Re: [PATCH net] sctp: clear new_transport when removing a peer
From: Xin Long <lucien.xin@gmail.com>
Date: 2026-08-11 15:45:02
Also in:
linux-sctp, lkml, stable
On Tue, Aug 11, 2026 at 11:28 AM Qing Ming [off-list ref] wrote:
sctp_process_asconf_param() stores a newly added peer transport in asoc->new_transport. After all parameters in the ASCONF chunk have been processed, sctp_sf_do_asconf() uses this pointer to send a HEARTBEAT to the new transport. An authenticated ASCONF from a remote SCTP peer can add a transport and remove it again with a wildcard DEL-IP parameter in the same chunk. The wildcard deletion preserves the transport on which the ASCONF arrived, but removes the newly added transport through sctp_assoc_del_nonprimary_peers(). The removal does not clear asoc->new_transport, leaving it pointing to the removed transport. sctp_sf_do_asconf() then creates a HEARTBEAT whose chunk->transport points to the removed transport without holding a transport reference. During local address replacement, src_out_of_asoc_ok keeps this HEARTBEAT on control_chunk_list. After the transport is freed by RCU, a successful ASCONF_ACK for the replacement address releases the queued HEARTBEAT and sctp_outq_select_transport() reads the freed transport's state. The issue was found during a static audit of SCTP objects. With an authenticated peer, the reproducer triggered the same KASAN report in 2 of 2 unpatched runs on a KASAN-enabled netdev/main kernel: BUG: KASAN: slab-use-after-free in sctp_outq_select_transport Read of size 4 at addr ffff88800b9bd95c by task python3/197 Call Trace: sctp_outq_select_transport+0x549/0x8b0 [sctp] sctp_outq_flush+0x306/0x2c60 [sctp] sctp_transport_immediate_rtx+0xaf/0x260 [sctp] sctp_process_asconf_ack+0xa48/0xf70 [sctp] Allocated by task 197: sctp_transport_new+0x68/0x650 [sctp] sctp_assoc_add_peer+0x258/0x12a0 [sctp] sctp_process_asconf+0x5e9/0x1090 [sctp] Last potentially related work creation: __call_rcu_common.constprop.0+0x77/0xb70 sctp_assoc_del_nonprimary_peers+0x7c/0xd0 [sctp] sctp_process_asconf+0xd9c/0x1090 [sctp] The first invalid access was a four-byte read of transport->state at net/sctp/outqueue.c:833. The same reproducer completed the full authenticated ASCONF and local-address replacement sequence with this change without a KASAN report or oops.
Could you also post the reproducer, at least to the maintainers? Thanks.
quoted hunk ↗ jump to hunk
Clear new_transport when its peer is removed, before it can be used to create the HEARTBEAT. Fixes: 6af29ccc223b ("sctp: Bundle HEAERTBEAT into ASCONF_ACK") Cc: stable@vger.kernel.org Assisted-by: Codex:gpt-5 Signed-off-by: Qing Ming <redacted> --- net/sctp/associola.c | 3 +++ 1 file changed, 3 insertions(+)diff --git a/net/sctp/associola.c b/net/sctp/associola.c index 5b0ae616e1ff..c65c83638cce 100644 --- a/net/sctp/associola.c +++ b/net/sctp/associola.c@@ -543,6 +543,9 @@ void sctp_assoc_rm_peer(struct sctp_association *asoc, asoc->addip_last_asconf->transport == peer) asoc->addip_last_asconf->transport = NULL; + if (asoc->new_transport == peer) + asoc->new_transport = NULL; + /* If we have something on the transmitted list, we have to * save it off. The best place is the active path. */base-commit: cba9ccb47e9fa4cc77692fb896cc5ab57a667882 -- 2.53.0