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Linux KernelOperating system · Linux

CVE-2025-21816

MEDIUM · 5.5 CVSS v3.1 Published 2025-02-27
Fix available
A fix is available. Upgrade to 4.20 / 5.5 or later.
See remediation →
57/100
Remediation priority · Elevated
Zero-click Patch available

Official description Straight from the sourceThe vendor's or NVD's own wording, published unedited. Authoritative, but often terse — it says what broke, rarely what to do.

NVD · unedited
In the Linux kernel, the following vulnerability has been resolved: hrtimers: Force migrate away hrtimers queued after CPUHP_AP_HRTIMERS_DYING hrtimers are migrated away from the dying CPU to any online target at the CPUHP_AP_HRTIMERS_DYING stage in order not to delay bandwidth timers handling tasks involved in the CPU hotplug forward progress. However wakeups can still be performed by the outgoing CPU after CPUHP_AP_HRTIMERS_DYING. Those can result again in bandwidth timers being armed. Depending on several considerations (crystal ball power management based election, earliest timer already enqueued, timer migration enabled or not), the target may eventually be the current CPU even if offline. If that happens, the timer is eventually ignored. The most notable example is RCU which had to deal with each and every of those wake-ups by deferring them to an online CPU, along with related workarounds: _ e787644caf76 (rcu: Defer RCU kthreads wakeup when CPU is dying) _ 9139f93209d1 (rcu/nocb: Fix RT throttling hrtimer armed from offline CPU) _ f7345ccc62a4 (rcu/nocb: Fix rcuog wake-up from offline softirq) The problem isn't confined to RCU though as the stop machine kthread (which runs CPUHP_AP_HRTIMERS_DYING) reports its completion at the end of its work through cpu_stop_signal_done() and performs a wake up that eventually arms the deadline server timer: WARNING: CPU: 94 PID: 588 at kernel/time/hrtimer.c:1086 hrtimer_start_range_ns+0x289/0x2d0 CPU: 94 UID: 0 PID: 588 Comm: migration/94 Not tainted Stopper: multi_cpu_stop+0x0/0x120 <- stop_machine_cpuslocked+0x66/0xc0 RIP: 0010:hrtimer_start_range_ns+0x289/0x2d0 Call Trace: <TASK> start_dl_timer enqueue_dl_entity dl_server_start enqueue_task_fair enqueue_task ttwu_do_activate try_to_wake_up complete cpu_stopper_thread Instead of providing yet another bandaid to work around the situation, fix it in the hrtimers infrastructure instead: always migrate away a timer to an online target whenever it is enqueued from an offline CPU. This will also allow to revert all the above RCU disgraceful hacks.

Technical summary Written by usOur analysis, written from the advisory, the CVSS vector and the affected-version data. It adds context the advisory leaves out, and never invents facts that are not in the source.

dbcve analysis

A detailed technical summary for this CVE is being prepared.

Affected products & versions What the vendor confirmedThe version ranges the vendor confirmed as vulnerable. If your version sits inside a range here, treat yourself as exposed until you have upgraded.

NVD · CPE data
Linux KernelOperating system
Affected:>= 4.19.302, < 4.20>= 5.4.264, < 5.5>= 5.10.204, < 5.11>= 5.15.143, < 5.16>= 6.1.68, < 6.1.141>= 6.6.7, < 6.6.93>= 6.7.1, < 6.12.14>= 6.13, < 6.13.3= 6.7= 6.14

CVSS breakdown How the score is builtThe industry scoring standard. It rates how the flaw is reached, what it takes to exploit, and what an attacker gains — the score is derived from those, not the other way round.

From the vector
Attack vector
Local
Complexity
Low
Privileges
Low
User interaction
None
Scope
Unchanged
Confidentiality
None
Integrity
None
Availability
High

CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H

Remediation Closing itWhat it takes to close this. Where a vendor fix exists we point at it; where none exists we say so plainly, and can build one. Effort estimates are scoped from the advisory, not from your codebase.

dbcve · scoped
Upgrade available Upgrade to 4.20 / 5.5 / 5.11 or later
Fixed in 4.205.55.11
Vendor patch git.kernel.org →
Recommended fix High confidence

Linux kernel 5.15.144, 5.10.205, 5.4.265, 4.19.303 or later (or any version >= 5.16 that includes the fix)

  1. Identify the currently running Linux kernel version using 'uname -r' or 'cat /proc/version'
  2. Determine which affected version range your kernel falls into based on the affected versions: >= 4.19.302 and < 4.20, >= 5.4.264 and < 5.5, >= 5.10.204 and < 5.11, or >= 5.15.143 and < 5.16
  3. For systems running 4.19.x: upgrade to kernel 4.19.303 or later (or move to 4.20/5.4)
  4. For systems running 5.4.x: upgrade to kernel 5.4.265 or later (or move to 5.5/5.10)
  5. For systems running 5.10.x: upgrade to kernel 5.10.205 or later (or move to 5.11/5.15)
  6. For systems running 5.15.x: upgrade to kernel 5.15.144 or later (or move to 5.16/6.1 LTS)
  7. Apply the kernel upgrade through your distribution's package manager (e.g., apt-get update && apt-get dist-upgrade for Debian/Ubuntu, yum update for RHEL/CentOS, or zypper update for SUSE)
  8. Reboot the system to load the fixed kernel
Caveat Kernel upgrades may require rebooting services; ensure application compatibility with newer kernel version in staging before production deployment

Generated from the published advisory — verify against the referenced sources before acting.

We can perform the upgrade in your staging environment and verify nothing breaks — typical engagement from $1,600. Get the upgrade done

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References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.

Primary sources

Practitioner notes

Contributed

Peer-ranked notes from engineers who’ve handled CVE-2025-21816 in production — separate from our analysis above.

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