010:timer_delete+0x67/0x110
>   Call Trace:
>    <IRQ>
>    sctp_process_strreset_addstrm_out (net/sctp/stream.c:832)
>    sctp_sf_do_reconf (net/sctp/sm_statefuns.c:4212)
>    sctp_do_sm (net/sctp/sm_sideeffect.c:1172)
>    sctp_assoc_bh_rcv (net/sctp/associola.c:1044)
>    sctp_rcv (net/sctp/input.c:243)
>    ip_local_deliver (net/ipv4/ip_input.c:262)
>    process_backlog (net/core/dev.c:6680)
>    </IRQ>
>
> A response can only acknowledge a request that was actually sent, so do
> not match asoc->strreset_chunk while chunk->transport is NULL. Guarding
> the lookup covers all three completion sites.
>
> Fixes: 810544764536 ("sctp: implement receiver-side procedures for the Outgoing SSN Reset Request Parameter")
> Cc: [email protected]
> Reported-by: Xiang Mei <[email protected]>
> Suggested-by: Xin Long <[email protected]>
> Assisted-by: Claude:claude-opus-5
> Signed-off-by: Weiming Shi <[email protected]>
> ---
>
> v3:
> - No code change; v2's changelog linked the wrong v1 posting.
>
> v2: https://lore.kernel.org/linux-sctp/[email protected]/
> v1: https://lore.kernel.org/linux-sctp/[email protected]/
>  net/sctp/stream.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/net/sctp/stream.c b/net/sctp/stream.c
> index 34ffe6c945a4..2012f61e250e 100644
> --- a/net/sctp/stream.c
> +++ b/net/sctp/stream.c
> @@ -488,7 +488,7 @@ static struct sctp_paramhdr *sctp_chunk_lookup_strreset_param(
>         struct sctp_reconf_chunk *hdr;
>         union sctp_params param;
>
> -       if (!chunk)
> +       if (!chunk || !chunk->transport)
>                 return NULL;
>
>         hdr = (struct sctp_reconf_chunk *)chunk->chunk_hdr;
> --
> 2.55.0
>

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