CVE-2026-89705 in Linux
Summary
by MITRE • 09/11/2026
In the Linux kernel, the following vulnerability has been resolved:
nfsd: restore rq_status_counter to even on all nfsd_dispatch() exit paths
nfsd_dispatch() sets rq_status_counter to an odd value once a request has been decoded, and back to an even value once it has been fully processed, forming a seq-lock like protocol with the lockless reader in nfsd_nl_rpc_status_get_dumpit().
Only the fully successful path restored the counter to even. The cache-hit (RC_REPLY), drop (RC_DROPIT / RQ_DROPME) and encode-error paths all return after the odd-valued store without ever bringing the counter back to even. Once one of those paths is taken, rq_status_counter is left odd: the next request's decode ORs in 1 (still odd) and only a subsequent successful encode restores even. While stuck odd, the dumpit reader treats the rqstp fields as stable and its retry check compares against the same unchanging odd value, so it never detects concurrent mutation. This exposes actively mutating fields (e.g. args->ops / args->opcnt during compound decode and release) to the lockless reader, which can read past the end of the 8-element inline ops array.
Add a helper that advances the counter to the next even value and call it on every return path that follows the odd-valued store. The decode-error path is left untouched as it is reached before the counter is set odd.
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Analysis
by VulDB Data Team • 09/11/2026
The vulnerability in question resides within the Network File System daemon (nfsd) component of the Linux kernel, specifically affecting the request dispatching mechanism known as nfsd_dispatch(). This function manages the lifecycle of incoming NFS requests, handling decoding, processing, and encoding phases. A critical aspect of this process involves a synchronization primitive implemented via rq_status_counter, which functions similarly to a seq-lock protocol designed to protect data accessed by lockless readers in the netlink RPC status dump iterator (nfsd_nl_rpc_status_get_dumpit). The intended behavior is for rq_status_counter to be set to an odd value immediately after a request has been successfully decoded and then restored to an even value once processing is complete. This alternating parity allows the reader to detect if it started reading during a mutation phase by checking if the counter changed between read attempts, thereby ensuring data consistency without requiring traditional locks for every access.
The core technical flaw arises from incomplete error handling within nfsd_dispatch(). While the function correctly sets rq_status_counter to an odd value upon successful decoding, several exit paths fail to restore it to an even state before returning control. Specifically, when a request results in a cache hit (RC_REPLY), is dropped due to resource constraints or policy decisions (RC_DROPIT / RQ_DROPME), or encounters an error during the encoding phase, the function returns immediately after setting the counter to odd but without executing the code that flips it back to even. Consequently, rq_status_counter remains stuck in an odd state until a subsequent request is successfully encoded and processed. This creates a window of vulnerability where the synchronization mechanism fails because the reader perceives no change in the status counter between its read attempts, assuming data stability when none exists.
The operational impact of this flaw is significant as it exposes actively mutating kernel structures to lockless readers that are not equipped to handle concurrent modifications safely. During the period when rq_status_counter remains odd, fields such as args->ops and args->opcnt within compound decode operations continue to mutate as new requests arrive or existing ones progress through different states. The nfsd_nl_rpc_status_get_dumpit() function, relying on the faulty assumption of stability due to unchanged counter values, proceeds to read these volatile fields without proper synchronization barriers. This lack of protection can lead to out-of-bounds memory accesses, specifically allowing the reader to access data past the end of the eight-element inline operations array embedded within the request structure. Such buffer over-reads constitute a serious security risk that could potentially be leveraged for information disclosure or further exploitation depending on what kernel memory lies adjacent to this array and how it is interpreted by subsequent code paths.
From a classification perspective, this vulnerability aligns with CWE-362: Concurrent Execution using Shared Resource with Improper Synchronization ('Race Condition'). The failure to properly synchronize access to shared data structures during error handling scenarios creates a race condition where the reader accesses inconsistent or invalid state. Additionally, the potential for out-of-bounds reads maps directly to CWE-125: Out-of-bounds Read. In terms of attack vectors and techniques, this flaw relates to ATT&CK technique T1083: File and Directory Discovery if exploited for information gathering, although its primary severity lies in the memory safety violation rather than mere discovery. The root cause is a logic error in control flow management where exceptional paths were not treated with the same rigor as successful execution paths regarding state cleanup.
Mitigation strategies primarily involve applying vendor-provided kernel patches that correct the nfsd_dispatch() function to ensure rq_status_counter is always restored to an even value on all exit paths following the initial odd-valued store. Developers should implement a helper routine that advances the counter to the next even number and invoke it uniformly across RC_REPLY, RC_DROPIT, RQ_DROPME, and encode-error returns. For system administrators unable to immediately patch, monitoring for unusual NFS daemon behavior or increased error rates in rpc status dumps may provide early indicators of exploitation attempts, though detection is difficult due to the subtle nature of race conditions. Long-term remediation requires rigorous code review practices that mandate symmetry between entry and exit state management in concurrent data structures, ensuring that all paths maintain invariant properties required by lockless synchronization protocols.