CVE-2025-39821

/// Technical 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 P

/// Remediation

Check the references section for vendor advisories, patches, and mitigation guidance. If immediate patching is not possible, review the CVSS vector to understand the attack surface and apply compensating controls such as network segmentation or access restrictions.

/// References