CVE-2025-39821

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:

This vulnerability is rated as Moderate severity because exploitation requires local user execution privileges to configure specific performance event sampling groups, limiting the potential impact to a local denial of service. Triggering the flaw requires configuring aggressively sampled leader events alongside disabled sibling events to force the throttling logic to act on uninitialized hardware indices. Red Hat Enterprise Linux releases are not affected as the vulnerable code introduced in recent upstream kernels is not present in shipped Red Hat kernels.

The CVSS score(s) listed for this vulnerability do not reflect the associated product's status, and are included for informational purposes to better understand the severity of this vulnerability.