CVE-2025-21850 in Linux
Summary
by MITRE • 03/12/2025
In the Linux kernel, the following vulnerability has been resolved:
nvmet: Fix crash when a namespace is disabled
The namespace percpu counter protects pending I/O, and we can only safely diable the namespace once the counter drop to zero. Otherwise we end up with a crash when running blktests/nvme/058 (eg for loop transport):
[ 2352.930426] [ T53909] Oops: general protection fault, probably for non-canonical address 0xdffffc0000000005: 0000 [#1] PREEMPT SMP KASAN PTI
[ 2352.930431] [ T53909] KASAN: null-ptr-deref in range [0x0000000000000028-0x000000000000002f]
[ 2352.930434] [ T53909] CPU: 3 UID: 0 PID: 53909 Comm: kworker/u16:5 Tainted: G W 6.13.0-rc6 #232
[ 2352.930438] [ T53909] Tainted: [W]=WARN
[ 2352.930440] [ T53909] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.16.3-3.fc41 04/01/2014
[ 2352.930443] [ T53909] Workqueue: nvmet-wq nvme_loop_execute_work [nvme_loop]
[ 2352.930449] [ T53909] RIP: 0010:blkcg_set_ioprio+0x44/0x180
as the queue is already torn down when calling submit_bio();
So we need to init the percpu counter in nvmet_ns_enable(), and wait for it to drop to zero in nvmet_ns_disable() to avoid having I/O pending after the namespace has been disabled.
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Analysis
by VulDB Data Team • 12/14/2025
The vulnerability CVE-2025-21850 addresses a critical race condition in the Linux kernel's NVMe over Fabrics target implementation that can lead to system crashes and potential denial of service conditions. This issue specifically affects the nvmet subsystem which handles NVMe target operations for networked storage environments. The flaw manifests when attempting to disable a namespace while pending I/O operations are still active, creating a scenario where the kernel attempts to access already freed memory structures. The problem occurs during the execution of blktests/nvme/058 test case which utilizes loop transport mechanisms, making it particularly concerning for storage virtualization environments that rely heavily on NVMe over Fabrics implementations.
The technical root cause stems from improper synchronization between namespace enablement and disablement operations within the NVMe target subsystem. The vulnerability exploits a missing safety check in the namespace disablement process where the kernel fails to verify that all pending I/O operations have completed before proceeding with namespace deactivation. This percpu counter mechanism, designed to track pending I/O operations, is not properly initialized during namespace enablement or adequately synchronized during disablement, leading to a scenario where I/O submission occurs against already torn-down queue structures. The crash occurs in the blkcg_set_ioprio function when attempting to access a null pointer dereference, specifically at address 0x0000000000000028, which indicates that the kernel attempted to operate on freed or invalid memory structures during the I/O submission process.
The operational impact of this vulnerability extends beyond simple system crashes to potentially compromise storage reliability in production environments. When exploited, the vulnerability can cause kernel oops messages and general protection faults that result in system instability, particularly in virtualized environments using NVMe loop transport mechanisms. The vulnerability affects systems running Linux kernel versions that include the affected nvmet subsystem, making it relevant to enterprise storage solutions, cloud infrastructure providers, and any environment relying on NVMe over Fabrics for high-performance storage networking. The specific test case mentioned in the vulnerability description demonstrates that this issue is reproducible under controlled conditions, suggesting that it could be exploited by malicious actors to cause system instability or denial of service attacks against storage services.
Mitigation strategies for this vulnerability require immediate kernel updates to versions containing the patched implementation that properly initializes percpu counters during namespace enablement and enforces proper synchronization during disablement operations. The fix implements a mechanism to ensure that all pending I/O operations complete before namespace disablement proceeds, preventing the race condition that leads to the null pointer dereference. System administrators should prioritize applying kernel patches that address this specific vulnerability, particularly in environments where NVMe over Fabrics targets are actively used. Additionally, monitoring systems should be configured to detect kernel oops messages or general protection faults that may indicate exploitation attempts. Organizations should also review their NVMe target configurations to ensure that namespace enablement and disablement operations are properly synchronized and avoid rapid succession of these operations that could trigger the race condition. This vulnerability aligns with CWE-362, which describes a race condition in concurrent programming, and represents a specific instance of improper synchronization in kernel-level storage subsystems that can lead to critical system failures.