CVE-2025-23142

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

sctp: detect and prevent references to a freed transport in sendmsg

sctpsendmsg() re-uses associations and transports when possible by doing a lookup based on the socket endpoint and the message destination address, and then sctpsendmsgtoasoc() sets the selected transport in all the message chunks to be sent.

There's a possible race condition if another thread triggers the removal of that selected transport, for instance, by explicitly unbinding an address with setsockopt(SCTPSOCKOPTBINDXREM), after the chunks have been set up and before the message is sent. This can happen if the send buffer is full, during the period when the sender thread temporarily releases the socket lock in sctpwaitforsndbuf().

This causes the access to the transport data in sctpoutqselect_transport(), when the association outqueue is flushed, to result in a use-after-free read.

This change avoids this scenario by having sctptransportfree() signal the freeing of the transport, tagging it as "dead". In order to do this, the patch restores the "dead" bit in struct sctptransport, which was removed in commit 47faa1e4c50e ("sctp: remove the dead field of sctptransport").

Then, in the scenario where the sender thread has released the socket lock in sctpwaitfor_sndbuf(), the bit is checked again after re-acquiring the socket lock to detect the deletion. This is done while holding a reference to the transport to prevent it from being freed in the process.

If the transport was deleted while the socket lock was relinquished, sctpsendmsgto_asoc() will return -EAGAIN to let userspace retry the send.

The bug was found by a private syzbot instance (see the error report [1] and the C reproducer that triggers it [2]).