high CVSS 8.8 EPSS <0.1%

CVE-2025-21876·Kernel vulnerability

In the Linux kernel, the following vulnerability has been resolved: iommu/vt-d: Fix suspicious RCU usage Commit <d74169ceb0d2> ("iommu/vt-d: Allocate DMAR fault interrupts locally") moved the call to enable_drhd_fault_handling() to a code path that does not hold any lock while traversing the drhd list. Fix it by ensuring the dmar_global_lock lock is held when traversing the drhd list. Without this fix, the following warning is triggered: ============================= WARNING: suspicious RCU usage 6.14.0-rc3 #55 Not tainted ----------------------------- drivers/iommu/intel/dmar.c:2046 RCU-list traversed in non-reader section!! other info that might help us debug this: rcu_scheduler_active = 1, debug_locks = 1 2 locks held by cpuhp/1/23: #0: ffffffff84a67c50 (cpu_hotplug_lock){++++}-{0:0}, at: cpuhp_thread_fun+0x87/0x2c0 #1: ffffffff84a6a380 (cpuhp_state-up){+.+.}-{0:0}, at: cpuhp_thread_fun+0x87/0x2c0 stack backtrace: CPU: 1 UID: 0 PID: 23 Comm: cpuhp/1 Not tainted 6.14.0-rc3 #55 Call Trace: <TASK> dump_stack_lvl+0xb7/0xd0 lockdep_rcu_suspicious+0x159/0x1f0 ? __pfx_enable_drhd_fault_handling+0x10/0x10 enable_drhd_fault_handling+0x151/0x180 cpuhp_invoke_callback+0x1df/0x990 cpuhp_thread_fun+0x1ea/0x2c0 smpboot_thread_fn+0x1f5/0x2e0 ? __pfx_smpboot_thread_fn+0x10/0x10 kthread+0x12a/0x2d0 ? __pfx_kthread+0x10/0x10 ret_from_fork+0x4a/0x60 ? __pfx_kthread+0x10/0x10 ret_from_fork_asm+0x1a/0x30 </TASK> Holding the lock in enable_drhd_fault_handling() triggers a lockdep splat about a possible deadlock between dmar_global_lock and cpu_hotplug_lock. This is avoided by not holding dmar_global_lock when calling iommu_device_register(), which initiates the device probe process.

Severity
high
Software
Kernel
Fixed in
6.13.6
Published

How likely is exploitation

FIRST's Exploit Prediction Scoring System puts the probability of this CVE being exploited in the next 30 days at <0.1% — higher than 15% of all published CVEs. EPSS is an independent forecast of exploitation, not a severity rating: it says nothing about how much damage a successful attack does, which is what a CVSS score measures.

Read the two together. CVE-2025-21876 scores 8.8 on CVSS, so a successful attack would be serious — but at <0.1%, almost nobody is currently attempting it. That is a different situation from a flaw under mass exploitation, and it is usually the difference between patching on your next maintenance window and patching tonight.

Affected versions

From: 6.13

Until: 6.13.6

Fixed in: 6.13.6

How to fix this CVE

Update your Linux kernel to version 6.13.6 or later to resolve a synchronization issue in the Intel IOMMU driver's fault handling initialization. This patch ensures proper locking during device enumeration on multi-core systems and prevents kernel warnings and potential stability issues during CPU hotplug operations.

sudo dnf update kernel kernel-devel

Defensia detects this vulnerability

How to check if you are affected

  1. Check your kernel version: uname -r — confirm if it is between 6.13 and 6.13.6
  2. Verify IOMMU presence: grep -i iommu /proc/cmdline — check if Intel DMAR (VT-d) is enabled in your boot parameters
  3. Review kernel logs for RCU warnings: sudo dmesg | grep -A 5 'suspicious RCU usage' — look for warnings referencing drivers/iommu/intel/dmar.c:2046
  4. Confirm patch status: grep -i vt-d /proc/version — if kernel is ≥6.13.6, the fix is applied

FAQ

What is CVE-2025-21876?

This vulnerability is a synchronization bug in the Linux kernel's Intel IOMMU (VT-d) driver where a critical list traversal was performed without proper locking, causing RCU (Read-Copy-Update) warnings and potential race conditions during CPU hotplug events.

Is CVE-2025-21876 being actively exploited?

No, this vulnerability is not being actively exploited in the wild. There are no public exploits available, and it is not tracked by CISA's Known Exploited Vulnerabilities (KEV) catalog.

What versions of Kernel are affected by CVE-2025-21876?

Linux kernel versions 6.13 through 6.13.5 are affected. The vulnerability was introduced in kernel 6.13 and patched in version 6.13.6.

How do I check if my server is vulnerable to CVE-2025-21876?

Run 'uname -r' to check your kernel version. If the output shows a version between 6.13 and 6.13.5 (e.g., 6.13.0, 6.13.3, 6.13.5), your system is vulnerable. Also verify VT-d is enabled with 'grep iommu /proc/cmdline'.

Does Defensia detect CVE-2025-21876?

Yes — Defensia's CVE advisory scanner compares installed package versions against the NVD database. If the Linux kernel is installed on a monitored server, CVE-2025-21876 will appear in your dashboard with remediation steps.

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References

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