# CVE-2025-39821: linux: In the Linux kernel, the following vulnerability has been resolved:  perf: Avoid undefined behavior from stopping/sta...

CVE-2025-39821 is fixed in linux on 1 platform by DEBIAN-CVE-2025-39821 (version 6.16.5-1), released September 16, 2025. It is not reported as exploited. Patch Radar priority: High.

- Web page: https://patchblox.com/security/cve/CVE-2025-39821
- Priority: High
- CVSS: 7.8 (high)
- EPSS: 0.0016
- CISA KEV: not listed
- Exploited (vendor report): no
- Publicly disclosed: no
- Published: Sep 16, 2025

## Description

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:

## Where it's fixed

| Platform | Product | Fixed in | Fix | Released |
|---|---|---|---|---|
| Linux | linux Debian 14 | 6.16.5-1 | [DEBIAN-CVE-2025-39821](https://patchblox.com/security/data/fix/linux/debian-cve-2025-39821.md) | Sep 16, 2025 |

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