CVE-2025-39821CVE-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 P