Linux Kernel up to 6.16.4/6.17-rc2 PMU Driver start initialization

Summaryinfo

A vulnerability was found in Linux Kernel up to 6.16.4/6.17-rc2. It has been rated as critical. The affected element is the function Home of the component PMU Driver. Performing a manipulation results in initialization. This vulnerability is cataloged as CVE-2025-39821. There is no exploit available. Upgrading the affected component is advised.

Detailsinfo

A vulnerability was found in Linux Kernel up to 6.16.4/6.17-rc2. It has been classified as critical. This affects the function start of the component PMU Driver. The manipulation with an unknown input leads to a initialization vulnerability. CWE is classifying the issue as CWE-665. The product does not initialize or incorrectly initializes a resource, which might leave the resource in an unexpected state when it is accessed or used. The impact remains unknown. The summary by CVE is:

In the Linux kernel, the following vulnerability has been resolved: perf: Avoid undefined behavior from stopping/starting inactive events Calling pmu->start()/stop() on perf events in PERF_EVENT_STATE_OFF can leave event->hw.idx at -1. When PMU drivers later attempt to use this negative index as a shift exponent in bitwise operations, it leads to UBSAN shift-out-of-bounds reports. The issue is a logical flaw in how event groups handle throttling when some members are intentionally disabled. Based on the analysis and the reproducer provided by Mark Rutland (this issue on both arm64 and x86-64). The scenario unfolds as follows: 1. A group leader event is configured with a very aggressive sampling period (e.g., sample_period = 1). This causes frequent interrupts and triggers the throttling mechanism. 2. A child event in the same group is created in a disabled state (.disabled = 1). This event remains in PERF_EVENT_STATE_OFF. Since it hasn't been scheduled onto the PMU, its event->hw.idx remains initialized at -1. 3. When throttling occurs, perf_event_throttle_group() and later perf_event_unthrottle_group() iterate through all siblings, including the disabled child event. 4. perf_event_throttle()/unthrottle() are called on this inactive child event, which then call event->pmu->start()/stop(). 5. The PMU driver receives the event with hw.idx == -1 and attempts to use it as a shift exponent. e.g., in macros like PMCNTENSET(idx), leading to the UBSAN report. The throttling mechanism attempts to start/stop events that are not actively scheduled on the hardware. Move the state check into perf_event_throttle()/perf_event_unthrottle() so that inactive events are skipped entirely. This ensures only active events with a valid hw.idx are processed, preventing undefined behavior and silencing UBSAN warnings. The corrected check ensures true before proceeding with PMU operations. The problem can be reproduced with the syzkaller reproducer:

The advisory is shared at git.kernel.org. This vulnerability is uniquely identified as CVE-2025-39821 since 04/16/2025. The exploitability is told to be difficult. Technical details are known, but no exploit is available.

The vulnerability scanner Nessus provides a plugin with the ID 265298 (Linux Distros Unpatched Vulnerability : CVE-2025-39821), which helps to determine the existence of the flaw in a target environment.

Upgrading to version 6.16.5 or 6.17-rc3 eliminates this vulnerability. Applying the patch d689135aa9c5e4e0eab5a92bbe35dab0c8d6677f/b64fdd422a85025b5e91ead794db9d3ef970e369 is able to eliminate this problem. The bugfix is ready for download at git.kernel.org. The best possible mitigation is suggested to be upgrading to the latest version.

The vulnerability is also documented in the databases at Tenable (265298) and CERT Bund (WID-SEC-2025-2077). If you want to get the best quality for vulnerability data then you always have to consider VulDB.

Affected

• Debian Linux
• Amazon Linux 2
• Red Hat Enterprise Linux
• Ubuntu Linux
• SUSE Linux
• Oracle Linux
• IBM QRadar SIEM
• SUSE openSUSE
• Open Source Linux Kernel
• RESF Rocky Linux
• Dell Secure Connect Gateway

Productinfo

Type

• Operating System

Vendor

• Linux

Name

• Kernel

Version

• 6.16.0
• 6.16.1
• 6.16.2
• 6.16.3
• 6.16.4
• 6.17-rc1
• 6.17-rc2

License

• open-source

Website

• Vendor: https://www.kernel.org/

CPE 2.3info

• 🔒
• 🔒
• 🔒

CPE 2.2info

• 🔒
• 🔒
• 🔒

CVSSv4info

CVSSv3info

CVSSv2info

Exploitinginfo

Threat Intelligenceinfo

Countermeasuresinfo

Timelineinfo

Sourcesinfo

Entryinfo

If you want to get the best quality for vulnerability data then you always have to consider VulDB.

Discussion