CVE-2025-39821 perf: Avoid undefined behavior from stopping/starting inactive events
A logic flaw in Linux kernel perf event throttling causes undefined behavior when processing inactive events, potentially leading to system instability or crashes.
- Exploited in the wild
- Not known
- Login needed
- Any signed-in user
- Attackable
- Local or same network only
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2025-39821?
CVE-2025-39821 is a high-severity vulnerability in Linux kernel, published on 16 Sep 2025. It can only be exploited from the same machine or local network, with no action from the victim.
Official 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:
Weakness: CWE-787
Which versions are affected by CVE-2025-39821?
It affects 1 product.
Products include Linux kernel.
How do I fix CVE-2025-39821?
Apply the vendor's patch.
Is CVE-2025-39821 being exploited?
There is no known exploitation of CVE-2025-39821 in the wild and no public exploit code.
Other vulnerabilities in Linux kernel
Sources
Also known as BELL-CVE-2025-39821, DEBIAN-CVE-2025-39821, GHSA-cqwc-52gw-4x53, UBUNTU-CVE-2025-39821.