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

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: stable@vger.kernel.org
Suggested-by: Xin Long <lucien.xin@gmail.com>
Assisted-by: Claude-Code:GLM-5.2
Signed-off-by: Yuxiang Yang <redacted>
---
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/20260808005748.1981039-1-yangyx22@mails.tsinghua.edu.cn/ (local)

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