CVE-2025-23142

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

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

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

Theres a possible race condition if another thread triggers the removal of that selected transport, for instance, by explicitly unbinding an address with setsockopt(SCTP_SOCKOPT_BINDX_REM), 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 sctp_wait_for_sndbuf().

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

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

Then, in the scenario where the sender thread has released the socket lock in sctp_wait_for_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, sctp_sendmsg_to_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]).

Weakness

The product reuses or references memory after it has been freed. At some point afterward, the memory may be allocated again and saved in another pointer, while the original pointer references a location somewhere within the new allocation. Any operations using the original pointer are no longer valid because the memory “belongs” to the code that operates on the new pointer.

Affected Software

Potential Mitigations

References

• https://git.kernel.org/stable/c/0f7df4899299ce4662e5f95badb9dbc57cc37fa5
• https://git.kernel.org/stable/c/2e5068b7e0ae0a54f6cfd03a2f80977da657f1ee
• https://git.kernel.org/stable/c/3257386be6a7eb8a8bfc9cbfb746df4eb4fc70e8
• https://git.kernel.org/stable/c/547762250220325d350d0917a7231480e0f4142b
• https://git.kernel.org/stable/c/5bc83bdf5f5b8010d1ca5a4555537e62413ab4e2
• https://git.kernel.org/stable/c/7a63f4fb0efb4e69efd990cbb740a848679ec4b0
• https://git.kernel.org/stable/c/9e7c37fadb3be1fc33073fcf10aa96d166caa697
• https://git.kernel.org/stable/c/c6fefcb71d246baaf3bacdad1af7ff50ebcfe652
• https://git.kernel.org/stable/c/f1a69a940de58b16e8249dff26f74c8cc59b32be
• https://lists.debian.org/debian-lts-announce/2025/05/msg00030.html
• https://lists.debian.org/debian-lts-announce/2025/05/msg00045.html