CVE-2026-53399 in Linux
Summary
by MITRE • 07/19/2026
In the Linux kernel, the following vulnerability has been resolved:
nfsd: release layout stid on setlease failure
nfs4_alloc_stid() publishes the new stid into cl->cl_stateids via idr_alloc_cyclic() under cl_lock before returning to nfsd4_alloc_layout_stateid(). When nfsd4_layout_setlease() then fails, the error path frees the layout stateid directly with kmem_cache_free() without ever calling idr_remove(), leaving the IDR slot pointing at freed slab memory. Any subsequent IDR walker (states_show, client teardown) dereferences the dangling pointer.
The correct teardown for an IDR-published stid is nfs4_put_stid(), which removes the IDR slot under cl_lock, dispatches sc_free (nfsd4_free_layout_stateid) to release ls->ls_file via nfsd4_close_layout(), and drops the nfs4_file reference in its tail.
A second issue blocks that switch: nfsd4_free_layout_stateid() unconditionally inspects ls->ls_fence_work via delayed_work_pending() under ls_lock, but INIT_DELAYED_WORK(&ls->ls_fence_work, ...) currently runs only after the setlease call. On the setlease-failure path the destructor would touch an uninitialized delayed_work.
nfsd4_alloc_layout_stateid() nfs4_alloc_stid() /* idr_alloc_cyclic under cl_lock */ nfsd4_layout_setlease() /* fails */ nfs4_put_stid() nfsd4_free_layout_stateid() delayed_work_pending(&ls->ls_fence_work) /* needs INIT */ nfsd4_close_layout() /* nfsd_file_put(ls->ls_file) */ put_nfs4_file()
Fix by hoisting the ls_fenced / ls_fence_delay / INIT_DELAYED_WORK initialization above the nfsd4_layout_setlease() call, and replace the manual nfsd_file_put + put_nfs4_file + kmem_cache_free cleanup with a single nfs4_put_stid(stp).
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Analysis
by VulDB Data Team • 07/19/2026
The vulnerability described involves a critical race condition and memory management flaw within the Linux kernel's NFS server implementation, specifically affecting the nfsd component that handles NFS version 4 operations. This issue stems from improper handling of state identifiers during layout allocation failures, creating potential for use-after-free conditions and system instability when processing NFS client requests.
The core technical flaw occurs in the nfsd4_alloc_layout_stateid function where a new state identifier is published into the client's stateids registry using idr_alloc_cyclic() while holding cl_lock. When the subsequent nfsd4_layout_setlease() operation fails, the error path attempts to clean up by directly calling kmem_cache_free() on the layout stateid structure without properly removing it from the IDR registry via idr_remove(). This leaves an invalid pointer reference in the IDR table that can be accessed during subsequent operations such as states_show or client teardown procedures.
The improper cleanup path creates a dangling pointer scenario that violates fundamental memory safety principles and can lead to kernel crashes or privilege escalation opportunities. According to CWE-415, this represents an explicit violation of proper resource management where freed memory is accessed after deallocation. The flaw also aligns with ATT&CK technique T1068 which involves exploiting weaknesses in privileged program execution.
The operational impact extends beyond simple memory corruption as this vulnerability affects the stability of NFS server operations and could be exploited by remote attackers to cause denial-of-service conditions or potentially gain elevated privileges within the kernel space. The issue is particularly concerning because it occurs during normal NFS operation when clients request layout changes, making it a persistent threat in production environments relying on NFS services.
The fix implements a proper resource cleanup sequence by introducing nfs4_put_stid() as the sole method for destroying state identifiers, which correctly handles all aspects of cleanup including IDR slot removal under proper locking, dispatching sc_free callbacks through nfsd4_free_layout_stateid(), and managing reference counts for nfs4_file structures. This approach ensures that all cleanup operations occur in the correct order and under appropriate synchronization primitives.
The second aspect of the fix addresses a dependency issue where delayed work initialization was occurring after the setlease call, meaning that on failure paths the fence work structure remained uninitialized. By moving the INIT_DELAYED_WORK() call before nfsd4_layout_setlease(), the fix ensures proper initialization of all required data structures regardless of whether the lease operation succeeds or fails. This modification aligns with best practices for resource initialization and prevents uninitialized memory access patterns that could lead to further system instability.
The corrected implementation follows established kernel patterns for state management where complex cleanup operations are centralized through well-defined interfaces rather than scattered manual memory management. The solution eliminates the need for multiple manual cleanup calls by leveraging the existing nfs4_put_stid() infrastructure, reducing complexity and potential for similar issues in the future while maintaining proper reference counting and locking semantics throughout the NFS server's layout state management subsystem.