CVE-2026-93073 in Linux
Summary
by MITRE • 09/17/2026
In the Linux kernel, the following vulnerability has been resolved:
dax: read holder_ops once in dax_holder_notify_failure()
dax_holder_notify_failure() reads dax_dev->holder_ops twice without READ_ONCE() -- once for the NULL check and once for the indirect notify_failure() call. A concurrent fs_put_dax() can clear holder_ops between the two reads, so the check can observe a non-NULL pointer while the call dereferences NULL. (kill_dax() also clears holder_ops, but only after synchronize_srcu(), so it cannot race a reader that is inside dax_read_lock(); fs_put_dax() does no such synchronization.)
Fetch holder_ops once into a local with READ_ONCE() so the NULL check and the indirect call observe the same value.
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Analysis
by VulDB Data Team • 09/18/2026
The Linux kernel's Direct Access (DAX) subsystem facilitates direct memory mapping of persistent storage, allowing applications to access non-volatile media as if it were main system RAM. This mechanism relies heavily on precise synchronization primitives to manage concurrent access and lifecycle management of device structures. A specific race condition vulnerability has been identified within the dax_holder_notify_failure function, which is responsible for notifying holders about failures in DAX devices. The core technical flaw lies in how this function accesses the holder_ops pointer stored within the dax_dev structure. Specifically, the code performs two separate reads of this pointer without employing atomic or ordered memory access semantics such as READ_ONCE. The first read occurs during a NULL check to ensure safety before proceeding, while the second read is used to dereference and invoke an indirect call through that same pointer.
This dual-read pattern creates a critical window for race conditions in multi-threaded environments. A concurrent execution of fs_put_dax can clear or nullify the holder_ops field between these two operations. Because there is no synchronization barrier like synchronize_srcu protecting this specific path, it is possible for one thread to observe that holder_ops is non-NULL during the initial check, only for another thread to set it to NULL before the second read occurs. Consequently, when the code attempts to dereference the pointer for the notify_failure call, it may access a memory address containing zero or garbage data, leading to a kernel panic or arbitrary code execution depending on how the null pointer is handled by the architecture and compiler optimizations. This scenario represents a classic Time-of-Check-to-Time-of-Use (TOCTOU) vulnerability where the state of the system changes between verification and usage due to lack of atomicity in reading shared data structures under concurrent modification.
From an industry standards perspective, this flaw aligns with CWE-362, which describes Concurrent Execution using Shared Resource with Improper Synchronization resulting in a Race Condition. The specific mechanism involves accessing a shared resource without proper locking or memory ordering constraints to ensure consistency across multiple execution contexts. In terms of the MITRE ATT&CK framework for enterprise security, this vulnerability could potentially be leveraged within techniques related to Privilege Escalation via Kernel Exploitation if an attacker can trigger the race condition from user space through crafted system calls that manipulate DAX device states. The lack of proper synchronization allows a local unprivileged user to crash the kernel or potentially escalate privileges by exploiting the undefined behavior resulting from dereferencing a null pointer, thereby compromising the integrity and availability of the operating system.
The operational impact of this vulnerability is significant for systems relying on persistent memory technologies such as Intel Optane DIMMs or other NVDIMM devices. A successful exploitation can lead to immediate kernel panics, causing denial of service by rendering the host machine unresponsive until a reboot occurs. In more severe scenarios where null pointer dereferences are exploited through specific compiler optimizations or hardware behaviors that allow execution flow redirection, it could result in arbitrary code execution with kernel-level privileges. This undermines the security boundary between user-space applications and the core operating system, potentially allowing attackers to bypass standard access controls and gain full control over the compromised host.
To mitigate this vulnerability, developers must ensure that shared pointers accessed by multiple threads are read atomically or under proper synchronization locks. The recommended fix involves fetching the holder_ops pointer into a local variable using READ_ONCE() at the beginning of the function. This ensures that both the NULL check and the subsequent indirect call operate on the same consistent snapshot of the data, eliminating the race window where the value could change between checks. Additionally, reviewing other areas of the DAX subsystem for similar patterns of unprotected concurrent access is advisable to prevent analogous vulnerabilities. System administrators should apply kernel updates containing this patch promptly to restore the integrity and stability of systems utilizing Direct Access features, ensuring that synchronization primitives are correctly applied to all critical data paths involving device lifecycle management.