Re: [PATCH v3] perf: Avoid undefined behavior from stopping/starting inactive events

[Peter Zijlstra <peterz@infradead.org>]

On Tue, Aug 12, 2025 at 04:51:28PM -0700, Liang, Kan wrote:
> 
> 
> On 2025-08-12 11:10 a.m., Yunseong Kim wrote:
> > 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:
> > Link: https://lore.kernel.org/lkml/714b7ba2-693e-42e4-bce4-feef2a5e7613@kzalloc.com/
> > 
> > Fixes: 9734e25fbf5a ("perf: Fix the throttle logic for a group")
> > Cc: Mark Rutland <mark.rutland@arm.com>
> > Signed-off-by: Yunseong Kim <ysk@kzalloc.com>
> 
> Thanks for the fix.
> 
> Reviewed-by: Kan Liang <kan.liang@linux.intel.com>

Thanks both!