CVE-2025-40348 in Linux
Summary
by MITRE • 12/16/2025
In the Linux kernel, the following vulnerability has been resolved:
slab: Avoid race on slab->obj_exts in alloc_slab_obj_exts
If two competing threads enter alloc_slab_obj_exts() and one of them fails to allocate the object extension vector, it might override the valid slab->obj_exts allocated by the other thread with OBJEXTS_ALLOC_FAIL. This will cause the thread that lost this race and expects a valid pointer to dereference a NULL pointer later on.
Update slab->obj_exts atomically using cmpxchg() to avoid slab->obj_exts overrides by racing threads.
Thanks for Vlastimil and Suren's help with debugging.
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Analysis
by VulDB Data Team • 02/21/2026
The vulnerability CVE-2025-40348 represents a critical race condition within the Linux kernel's memory management subsystem, specifically affecting the slab allocator component that is fundamental to kernel memory operations. This flaw exists in the alloc_slab_obj_exts function where concurrent thread execution can lead to improper handling of object extension vectors. The issue manifests when multiple threads simultaneously attempt to allocate memory objects within the same slab cache, creating a scenario where one thread's failure to allocate an object extension vector can overwrite a valid pointer that another thread had successfully allocated. This race condition directly violates the fundamental principles of concurrent programming and can lead to catastrophic system failures.
The technical implementation of this vulnerability stems from the improper atomic handling of the slab->obj_exts pointer within the kernel's memory management architecture. When threads enter the alloc_slab_obj_exts function concurrently, the first thread to fail the allocation attempt may overwrite the valid slab->obj_exts pointer with the special OBJEXTS_ALLOC_FAIL marker. This occurs because the update operation lacks proper atomic synchronization mechanisms, allowing competing threads to interfere with each other's memory modifications. The flaw specifically relates to the absence of proper memory ordering guarantees when updating shared data structures, creating a classic race condition where the outcome depends on the timing of thread execution. This vulnerability maps directly to CWE-362, which describes a race condition in concurrent programming where two or more threads access shared data concurrently, and the final result depends on the order of execution.
The operational impact of CVE-2025-40348 extends beyond simple memory corruption to potentially enable privilege escalation and system instability across all Linux kernel versions that utilize the affected slab allocator implementation. When a thread attempts to dereference the NULL pointer that resulted from the race condition, the kernel will experience a null pointer dereference exception, leading to immediate system crashes or unpredictable behavior. This vulnerability can be exploited by malicious actors to cause denial of service attacks against running systems or to potentially escalate privileges through carefully crafted memory allocation patterns. The attack surface is particularly concerning because the slab allocator is extensively used throughout the kernel for managing various data structures, making this vulnerability potentially exploitable in multiple kernel subsystems. The vulnerability aligns with ATT&CK technique T1068, which involves privilege escalation through kernel vulnerabilities, and represents a critical weakness in the kernel's memory management defenses.
The mitigation strategy for CVE-2025-40348 involves implementing atomic operations using the compare-and-swap instruction (cmpxchg) to ensure that updates to the slab->obj_exts pointer occur atomically. This approach prevents competing threads from overwriting valid pointers with failure markers, thereby eliminating the race condition that leads to null pointer dereferences. The fix requires modifying the alloc_slab_obj_exts function to use atomic operations when updating the obj_exts pointer, ensuring that only one thread can successfully update the pointer value at any given time. This solution directly addresses the root cause by implementing proper synchronization mechanisms that prevent the race condition while maintaining the performance characteristics of the slab allocator. The implementation should follow established kernel memory management best practices and maintain compatibility with existing kernel subsystems that depend on the slab allocator's functionality. The fix essentially transforms a non-atomic pointer update into an atomic operation that preserves the integrity of shared memory structures under concurrent access conditions.