CVE-2026-74586: sctp: clear new_transport when removing a peer

In the Linux kernel, the following vulnerability has been resolved:

sctp: clear newtransport when removing a peer

sctpprocessasconfparam() stores a newly added peer transport in asoc->newtransport. After all parameters in the ASCONF chunk have been processed, sctpsfdoasconf() 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 sctpassocdelnonprimarypeers(). The removal does not clear asoc->newtransport, leaving it pointing to the removed transport.

sctpsfdoasconf() then creates a HEARTBEAT whose chunk->transport points to the removed transport without holding a transport reference. During local address replacement, srcoutofasocok keeps this HEARTBEAT on controlchunklist. After the transport is freed by RCU, a successful ASCONFACK for the replacement address releases the queued HEARTBEAT and sctpoutqselecttransport() 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 sctpoutqselecttransport Read of size 4 at addr ffff88800b9bd95c by task python3/197

Call Trace: sctpoutqselecttransport+0x549/0x8b0 [sctp] sctpoutqflush+0x306/0x2c60 [sctp] sctptransportimmediatertx+0xaf/0x260 [sctp] sctpprocessasconfack+0xa48/0xf70 [sctp]

Allocated by task 197: sctptransportnew+0x68/0x650 [sctp] sctpassocaddpeer+0x258/0x12a0 [sctp] sctpprocessasconf+0x5e9/0x1090 [sctp]

Last potentially related work creation: callrcucommon.constprop.0+0x77/0xb70 sctpassocdelnonprimarypeers+0x7c/0xd0 [sctp] sctpprocessasconf+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.

Clear newtransport when its peer is removed, before it can be used to create the HEARTBEAT.