CVE-2026-90149 in Linux
Summary
by MITRE • 09/17/2026
In the Linux kernel, the following vulnerability has been resolved:
NFSv4/flexfiles: fix NULL dereference for NFSv4.0 data servers
flexfiles accepts NFSv4.0 data servers, but two NFSv4 code paths assume the data server client has a session. Unlike NFSv4.1+, an NFSv4.0 client has no session (clp->cl_session is NULL; it uses clp->cl_slot_tbl), so I/O to a v4.0 flexfiles DS oopses:
- nfs4_init_ds_session() dereferences clp->cl_session->session_state while seeding the DS lease. It also only seeds cl_lease_time when NFS4_SESSION_INITING is set; without a session that never happens, so cl_lease_time stays 0 and nfs4_renew_state() busy-loops, requeuing every 5 seconds. Seed the lease whenever there is no session and return before touching session state.
- ff_layout_async_handle_error_v4() dereferences clp->cl_session->fc_slot_table on every DS I/O error. Fall back to the v4.0 transport slot table (clp->cl_slot_tbl) when there is no session.
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Analysis
by VulDB Data Team • 09/17/2026
The Linux kernel contains a critical flaw within the NFSv4 flexfiles layout driver that results in null pointer dereferences and subsequent system instability when interacting with NFS version 4.0 data servers. This vulnerability arises from an architectural assumption embedded in the codebase where specific functions expect the presence of an NFS session structure, which is fundamentally absent in NFSv4.0 implementations. While newer versions like NFSv4.1 and beyond utilize a dedicated session object to manage state and slot tables, NFSv4.0 relies on a legacy client structure that does not include this session component. Consequently, any attempt by the flexfiles driver to access session-specific fields triggers a kernel panic due to dereferencing a null pointer, effectively crashing the system or causing severe service disruption for users relying on this configuration.
The first manifestation of this flaw occurs during the initialization phase when seeding the data server lease through the nfs4_init_ds_session function. The code attempts to access clp->cl_session->session_state without verifying that clp->cl_session is non-null, leading directly to a null pointer dereference crash. Furthermore, even if the initial dereference were avoided, the logic for setting the lease time contains a logical error tied to session states. The function only updates cl_lease_time when the NFS4_SESSION_INITING flag is set, but since an NFSv4.0 client lacks a session entirely, this condition never evaluates to true. This leaves the lease timer at zero, causing nfs4_renew_state to enter a busy loop that requeues operations every five seconds indefinitely, consuming CPU resources and preventing normal I/O progress until the system becomes unresponsive or is manually restarted.
The second area of vulnerability involves error handling during asynchronous I/O operations via ff_layout_async_handle_error_v4. This function assumes the existence of a session slot table by dereferencing clp->cl_session->fc_slot_table whenever an error occurs on a data server connection. In NFSv4.0 environments, this pointer is null, resulting in another immediate kernel oops or panic upon encountering any I/O error condition. The lack of proper fallback logic means that transient network issues or storage errors can escalate into system crashes rather than being handled gracefully through the appropriate legacy slot table mechanism designed for version 4.0 clients.
From a classification perspective, this vulnerability aligns with CWE-476, which denotes NULL Pointer Dereference, as the root cause is the failure to validate pointer validity before access. It also relates to CWE-253, Incorrect Check of Function Return Value or Status, insofar as the code fails to properly check for the absence of a session object before proceeding with operations that require it. In terms of attack vectors and behavior mapping under MITRE ATT&CK, this issue falls under T1079, Application Window Discovery, if considered from a persistence standpoint due to system instability, but more accurately reflects aspects of Denial of Service through resource exhaustion or crash induced by local configuration triggers, potentially mapped to TA0040 Impact. The scenario is primarily exploitable locally when an administrator configures flexfiles with NFSv4.0 data servers, making it a high-severity stability issue rather than a remote code execution vector.
To mitigate this vulnerability, the Linux kernel maintainers have implemented fixes that introduce explicit checks for session existence before accessing session-specific structures. For the lease initialization path, the logic has been adjusted to seed the lease time even when no session is present and to return early if session state access would be invalid. Similarly, in the error handling routine, a fallback mechanism now directs traffic to clp->cl_slot_tbl instead of attempting to use fc_slot_table when clp->cl_session is null. Administrators running affected kernel versions must apply these upstream patches or update their systems to patched releases that include this correction. Until such updates are applied, organizations utilizing NFSv4 flexfiles layouts should consider avoiding data servers configured with NFS version 4.0 protocols and instead upgrade the server configuration to support NFSv4.1 or higher, which natively supports session-based operations and avoids these legacy code paths entirely.