CVE-2026-64281 in Linux
Summary
by MITRE • 07/25/2026
In the Linux kernel, the following vulnerability has been resolved:
svcrdma: wake sq waiters when the transport closes
Threads parked in svc_rdma_sq_wait() on sc_sq_ticket_wait or sc_send_wait can hang indefinitely in TASK_UNINTERRUPTIBLE state across transport teardown, pinning svc_xprt references and blocking svc_rdma_free().
The close path sets XPT_CLOSE before invoking xpo_detach and both wait_event predicates include an XPT_CLOSE term, but the predicates are re-evaluated only on wakeup. sc_sq_ticket_wait has no completion-driven wake path; it is advanced solely by the chained ticket handoff inside svc_rdma_sq_wait() itself. Without an explicit wake at close, parked threads never observe XPT_CLOSE, hold their svc_xprt_get reference forever, and svc_rdma_free() blocks on xpt_ref dropping to zero.
Two close entry points reach this transport. Local teardown runs svc_rdma_detach() from svc_handle_xprt() -> svc_delete_xprt() -> xpo_detach() on a worker thread. A remote disconnect arrives at svc_rdma_cma_handler(), which calls svc_xprt_deferred_close(): that sets XPT_CLOSE and enqueues the transport but does not access either RDMA waitqueue, so a worker already parked in svc_rdma_sq_wait() never re-evaluates its predicate. With every worker parked on this transport, no thread is available to run the local teardown either, and the wake site there is unreachable.
Introduce svc_rdma_xprt_deferred_close(), a thin svcrdma wrapper that calls svc_xprt_deferred_close() and then wakes both sc_sq_ticket_wait and sc_send_wait. Convert the svcrdma producers that called svc_xprt_deferred_close() directly: svc_rdma_cma_handler(), qp_event_handler(), svc_rdma_post_send_err(), svc_rdma_wc_send(), the sendto drop path, the rw completion error paths, and the recvfrom flush and read-list error paths.
Wake both waitqueues from svc_rdma_detach() as well. The synchronous svc_xprt_close() path (backchannel ENOTCONN, device removal via svc_rdma_xprt_done) reaches detach without flowing through svc_xprt_deferred_close() and therefore does not invoke the new helper.
[ cel: add svc_rdma_xprt_deferred_close() to complete the fix ]
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Analysis
by VulDB Data Team • 07/25/2026
The vulnerability described represents a critical race condition and resource deadlock scenario within the Linux kernel's svcrdma subsystem, which handles RDMA transport operations for NFS over RDMA implementations. This flaw manifests when transport teardown occurs while threads are blocked in specific wait states, creating an indefinite hang situation that prevents proper cleanup of transport resources. The core issue lies in the asynchronous nature of transport closure combined with insufficient wake-up mechanisms for parked threads, resulting in persistent reference counting and blocked resource deallocation.
The technical root cause stems from improper handling of wait queue predicates during transport shutdown sequences. Threads waiting in svc_rdma_sq_wait() function on either sc_sq_ticket_wait or sc_send_wait queues can become indefinitely suspended in TASK_UNINTERRUPTIBLE state when the underlying transport closes. While the close path correctly sets XPT_CLOSE flag and invokes xpo_detach, the wait event predicates that check this flag are only re-evaluated upon wakeup events rather than during the teardown process itself. This design gap means that threads parked on these specific queues never observe the XPT_CLOSE condition, maintaining their svc_xprt_get reference and preventing svc_rdma_free() from completing its cleanup operations.
The problem extends beyond simple thread blocking to encompass broader system resource management issues that align with CWE-362 (Concurrent Execution using Shared Resource with Improper Synchronization) and CWE-704 (Incorrect Type Conversion or Cast). The vulnerability affects both local teardown scenarios, such as those initiated through svc_handle_xprt() -> svc_delete_xprt() -> xpo_detach() on worker threads, and remote disconnect handling via svc_rdma_cma_handler() which calls svc_xprt_deferred_close(). The latter path particularly compounds the issue because it sets XPT_CLOSE but does not access the RDMA waitqueues, leaving already parked threads unaware of the transport state change.
The operational impact of this vulnerability is severe as it creates a deadlock condition that prevents system cleanup and resource reclamation. When all worker threads become parked on a transport with pending close operations, no thread remains available to process the local teardown sequence, effectively creating a circular dependency where the system cannot progress past the shutdown phase. This behavior directly violates the principles of proper resource management as outlined in ATT&CK technique T1490 (Inhibit System Recovery) and demonstrates a failure in proper synchronization mechanisms for shared resources. The persistent reference counting prevents svc_rdma_free() from dropping xpt_ref to zero, which is essential for proper transport cleanup and memory deallocation.
The fix implemented addresses this through the introduction of svc_rdma_xprt_deferred_close(), a specialized wrapper function that extends the existing svc_xprt_deferred_close() functionality by explicitly waking both sc_sq_ticket_wait and sc_send_wait queues. This modification ensures that all code paths previously calling svc_xprt_deferred_close() directly now properly notify waiting threads of transport closure, including svc_rdma_cma_handler(), qp_event_handler(), svc_rdma_post_send_err(), svc_rdma_wc_send(), sendto drop path handlers, and various completion error paths for read-list and receive operations. Additionally, the fix ensures that svc_rdma_detach() also wakes these waitqueues, covering synchronous close paths that bypass the deferred close mechanism entirely. This comprehensive approach addresses both the asynchronous and synchronous transport closure scenarios while maintaining backward compatibility with existing system behavior patterns. The solution effectively breaks the deadlock cycle by ensuring proper predicate re-evaluation for all parked threads during transport teardown operations, thereby enabling proper resource cleanup and preventing indefinite hanging conditions across all svcrdma transport management paths.