[PATCH net v2] sctp: fix NULL deref on untransmitted RECONF completion

[Weiming Shi <bestswngs@gmail.com>]

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: stable@vger.kernel.org
Reported-by: Xiang Mei <xmei5@asu.edu>
Suggested-by: Xin Long <lucien.xin@gmail.com>
Assisted-by: Claude:claude-opus-5
Signed-off-by: Weiming Shi <bestswngs@gmail.com>
---

v2:
- Guard sctp_chunk_lookup_strreset_param() instead of NULL-checking
  chunk->transport at the three completion sites (Xin Long).
- Note the peer-only path into the same dereference.
- The src_out_of_asoc_ok latch is still not addressed; happy to follow
  up for net-next.

v1: https://lore.kernel.org/linux-sctp/20260820161400.2649968-1-bestswngs@gmail.com/
 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