Linux Kernel CVE-2025-68775

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

net/handshake: duplicate handshake cancellations leak socket

When a handshake request is cancelled it is removed from the handshake_net->hn_requests list, but it is still present in the handshake_rhashtbl until it is destroyed.

If a second cancellation request arrives for the same handshake request, then remove_pending() will return false... and assuming HANDSHAKE_F_REQ_COMPLETED isn't set in req->hr_flags, we'll continue processing through the out_true label, where we put another reference on the sock and a refcount underflow occurs.

This can happen for example if a handshake times out - particularly if the SUNRPC client sends the AUTH_TLS probe to the server but doesn't follow it up with the ClientHello due to a problem with tlshd. When the timeout is hit on the server, the server will send a FIN, which triggers a cancellation request via xs_reset_transport(). When the timeout is hit on the client, another cancellation request happens via xs_tls_handshake_sync().

Add a test_and_set_bit(HANDSHAKE_F_REQ_COMPLETED) in the pending cancel path so duplicate cancels can be detected.

The Linux kernel is affected in versions that include the net/handshake TLS upcall subsystem (present since the ~6.5 series when kernel TLS handshake support and SUNRPC-over-TLS landed). No exact vulnerable version range or CPE was supplied in the input. Distribution-level exposure is confirmed for Ubuntu, whose advisory USN-8177-1 (https://ubuntu.com/security/notices/USN-8177-1) tracks fixed kernel packages. Upstream stable fixes are recorded at git.kernel.org for multiple stable branches via commits 011ae80c49d9bfa5b4336f8bd387cd25c7593663, 15564bd67e2975002f2a8e9defee33e321d3183f, 3c330f1dee3cd92b57e19b9d21dc8ce5970b09be, and e1641177e7fb48a0a5a06658d4aab51da6656659. A third-party summary is available at VulDB (https://vuldb.com/?id.340667).