CVE-2026-89546
CriticalCVSS 9.8Exploitation Probability (EPSS)
Low risk9th percentile - higher than 9% of all known CVEs
Summary
In the Linux kernel, a race occurs during backchannel receive when the NFS callback service is being torn down. xprt_enqueue_bc_request() may skip enqueueing or freeing the request if xprt->bc_serv has already been cleared, leading to a leak or use-after-free.
Risk Assessment
This can lead to memory leak or use-after-free, potentially causing kernel crash or privilege escalation. However, it requires a remote NFS callback invocation at a critical moment.
Recommendation
Update the Linux kernel to a version containing the fix that closes the producer side before stopping callback threads. Check NFS servers with backchannel enabled.
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Original NVD description (English source)
In the Linux kernel, the following vulnerability has been resolved: SUNRPC: close backchannel before destroying callback service A backchannel receive can complete a request while the NFS callback service is being torn down. xprt_complete_bc_request() removes the request from bc_pa_list, drops bc_alloc_count, marks the request in use, and then asks xprt_enqueue_bc_request() to hand it to the callback service. If teardown has already cleared xprt->bc_serv, xprt_enqueue_bc_request() currently returns without enqueueing or freeing the committed request. The xprt_get() taken on entry is leaked as well. If the producer wins the race before bc_serv is cleared, it can also enqueue onto sv_cb_list after nfs_callback_down() has stopped the callback threads, leaving the request linked to a svc_serv that is about to be freed. Close the producer side before callback threads are stopped. Add xprt_svc_shutdown_bc() to clear xprt->bc_serv under bc_pa_lock, and call it on callback shutdown and callback-start failure before stopping the service threads. Requests that lose the NULL transition in xprt_enqueue_bc_request() are released through the normal backchannel free path after balancing bc_slot_count. Finally, drain any remaining sv_cb_list requests after the callback threads have stopped and before svc_destroy() frees the service.