[PATCH net v3] sctp: don't free the ASCONF's own transport in DEL-IP processing

[PATCH net v3] sctp: don't free the ASCONF's own transport in DEL-IP processing

[Jun Yang]

From: Jun Yang <junvyyang@tencent.com>

sctp_process_asconf() caches the transport the ASCONF chunk is processed
against in asconf->transport (== chunk->transport, set once in sctp_rcv()).
For an ASCONF located through its Address Parameter by
__sctp_rcv_asconf_lookup(), that cached transport corresponds to the
Address Parameter, which need not be the packet's source address.

sctp_process_asconf_param() rejects a DEL-IP for the packet source address
(ADDIP D8, SCTP_ERROR_DEL_SRC_IP), but nothing protects asconf->transport.
A single ASCONF can therefore carry, in order:

    [Address Parameter L] [DEL-IP L] [DEL-IP 0.0.0.0]

where L differs from the source. The DEL-IP for L passes the D8 check and
calls sctp_assoc_rm_peer() on the transport that asconf->transport still
points at, freeing it (RCU-deferred). The following wildcard DEL-IP then
reuses the now-dangling asconf->transport in sctp_assoc_set_primary() and
sctp_assoc_del_nonprimary_peers(): set_primary() dereferences the freed
transport (->ipaddr, ->state) and plants the dangling pointer into
asoc->peer.primary_path / active_path, and del_nonprimary_peers(), keeping
only the pointer that is no longer on the list, removes every real
transport, leaving the association with a transport_count of 0 and
primary_path/active_path pointing at freed memory.

Reject a DEL-IP that targets the transport the ASCONF is being processed
against, mirroring the existing source-address guard, so the wildcard
branch can never reuse a freed transport.

Fixes: 42e30bf3463c ("[SCTP]: Handle the wildcard ADD-IP Address parameter")
Cc: stable@kernel.org
Signed-off-by: Jun Yang <junvyyang@tencent.com>
Acked-by: Xin Long <lucien.xin@gmail.com>
---
v3: return Request Refused instead of Request to Delete Source IP Address,
    as the protected transport need not be the packet's source address.
v2: add [net] subject prefix to target the net tree.

 net/sctp/sm_make_chunk.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/net/sctp/sm_make_chunk.c b/net/sctp/sm_make_chunk.c
index 8adac9e0cd66..b14251214896 100644
--- a/net/sctp/sm_make_chunk.c
+++ b/net/sctp/sm_make_chunk.c
@@ -3153,6 +3153,12 @@ static __be16 sctp_process_asconf_param(struct sctp_association *asoc,
 		if (!peer)
 			return SCTP_ERROR_DNS_FAILED;

+		/* Don't free asconf->transport; a later wildcard DEL-IP
+		 * parameter reuses it.
+		 */
+		if (peer == asconf->transport)
+			return SCTP_ERROR_REQ_REFUSED;
+
 		sctp_assoc_rm_peer(asoc, peer);
 		break;
 	case SCTP_PARAM_SET_PRIMARY:
--
2.55.0

Re: [PATCH net v3] sctp: don't free the ASCONF's own transport in DEL-IP processing

[Jakub Kicinski]

On Tue, 21 Jul 2026 21:14:05 +0800 Jun Yang wrote:
> Acked-by: Xin Long <lucien.xin@gmail.com>

Could you point me to where Xin Long acked this?
I don't see the ack in replies to v1 or v2.

Re: [PATCH net v3] sctp: don't free the ASCONF's own transport in DEL-IP processing

[Xin Long]

On Thu, Jul 23, 2026 at 10:29 AM Jakub Kicinski <kuba@kernel.org> wrote:
>
> On Tue, 21 Jul 2026 21:14:05 +0800 Jun Yang wrote:
> > Acked-by: Xin Long <lucien.xin@gmail.com>
>
> Could you point me to where Xin Long acked this?
> I don't see the ack in replies to v1 or v2.
Hi Jakub,

He initially sent the patch to security@kernel.org, and I asked him to
submit it to netdev with my Ack.

By the way, there's a question about whether patches for security fixes
should include the PoC in the patch/cover-letter when they're posted to
the netdev ML?

Thanks,