CVE-2026-72317

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

SUNRPC: pin upper rpc_clnt across the TLS connect_worker

The TLS connect path has a use-after-free: nothing pins the

upper rpc_clnt across the delayed connect_worker. xs_connect()

stores task->tk_client in sock_xprt::clnt as a raw pointer

and queues the worker; for TLS-secured transports that worker

is xs_tcp_tls_setup_socket(), which reads several fields out

of the saved pointer (cl_timeout, cl_program, cl_prog,

cl_vers, cl_cred, cl_stats) to construct the args for the

inner handshake rpc_clnt.

The xprt does not reference the rpc_clnt; the rpc_clnt

references the xprt. xs_destroy() does cancel the

connect_worker, but it runs only when the xprt's refcount

drops to zero, which cannot happen until the rpc_clnt

releases its cl_xprt reference in rpc_free_client_work().

When a TLS handshake fails fatally (for example, an mTLS

mount whose client cert does not match the server), the

connecting task is woken with -EACCES and exits, the mount

caller invokes rpc_shutdown_client(), and the upper rpc_clnt

is freed before the queued connect_worker fires.

xs_tcp_tls_setup_socket() then dereferences the freed clnt,

producing the refcount_t underflow Michael Nemanov reported.

Take a reference on the upper rpc_clnt in xs_connect() for

TLS transports via a new rpc_hold_client() helper, and drop

it in the connect_worker's exit path with rpc_release_client().

The xprt_lock_connect() / xprt_unlock_connect() pairing

already serialises xs_connect() with xs_tcp_tls_setup_socket(),

so the take and release are balanced one-for-one.

The non-TLS connect worker (xs_tcp_setup_socket) never reads

sock_xprt::clnt, so leave that path alone and avoid the

clnt-holds-xprt-holds-clnt cycle that would otherwise prevent

xprt destruction.