CVE-2026-98243 in Linux
Summary
by MITRE • 10/06/2026
In the Linux kernel, the following vulnerability has been resolved:
dma-buf/dma-fence: fix checking signaling bit for timeline and driver name v3
The patch "dma-buf: dma-fence: Fix potential NULL pointer dereference" changed the check to test for the ops pointer instead of the signaled bit to avoid a potential NULL dereference when the ops pointer has been cleared.
The problem is now that the ops pointer is cleared only when neither the release nor the wait callback is implemented and this isn't true for a lot of dma_fence implementations yet. So those implementations lost the RCU protection after signaling of the returned string resulting in potential use after free.
Add the signaling check additional to the ops pointer check so that we have both the protection against NULL dereference as well as the RCU protection after signaling for the returned string.
v2: improve comments to note RCU protection and explain why we check both signaling state and ops pointer v3: some comment improvements suggested by Philip
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Analysis
by VulDB Data Team • 10/06/2026
The Linux kernel's dma-buf subsystem, which facilitates buffer sharing between different drivers and devices, relies heavily on the dma-fence infrastructure for synchronization. A recent patch intended to fix a potential NULL pointer dereference in this subsystem inadvertently introduced a new class of vulnerabilities related to use-after-free conditions. The original issue arose because code was checking the signaled bit directly without verifying if the associated operations structure remained valid, leading to crashes when that structure had been freed. To address this, developers modified the check to verify the ops pointer instead, assuming that clearing this pointer would indicate the fence is no longer active and thus safe from concurrent access issues.
However, this fix was based on an incorrect assumption about how dma_fence implementations manage their lifecycle. The ops pointer is only cleared when neither the release nor the wait callback functions are implemented by a specific driver. Many existing dma-fence implementations do implement these callbacks to handle complex synchronization scenarios or resource cleanup. Consequently, for those drivers that retain their ops pointers even after signaling completion, the RCU (Read-Copy-Update) protection mechanism is not properly applied to the returned string data. This creates a window where user-space applications or other kernel components might access memory that has already been freed by the driver's release callback, resulting in use-after-free vulnerabilities.
This vulnerability aligns with CWE-416, Use After Free, as it involves accessing memory after it has been made available for reuse without proper synchronization checks. From a threat modeling perspective using MITRE ATT&CK techniques, this flaw could potentially be leveraged for privilege escalation or denial of service attacks if an attacker can trigger the race condition between signaling and release operations in specific driver implementations. The lack of RCU protection means that concurrent readers may dereference stale pointers, leading to kernel panics or arbitrary code execution depending on memory layout exploitation possibilities.
The resolution involves adding a check for the signaling state alongside the existing ops pointer validation. By ensuring both conditions are met before accessing the fence data, the patch restores proper RCU semantics and prevents access to freed memory regions. This dual-check approach guarantees that even if an implementation retains its ops structure after signaling, the system correctly identifies that the fence is no longer active for synchronization purposes. Developers should ensure all dma-fence implementations adhere to these updated checks during code reviews to maintain kernel stability across diverse hardware drivers.
Mitigation strategies include applying the latest kernel patches that incorporate this fix and auditing custom or out-of-tree driver implementations to verify they follow proper lifecycle management protocols. System administrators running affected kernels should prioritize updates to mitigate risks associated with local privilege escalation through dma-buf interactions. Long-term, enforcing stricter coding standards for RCU usage in synchronization primitives within the graphics and multimedia subsystems will help prevent similar logical errors from recurring in future kernel versions.