CVE-2026-89798 in Linux
Summary
by MITRE • 09/16/2026
In the Linux kernel, the following vulnerability has been resolved:
rpcrdma: arm rn_done before publishing the notification
rpcrdma_rn_register() inserts @rn into rd_xa with xa_alloc() before storing the caller's callback in rn->rn_done. The xarray makes @rn reachable to rpcrdma_remove_one(), which walks rd_xa and invokes rn->rn_done(rn) for every registered notification. A device removal that races a fresh registration can therefore observe @rn with rn_done still NULL, because the notification objects are zero allocated by their owners, and call through a NULL function pointer.
Store rn->rn_done before xa_alloc() publishes @rn. The xarray's store-side and load-side ordering then guarantees that any CPU which finds @rn in rd_xa also observes the armed callback.
rpcrdma_rn_unregister() treats a non-NULL rn_done as the sentinel for a completed registration, so the early store must not survive a failed registration. Clear rn_done again when xa_alloc() fails. Were it left set, the failed-accept cleanup path would call rpcrdma_rn_unregister() on an @rn that was never inserted, erasing an unrelated rd_xa slot and underflowing rd_kref.
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Analysis
by VulDB Data Team • 09/16/2026
The vulnerability identified in the Linux kernel's RPC over RDMA subsystem stems from a critical race condition involving memory ordering and pointer publication within the rpcrdma_rn_register function. The core issue arises because the registration process inserts a notification object into an xarray data structure using xa_alloc before storing the caller's callback function pointer into the rn_done field of that object. This sequence creates a window where the notification object becomes reachable to other parts of the kernel, specifically rpcrdma_remove_one, which iterates through the rd_xa array and attempts to invoke the registered callbacks. Since notification objects are typically zero-allocated by their owners, the rn_done pointer is initially NULL. If a device removal event occurs concurrently with this fresh registration attempt, the removal logic may locate the newly inserted object in the xarray but observe that its callback pointer remains NULL due to the incomplete initialization sequence.
This race condition results in a null pointer dereference when rpcrdma_remove_one attempts to call through the rn->rn_done function pointer. In kernel space, executing code at address zero typically leads to an immediate system crash or panic, causing a denial of service for the entire host system. The root cause is fundamentally a lack of proper memory synchronization between publishing the object's reference in the shared data structure and initializing its internal state fields. Without explicit ordering guarantees, different CPU cores can observe inconsistent views of memory, allowing one core to see the published pointer while another sees uninitialized or zeroed-out member variables within that same object.
The resolution involves reordering the operations within rpcrdma_rn_register to store the rn->rn_done callback before calling xa_alloc to publish the notification object into the xarray. This change leverages the inherent load-side and store-side memory ordering guarantees provided by the xarray implementation, ensuring that any CPU which successfully finds the object in rd_xa will also observe the correctly armed callback function pointer. By establishing this strict ordering, the race condition is eliminated because the object cannot be accessed for execution until its initialization is fully complete from a visibility perspective across all cores.
Additionally, the fix addresses a secondary logic error related to failed registration attempts. The rpcrdma_rn_unregister function relies on checking whether rn_done is non-NULL as a sentinel to determine if a registration was successfully completed. If xa_alloc fails and the code path does not clear the previously stored rn_done value, subsequent cleanup routines might incorrectly assume that the object was registered. This would lead to calling rpcrdma_rn_unregister on an object that was never inserted into the xarray. Such erroneous unregistration can erase unrelated slots in the rd_xa array or cause reference count underflows by decrementing rd_kref for objects not currently tracked, leading to potential memory corruption and further instability. The fix ensures that rn_done is cleared if xa_alloc fails, preventing these invalid cleanup paths from executing on uninitialized data structures.
From a classification standpoint, this vulnerability aligns with CWE-362, which describes concurrent execution using shared resources with improper synchronization, specifically manifesting as a race condition leading to a null pointer dereference classified under CWE-476. The attack vector allows for local denial of service by triggering the device removal and registration race, potentially requiring physical access or elevated privileges depending on system configuration. In terms of adversarial tactics, this scenario reflects techniques associated with resource manipulation where an attacker might attempt to exploit timing windows in kernel subsystems to cause instability, fitting within broader categories of availability impact found in ATT&CK frameworks related to system exploitation for denial of service.
Mitigation strategies primarily involve applying the upstream Linux kernel patch that corrects the initialization order and handles error paths correctly. System administrators should ensure their kernels are updated with this fix applied. For environments where immediate patching is not feasible, monitoring logs for null pointer dereference panics related to rpcrdma or xarray operations can help detect exploitation attempts. Furthermore, reducing the window of vulnerability by minimizing concurrent device hotplug events and registration calls during high-load periods may reduce the probability of triggering this specific race condition until a permanent fix is deployed.